Mapping of collision frequencies for stearic acid spin labels by saturation-recovery electron paramagnetic resonance.

Short pulse saturation-recovery electron paramagnetic resonance methods have been used to measure interactions of 14N:15N stearic acid spin label pairs in multilamellar liposomal dispersions composed of dimyristoyl-phosphatidylcholine (DMPC) and dielaidoylphosphatidylcholine (DEPC). Pairs consisting of various combinations of [14N]-16-, [14N]-12- or [14N]-5-doxylstearate, and [15N]-16-, [15N]-12-, or [15N]-5-doxylstearate were studied. SR experiments were performed at 27 degrees and 37 degrees C, and recovery signals were analyzed for initial conditions and multiexponential time constants by computer fitting using a damped least-squares approach. The time constants contain combinations of the electron spin lattice relaxation time, Tle, for each member of the spin-label pair, and the Heisenberg exchange rate constant, Kx. Spin-lattice relaxation times for each of the 14N and 15N stearic acid spin labels were determined, and it is noted that Tle for a given 15N-SASL was always slightly greater than that of the corresponding 14N-SASL. From Kx the bimolecular collision frequency was calculated, providing a detailed picture of molecular interactions. For both lipid systems the bimolecular collision rates were ordered as 12:5 less than 16:5 less than 5:5 less than 16:12 less than 12:12 less than 16:16. For all spin-label pairs studied, interaction frequencies were greater in DMPC than in DEPC. For the 16:16, 12:12, and 16:12 pairs, Kx was approximately 30% greater in DMPC than in DEPC, a significantly greater difference than is observed by conventional EPR methods.(ABSTRACT TRUNCATED AT 250 WORDS)     

Lipid lateral diffusion measurements in model membranes

The rate of lipid lateral diffusion has been investigated by computer simulation of electron spin resonance (ESR) spectra of spin-labelled dimyristoyl phosphatidylcholine (DMPC) vesicles. An optimization method has been developed to fit the experimental spectra to the theoretical ones calculated from the modified Bloch-equations in order to determine frequencies of probe-probe collisions and the lipid lateral diffusion coefficients. The main results of this study are: (i) Due to the sensitivity of our method to the extent of the overlapping of hyperfine spectral lines it is possible to determine the spin exchange contribution to linebroadening. (ii) It is obvious from these computer analyses that over a wide range of temperatures well above the phase transition both static dipolar interaction and dynamic spin exchange make significant contributions to the linebroadening. (iii) Lipid lateral diffusion coefficient in DMPC bilayers at 36 degrees C was (2.3 +/- 0.2) x 10(-11) m2 s-1.     

Dynamic imaging of lateral diffusion by electron spin resonance and study of rotational dynamics in model membranes. Effect of cholesterol.

The effects of cholesterol on the dynamics and the structural properties of two different spin probes, the sterol type CSL and the phospholipid type 16-PC, in POPC/cholesterol oriented multilayer model membranes were examined. Our results are consistent with a nonideal solution containing cholesterol-rich clusters created by the self association of cholesterol in POPC model membranes. The lateral diffusion coefficient D of the spin probes was measured over the temperature range of 15 to 60 degrees C and over the concentration range of 0 to 30 mol% of cholesterol in the model membrane by the electron spin resonance (ESR) imaging method. The rotational diffusion coefficients (including R perpendicular) and the order parameter S were determined utilizing a nonlinear least square ESR spectral simulation method. D, R perpendicular and S of CSL deviate considerably from linear dependence on mole percent cholesterol. The D of CSL was decreased by a factor of four at 15 degrees C and a factor of two at 60 degrees C for concentrations of cholesterol over 10 mol %, whereas those of 16-PC were hardly affected. Cholesterol decreased R perpendicular by a factor of 10 at 30 mol % of cholesterol, but it increased slightly that of 16-PC. A significant increase of S for CSL due to the presence of cholesterol was observed. It is shown how the difference in variation of S for CSL vs. 16-PC with composition may be interpreted in terms of their respective activity coefficients, and how a single universal linear relation is obtained for the S of both probes in terms of a scaled temperature. Simple but general correlations of D and of R perpendicular with S were also found, which aid in the interpretation of these diffusion coefficients.     

Evidence that the two free sulfhydryl groups of plasma fibronectin are in different local environments. Saturation-recovery electron spin resonance study.

Human plasma fibronectin is a dimer consisting of two subunits; each contains two cryptic thiol groups that were selectively labeled with an 15N,2H-maleimide spin label. Previous studies using conventional X-band electron spin resonance (ESR) methods showed that the spectrum of the labeled protein displays a single strongly immobilized component with an effective rotational correlation time of approximately 17 ns, suggesting that the physical environments of the two labeled sites per chain are indistinguishable. Here we have used saturation-recovery ESR to measure directly electron spin-lattice relaxation time (T1) of the labeled protein in solution at 27 degrees C. Interestingly, the time evolution of the signal was found to be biphasic, which was deconvoluted into two T1 values of 1.37 and 4.53 microseconds. Thus, the two spin-labeled sulfhydryl sites of plasma fibronectin (Fn), being similar in rates of rotational diffusion, differ by a factor of 3.2 in T1. Parallel experiments using various fibronectin fragments showed that the 1.37-microseconds component is associated with the label attached onto the thiol located in between the DNA-binding and the cell-binding domains, and the 4.53-microseconds component is associated with the label attached onto the thiol located within the carboxyl-terminal fibrin-binding domain. The data suggest that the saturation-recovery ESR is a useful method for differentiating multiple spin-labeled sites on macromolecules in which the labels undergo similar rates of rotational motion.     

Pulse EPR detection of lipid exchange between protein-rich raft and bulk domains in the membrane: methodology development and its application to studies of influenza viral membrane

A pulse saturation-recovery electron paramagnetic resonance (EPR) method has been developed that allows estimation of the exchange rates of a spin-labeled lipid between the bulk domain and the protein-rich membrane domain, in which the rate of collision between the spin label and molecular oxygen is reduced (slow-oxygen transport domain, or SLOT domain). It is based on the measurements of saturation-recovery signals of a lipid spin label as a function of concentrations of both molecular oxygen and the spin label. Influenza viral membrane, one of the simplest paradigms for the study of biomembranes, showed the presence of two membrane domains with slow and fast collision rates with oxygen (a 16-fold difference) at 30 degrees C. The outbound rate from and the inbound rate into the SLOT domain (or possibly the rate of the domain disintegration and formation) were estimated to be 7.7 x 10(4) and 4.6 x 10(4) s(-1), (15 micros residency time), respectively, indicating that the SLOT domain is highly dynamic and that the entire SLOT domain represents about one-third of the membrane area. Because the oxygen transport rate in the SLOT domain is a factor of two smaller than that in purple membrane, where bacteriorhodopsin is aggregated, we propose that the SLOT domain in the viral membrane is the cholesterol-rich raft domain stabilized by the trimers of hemagglutinin and/or the tetramers of neuraminidase.     

Exchange rates at the lipid-protein interface of myelin proteolipid protein studied by spin-label electron spin resonance

The electron spin resonance (ESR) spectra from spin-labeled phospholipids in recombinants of myelin proteolipid apoprotein with dimyristoylphosphatidylcholine have been simulated with the exchanged-coupled Bloch equations to obtain values for both the fraction of motionally restricted lipids and the exchange rate between the fluid and motionally restricted lipid populations. The rate of exchange between the two spin-labeled lipid components is found to lie in the slow exchange regime of nitroxide ESR spectroscopy. The values obtained for the fraction of motionally restricted component in the exchanged-coupled spectra are found to be in good agreement with those obtained previously by spectral subtraction for the same system [Brophy, P. J., Horváth, L. I., & Marsh, D. (1984) Biochemistry 23, 860-865]. The rate of lipid exchange off the protein is independent of lipid/protein ratio for a given spin-labeled phospholipid, as expected, and decreases with increasing selectivity of the various phospholipids for the protein. At 30 degrees C and for ionic strength 0.1 and pH 7.4, the off-rate constants are 4.6 X 10(6) s-1 for phosphatidic acid, 1.1 X 10(7) s-1 for phosphatidylserine, 1.6 X 10(7) s-1 for phosphatidylcholine, and 2.2 X 10(7) s-1 for phosphatidylethanolamine. These values are in the inverse ratio of the relative association constants of the various lipids for the protein (Brophy et al., 1984) and are appreciably slower than the rate of lipid lateral diffusion in dimyristoylphosphatidylcholine bilayers.(ABSTRACT TRUNCATED AT 250 WORDS)     

Exchange rates at the lipid-protein interface of the myelin proteolipid protein determined by saturation transfer electron spin resonance and continuous wave saturation studies.

The microwave saturation properties of various spin-labeled lipids in reconstituted complexes of the myelin proteolipid protein with dimyristoyl phosphatidylcholine have been studied both by conventional and saturation transfer electron spin resonance (ESR) spectroscopy. In the fluid phase, the conventional ESR spectra consist of a fluid and a motionally restricted (i.e., protein-associated) component, whose relative proportions can be determined by spectral subtractions and depend on the selectivity of the particular spin-labeled lipid for the protein. At 4 degrees C when the bulk lipid is in the gel phase, the integrated intensity of the saturation transfer ESR spectra displays a linear dependence on the fraction of motionally restricted lipid that is deduced from the conventional ESR spectra in the fluid phase, indicating the presence of distinct populations of free and protein-interacting lipid with no exchange between them on the saturation transfer ESR time scale in the gel phase. At 30 degrees C when the bulk lipid is in the fluid phase, the saturation transfer integral displays a nonlinear dependence on the fraction of motionally restricted lipid, consistent with exchange between the two lipid populations on the saturation transfer ESR time scale in the fluid phase. For lipid spin labels with different selectivities for the protein in complexes of fixed lipid/protein ratio, the data in the fluid phase are consistent with a constant (diffusion-controlled) on-rate for exchange at the lipid-protein interface. Values ranging between 1 and 9 x 10(6) s-1 are estimated for the intrinsic off-rates for exchange of spin-labeled stearic acid and phosphatidylcholine, respectively, at 30 degrees C. Conventional continuous wave saturation experiments lead to similar conclusions regarding the lipid exchange rates in the fluid and gel phases of the lipid/protein recombinants. The ESR saturation studies therefore demonstrate exchange on the time scale of the nitroxide spin-lattice relaxation at the lipid-protein interface of myelin proteolipid/dimyristoyl phosphatidylcholine complexes in the fluid phase but not in the gel phase.     

Studies on lipid membranes by two-dimensional Fourier transform ESR: Enhancement of resolution to ordering and dynamics.

The first two-dimensional Fourier-transform electron spin resonance (2D-FT-ESR) studies of nitroxide-labeled lipids in membrane vesicles are reported. The considerable enhancement this experiment provides for extracting rotational and translational diffusion rates, as well as orientational ordering parameters by means of ESR spectroscopy, is demonstrated. The 2D spectral analysis is achieved using theoretical simulations that are fit to experiments by an efficient and automated nonlinear least squares approach. These methods are applied to dispersions of 1-palmitoyl-2oleoyl-sn-glycerophosphatidylcholine (POPC) model membranes utilizing spin labels 1-palmitoyl-2-(16-doxyl stearoyl) phosphatidylcholine and the 3-doxyl derivative of cholestan-3-one (CSL). Generally favorable agreement is obtained between the results obtained by 2D-FT-ESR on vesicles with the previous results on similar systems studied by continuous wave (cw) ESR on aligned samples. The precision in determining the dynamic and ordering parameters is significantly better for 2D-FT-ESR, even though the cw ESR spectra from membrane vesicles are resolved more poorly than those from well aligned samples. Some small differences in results by the two methods are discussed in terms of limitations of the methods and/or theoretical models, as well as possible differences between dynamic molecular structure in vesicles versus aligned membranes. An interesting observation with CSL/POPC, that the apparent homogeneous linewidths seem to increase in "real time," is tentatively attributed to the effects of slow director fluctuations in the membrane vesicles.     

Lateral diffusion coefficients in membranes measured by resonance energy transfer and a new algorithm for diffusion in two dimensions

We describe measurements of lateral diffusion in membranes using resonance energy transfer. The donor was a rhenium (Re) metal-ligand complex lipid, which displays a donor decay time near 3 micros. The long donor lifetime resulted in an ability to measure lateral diffusion coefficient below 10(-8) cm(2)/s. The donor decay data were analyzed using a new numerical algorithm for calculation of resonance energy transfer for donors and acceptors randomly distributed in two dimensions. An analytical solution to the diffusion equation in two dimensions is not known, so the equation was solved by the relaxation method in Laplace space. This algorithm allows the donor decay in the absence of energy transfer to be multiexponential. The simulations show that mutual lateral diffusion coefficients of the donor and acceptor on the order of 10(-8) cm(2)/s are readily recovered from the frequency-domain data with donor decay times on the microsecond timescale. Importantly, the lateral diffusion coefficients and acceptor concentrations can be recovered independently despite correlation between these parameters. This algorithm was tested and verified using the donor decays of a long lifetime rhenium lipid donor and a Texas red-lipid acceptor. Lateral diffusion coefficients ranged from 4.4 x 10(-9) cm(2)/s in 1,2-dimyristoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (DMPG) at 10 degrees C to 1.7 x 10(-7) cm(2)/s in 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) at 35 degrees C. These results demonstrated the possibility of direct measurements of lateral diffusion coefficients using microsecond decay time luminophores.     

Electron spin resonance and electron-spin-echo study of oriented multilayers of L alpha-dipalmitoylphosphatidylcholine water systems.

A detailed electron spin resonance (ESR) study of spin-labeled-oriented multilayers of L alpha-dipalmitoylphosphatidylcholine (DPPC) water systems for low water content (2-10% by weight) is reported with the purpose of characterizing the dynamical and structural properties of model membrane systems. Emphasis is placed on the value of combining such experiments with detailed simulations based on current slow-motional theories. Information is obtained regarding ordering and anisotropic rotational diffusion rates via ESR lineshape analysis over the entire motional range, from the fast motional region through the moderately slow and slow to the rigid limit. This includes the low-temperature gel phase, the liquid crystalline L alpha (1) phase and what appears to be a third high-temperature phase above the L alpha phase. Cholestane (CSL) and spin-labeled DPPC (5-PC, 8-PC, and 16-PC) have been used to probe different depths of the bilayer. While CSL and 5-PC both reflect the high ordering of the bilayer close to the lipid-water interface, CSL appears to be located close enough to the water for the nitroxide to be involved in hydrogen bonding with water molecules. 16-PC reflects the relatively low ordering near the tail of the hydrocarbon chain in the bilayer. Quantitative estimates of ordering and motion are obtained for these cases. The results from CSL indicate that close to the lipid-water interface the DPPC molecule is oriented approximately perpendicular to the bilayer in these low water-content systems. However, all three labeled lipid probes indicate that the hydrocarbon chain of DPPC may be bent away from the bilayer normal by as much as 30 degrees and this evidence is stronger at low temperatures. When cholesterol is added to the DPPC-water system at a concentration greater than or equal to 2.5 mol %, the ordering is greatly increased although the rotational diffusion rate remains almost unaffected in the gel phase. Electron spin echoes (ESE) are observed for the first time from oriented lipid-water multilayers. Results obtained from cw ESR lineshape analysis are correlated with data from ESE experiments, which give a more direct measurement of relaxation times. These results indicate that for detection of very slow motions (close to the rigid limit) ESE experiments are more sensitive to dynamics than continuous wave ESR for which inhomogeneous broadening becomes a major problem.     

Dynamics and ordering in mixed model membranes of dimyristoylphosphatidylcholine and dimyristoylphosphatidylserine: a 250-GHz electron spin resonance study using cholestane.

We report here on a 250-GHz electron spin resonance (ESR) study of macroscopically aligned model membranes composed of mixtures of dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylserine (DMPS), utilizing the nixtroxide-labeled cholesterol analog cholestane (CSL). Two clearly resolved spectral components, distinct in both their ordering and dynamics, are resolved. The major component in membranes composed mostly of DMPC shows typical characteristics, with the long axis of CSL parallel to the bilayer normal with slow (10(6) </= R </= 10(7) s-1) rotational diffusion rates, as expected for cholesterol. The second component grows in as the mole fraction of DMPS increases. A detailed analysis shows that CSL senses a local, strongly biaxial environment. Our results imply that the inefficient packing between cholesterol and DMPS occurs probably because of the strong interactions between the PS headgroups, which provide the local biaxiality. Such a packing of the headgroups has been predicted by molecular dynamics simulations but had not been observed experimentally. The analysis of these spectral components was greatly aided by the excellent orientational resolution provided by the 250-GHz spectra. This enabled the key qualitative features of this interpretation to be "read" off the spectra before the detailed analysis.     

Dynamics of phosphatidylcholine-cholesterol mixed model membranes in the liquid crystalline state.

The effects of cholesterol on the dynamics of cholestane spin probe (CSL) in various phosphatidylcholine-cholesterol mixed model membranes are examined. The lateral diffusion, D of CSL in DMPC/POPC/cholesterol ternary mixtures, is measured utilizing an improved dynamic imaging electron spin resonance method. It shows a factor of two decrease at 10 mol % and 25 degrees C, whereas it shows only a 40% decrease at 50 mol % and 50 degrees C. A comparison with results in POPC/cholesterol mixtures, which show a stronger effect of cholesterol on D, indicates that acyl chain unsaturation leads to stronger self association of cholesterol in PC model membranes. An S2CSL dependence of the activation energy for D, has been confirmed for the DMPC/POPC/cholesterol mixtures. Here SCSL is the order parameter for CSL. A similar correlation of R perpendicular, the perpendicular component of the rotational diffusion coefficient, with SCSL, which is true for all three mixtures (DMPC/cholesterol, POPC/cholesterol, and DMPC/POPC/cholesterol) we have studied, is also found. These are associated with the effects of enhanced local ordering on the free volume needed for translation and reorientation. Such correlations of dynamic properties D and R perpendicular with the thermodynamic quantity S, as well as the consistent interpretations of the effect of acyl chain unsaturation on the dynamics in terms of the activity coefficients, strongly emphasize the interrelation between the dynamic structure and the thermodynamics of the PC/cholesterol mixtures.     

The effects of cholesterol on lateral diffusion and vertical fluctuations in lipid bilayers. An electron-electron double resonance (ELDOR) study.

Electron-electron double resonance (ELDOR) and saturation-recovery spectroscopy employing 14N:15N stearic acid spin-label pairs have been used to study the effects of cholesterol on lateral diffusion and vertical fluctuations in lipid bilayers. The 14N:15N continuous wave electron-electron double resonance (CW ELDOR) theory has been developed using rate equations based on the relaxation model. The collision frequency between 14N-16 doxyl stearate and 15N-16 doxyl stearate, WHex (16:16), is indicative of lateral diffusion of the spin probes, while the collision frequency between 14N-16 doxyl stearate and 15N-5 doxyl stearate, WHex (16:5), provides information on vertical fluctuations of the 14N-16 doxyl stearate spin probe toward the membrane surface. Our results show that: (a) cholesterol decreases the electron spin-lattice relaxation time Tle of 14N-16 doxyl stearate spin label in dimyristoylphosphatidylcholine (DMPC) and egg yolk phosphatidylcholine (egg PC). (b) Cholesterol increases the biomolecular collision frequency WHex (16:16) and decreases WHex (16:5), suggesting that incorporation of cholesterol significantly orders the part of the bilayer that it occupies and disorders the interior region of the bilayer. (c) Alkyl chain unsaturation of the host lipid moderates the effect of cholesterol on both vertical fluctuations and lateral diffusion of 14N-16 doxyl stearate. And (d), there are marked differences in the effects of cholesterol on lateral diffusion and vertical fluctuations between 0-30 mol% and 30-50 mol% of cholesterol that suggest an inhomogeneous distribution of cholesterol in the membrane.     

Stoichiometry, selectivity, and exchange dynamics of lipid-protein interaction with bacteriophage M13 coat protein studied by spin label electron spin resonance. Effects of protein secondary structure.

Bacteriophage M13 major coat protein has been isolated with cholate and reconstituted in dimyristoyl- and dioleoylphosphatidylcholine (DMPC and DOPC, respectively) bilayers by dialysis. Fourier transform infrared spectra of DMPC/coat protein recombinants confirmed that, whereas the protein isolated by phenol extraction was predominantly in a beta-sheet conformation, the cholate-isolated coat protein contained a higher proportion of the alpha-helical conformation [cf. Spruijt, R. B., Wolfs, C. J. A. M., & Hemminga, M. A. (1989) Biochemistry 28, 9158-9165]. The cholate-isolated coat protein/lipid recombinants gave different electron spin resonance (ESR) spectral line shapes of incorporated lipid spin labels, as compared with those from recombinants with the phenol-extracted protein that were studied previously [Wolfs, C. J. A. M., Horváth, L. I., Marsh, D., Watts, A., & Hemminga, M. A. (1989) Biochemistry 28, 9995-10001]. Plots of the ratio of the fluid/motionally restricted components in the ESR spectra of spin-labeled phosphatidylglycerol were linear with respect to the lipid/protein ratio in the recombinants up to 20 mol/mol. The corresponding values of the relative association constants, Kr, and number of association sites, N1, on the protein were Kr approximately 1 and N1 approximately 4 for DMPC recombinants and Kr approximately 1 and N1 approximately 5 for DOPC recombinants. Simulation of the two-component lipid spin label ESR spectra with the exchange-coupled Bloch equations gave values for the off-rate of the lipids leaving the protein surface of 2.0 x 10(7) s-1 at 27 degrees C in DMPC recombinants and 3.0 x 10(7) s-1 at 24 degrees C in DOPC recombinants.(ABSTRACT TRUNCATED AT 250 WORDS)     

Synthesis and biochemical applications of a solid cyclic nitrone spin trap: a relatively superior trap for detecting superoxide anions and glutathiyl radicals

A novel cyclic nitrone spin trap, 5-tert-butoxycarbonyl 5-methyl-1-pyrroline N-oxide (BMPO) as a pure white solid has been synthesized for the first time. BMPO offers several advantages over the existing spin traps in the detection and characterization of thiyl radicals, hydroxyl radicals, and superoxide anions in biological systems. The corresponding BMPO adducts exhibit distinct and characteristic electron spin resonance (ESR) spectral patterns. Unlike the 5,5-dimethyl-1-pyrroline N-oxide (DMPO)-derived superoxide adduct, the BMPO superoxide adduct does not non-enzymatically decompose to the BMPO hydroxyl adduct. This feature is clearly perceived as a definite advantage of BMPO in its biological applications. In addition, the ESR spectrum of the BMPO glutathionyl adduct (BMPO/*SG) does not fully overlap with the spectrum of its hydroxyl adduct. This spectral feature is again distinctly different from that of DMPO because the ESR spectral lines of DMPO glutathionyl and hydroxyl radical adducts largely overlap. Finally, the ESR spectra of BMPO-derived adducts exhibit a much higher signal-to-noise ratio in biological systems. These favorable chemical and spectroscopic features make BMPO ideal for the detection of superoxide anions, hydroxyl and thiyl radicals in biochemical oxidation and reduction.     

An electron spin resonance study of interactions between phosphatidylcholine and phosphatidylserine in oriented membranes.

A detailed electron spin resonance (ESR) study of mixtures of 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) and phosphatidylserine (POPS) in oriented multilayers in the liquid crystalline phase is reported with the purpose of characterizing the effects of headgroup mixing on the structural and dynamical properties of the acyl chains. These studies were performed over a range of blends of POPC and POPS and temperatures, utilizing the spin-labeled lipids 16-phosphatidylcholine and 5-phosphatidylcholine as well as cholestane (CSL). The ESR spectra were analyzed by nonlinear least-squares fitting using detailed spectral simulations. Whereas CSL shows almost no variation in ordering and rotational dynamics versus mole fraction POPS, (i.e. XPS), and 5-PC shows small effects, the weakly ordered end-chain labeled 16-PC shows large relative effects, such that the orientational order parameter, S is at a minimum for XPS = 0.5 where it is about one-third the value observed for XPS = 0 and 1. This is directly reflected in the ESR spectrum as a substantial variation in the hyperfine splitting with XPS. The least-squares analysis also shows a reduction in rotational diffusion coefficient, R perpendicular by a fractor of 2 for XPS = 0.5 and permits the estimation of S2, the ordering parameter representing deviations from cylindrically symmetric alignment. These results are contrasted with 2H NMR studies which were insensitive to effects of mixing headgroups on the acyl chains. The ESR results are consistent with a somewhat increased disorder in the end-chain region as well as a small amount of chain tilting upon mixing POPC and POPS.(ABSTRACT TRUNCATED AT 250 WORDS)     

ESR studies of spin-labeled membranes aligned by isopotential spin-dry ultracentrifugation: lipid-protein interactions.

Electron spin resonance (ESR) studies have been performed on spin-labeled model membranes aligned using the isopotential spin-dry ultracentrifugation (ISDU) method of Clark and Rothschild. This method relies on sedimentation of the membrane fragments onto a gravitational isopotential surface with simultaneous evaporation of the solvent in a vacuum ultracentrifuge to promote alignment. The degree of alignment obtainable using ISDU, as monitored by ESR measurements of molecular ordering for both lipid (16-PC) and cholestane spin labels (CSL), in dipalmitoylphosphatidylcholine (DPPC) model membranes compares favorably with that obtainable by pressure-annealing. The much gentler conditions under which membranes may be aligned by ISDU greatly extends the range of macroscopically aligned membrane samples that may be investigated by ESR. We report the first ESR study of an integral membrane protein, bacteriorhodopsin (BR) in well-aligned multilayers. We have also examined ISDU-aligned DPPC multilayers incorporating a short peptide gramicidin A' (GA), with higher water content than previously studied. 0.24 mol% BR/DPPC membranes with CSL probe show two distinct components, primarily in the gel phase, which can be attributed to bulk and boundary regions of the bilayer. The boundary regions show sharply decreased molecular ordering and spectral effects comparable to those observed from 2 mol% GA/DPPC membranes. The boundary regions for both BR and GA also exhibit increased fluidity as monitored by the rotational diffusion rates. The high water content of the GA/DPPC membranes reduces the disordering effect as evidenced by the reduced populations of the disordered components. The ESR spectra obtained slightly below the main phase transition of DPPC from both the peptide- and protein-containing membranes reveals a new component with increased ordering of the lipids associated with the peptide or protein. This increase coincides with a broad endothermic peak in the DSC, suggesting a disaggregation of both the peptide and the protein before the main phase transition of the lipid. Detailed simulations of the multicomponent ESR spectra have been performed by the latest nonlinear least-squares methods, which have helped to clarify the spectral interpretations. It is found that the simulations of ESR spectra from CSL in the gel phase for all the lipid membranes studied could be significantly improved by utilizing a model with CSL molecules existing as both hydrogen-bonded to the bilayer interface and non-hydrogen-bonded within the bilayer.     

Lipid lateral diffusion in oriented lipid/D2O multilayers by pulsed NMR

The temperature and hydration dependences of lipid lateral diffusion in model membrane/D₂O multilayers of dipalmitoyl (DPL), dilauryl (DLL) and egg yolk (EYPC) lecithins were measured using pulsed gradient proton nuclear magnetic resonance (NMR) spin echo techniques. Oriented samples were used to minimize anisotropic dipolar interactions and permit formation of a spin echo. Significantly lipid lateral diffusion is hydration dependent over the range studied (15–40% D₂O w/w), varying in DPL over this range for example by a factor of 2. For the saturated lipids at the same hydration and temperature, diffusion decreases monotonically as the chain length increases. The results tend to be larger, by factors of 2–5, than the earlier electron spin resonance (ESR) spin label results, the differences being attributable in part to the differences in hydration and to the absence of probe effects in this work. The addition of cholesterol (28.6 mol%) decreases diffusion of the lipids. Comparisons with other methods of lateral diffusion measurements are made.     

Electron spin resonance study of human alpha 1-acid glycoprotein interaction with a spin labelled steroid.

The interaction of human alpha 1-acid glycoprotein (AAG) with a corticosteroid was studied using nitroxide labeled deoxycorticosterone and electron spin resonance (ESR) spectroscopy. The ESR spectra of the spin labeled steroid in the presence of AAG could be used to characterize the ligand-protein interaction at equilibrium without the need of a separation between bound and free species. An association constant Ka of 6.10(5) M-1 at 20 degrees C and a binding capacity of one site per mole protein were found. ESR spectra recorded at equilibrium at various temperatures allowed the calculation of enthalpy and entropy variations for the steroid-protein interaction; these thermodynamic parameters exhibited a rapid change above 45 degrees C which may be related to a protein conformational modification above this temperature, as detected by circular dichroism study. The ESR spectra width could be used to define a polar character for the spin label environment in the steroid binding site of AAG and to calculate an apparent rotational correlation time of 2.8 x 10(-8) sec for the steroid-protein complex in aqueous solution at 20 degrees C. It can be concluded that spin labeling and ESR methodology is of value in the study of steroid-protein interactions of biological significance above all because it can provide direct physico-chemical information concerning the local environment of the ligand in its binding site at equilibrium.     

A multifrequency electron spin resonance study of T4 lysozyme dynamics.

Electron spin resonance (ESR) spectroscopy at 250 GHz and 9 GHz is utilized to study the dynamics and local structural ordering of a nitroxide-labeled enzyme, T4 lysozyme (EC 3.2.1.17), in aqueous solution from 10 degrees C to 35 degrees C. Two separate derivatives, labeled at sites 44 and 69, were analyzed. The 250-GHz ESR spectra are well described by a microscopic ordering with macroscopic disordering (MOMD) model, which includes the influence of the tether connecting the probe to the protein. In the faster "time scale" of the 250-GHz ESR experiment, the overall rotational diffusion rate of the enzyme is too slow to significantly affect the spectrum, whereas for the 9-GHz ESR spectra, the overall rotational diffusion must be accounted for in the analysis. This is accomplished by using a slowly relaxing local structure model (SRLS) for the dynamics, wherein the tether motion and the overall motion are both included. In this way a simultaneous fit is successfully obtained for both the 250-GHz and 9-GHz ESR spectra. Two distinct motional/ordering modes of the probe are found for both lysozyme derivatives, indicating that the tether exists in two distinct conformations on the ESR time scale. The probe diffuses more rapidly about an axis perpendicular to its tether, which may result from fluctuations of the peptide backbone at the point of attachment of the spin probe.