Molecular mobility of a fluorescent probe in binding sites of an albumin molecule

The molecular mobility of the fluorescent probe, N-(carboxymethyl)imide of 4-(dimethylamino)naphthalic acid (K-35), in three types of binding site on a human serum albumin (HSA) molecule has been studied. Study of the time-resolved decay of K-35 polarized fluorescence in HSA has shown that probe molecules bound to different sites have different fluorescence decay times, which poses problems in interpreting the polarization curves. However, it has been found that, in the case of rather slow thermal rotation of the probe, the decay of the vertical and the horizontal components of polarized fluorescence can each be approximated with three exponentials corresponding to three types of binding site. The mobility of the probe in different sites was estimated. The mobility was different but in all cases hindered by tens of times relative to the rotation of K-35 in water. The slowest motion occurred in the sites of the first type localized in the region of the well known drug site I: there the rotational correlation time was at least 72 ns. In the sites of the second type, this time was about 40 ns, and in the sites of the third type, about 10 ns. The faster was the rotation, the higher was the fluorescence quenching rate. Probably, it is this motion that is responsible for different fluorescence decay times in different HSA sites.     

K-35, a fluorescent probe for albumin study: optical properties

Fluorescent probe N-(carboxyphenyl)imide of 4-(dimethylamino)naphthalic acid, K-35, is used as an indicator of structural changes of human serum albumin molecules in pathology. The probe occupies albumin binding pockets where the probe environment is of very high polarity; probably, the pocket(s) contains protein polar groups and water molecules. At the same time rather small Stokes shift of K-35 fluorescence spectrum shows that the polar group motion is of one-two order of value lower than mobility of polar molecules in polar fluids. K-35 fluorescence decay in HSA can be described as a sum of three exponentials with time constants close to tau1=9 ns; tau2=3.6 ns and tau3=1.0 ns. A difference between excitation maxima of these three decay components shows that environment of these three species of K-35 molecules has been different before excitation. Different r values are probably a consequence of non-identical structure of several binding sites, or a binding site(s) can have a variable conformation.     

K-35, a fluorescent probe for albumin study: Optical properties

Fluorescent probe N-(carboxyphenyl)imide of 4-(dimethylamino)naphthalic acid, K-35, is used as an indicator of structural changes of human serum albumin molecules in pathology. The probe occupies albumin binding pockets where the probe environment is of very high polarity; probably, the pockets contain protein polar groups and also water molecules. At the same time the rather small Stokes shift of K-35 fluorescence spectrum shows that the polar group motion is one-two orders of magnitude lower than the mobility of polar molecules in polar fluids. K-35 fluorescence decay in HSA can be described as a sum of three exponentials with time constants close to τ₁ = 9 ns; τ₂ = 3.6 ns and τ₃ = 1.0 ns. The difference between excitation maxima of these three decay components shows that the environment of these three species of K-35 molecules has been different before excitation. Different τ values are probably a consequence of nonidentical structure of several binding sites, or the binding site(s) can have variable conformation.     

Features of the binding of the fluorescent probe K-35 to albumin

A study was made of the binding of a fluorescent probe K-35 (N-(carboxyphenyl)imide of 4-(dimethylamino)naphthalic acid), used as an indicator of albumin structural changes in pathology, to human serum albumin (HSA). Based on the data on the fluorescence decay of the probe, four types of site of K-35 binding to HSA have been recognized, which differ in fluorescence decay time (τ) and binding constant (K). Probe molecules bound to the first type of site have a decay time of 8–10 ns; this value corresponds to a high fluorescence quantum yield of about 0.7. These sites have a maximal binding constant, K ₁ = 5 · 10⁴ M⁻¹. The τ₂ of the second type of site is close to 3.6 ns and K ₂ = 1 · 10⁴ M⁻¹, which is much lower than K ₁; however, the number of these sites is several times greater. The number of sites of the third type and the binding constant are close to those of the second type, but the decay time τ₃ is 1 ns, which is significantly lower than τ₂. The binding of K-35 to sites of the second and the third types is characterized by a positive cooperativity. Their properties are similar but not completely identical. The total number of sites of these three types is about two per one HSA molecule. There are also one-two sites of the fourth type where bound K-35 molecules have a very short decay time τ₄ ≪ 1, i.e., are virtually nonfluorescent, and K ₄ = 1 · 10⁴ M⁻¹. The major contribution to the steady-state fluorescence is made by probe molecules bound to sites of the first and second types. As a rule, the concentration of albumin binding sites in blood is significantly higher than the concentration of metabolites and xenobiotics transferred by albumin. Therefore, the metabolite—or the probe in these experiments—is distributed among different sites in accordance with their K _i n _i values (n _i is the number of sites of the i-th type per albumin molecule). The low occupancy of the sites results in an approximately equal number of K-35 molecules bound to different sites of types 1, 2, and 3. The competition of K-35 with phenylbutazone, a marker of the albumin drug-binding site I, allows one to suggest that the K-35 site of the first type is localized exactly in the drug site I region, while the sites of the second and third types are close to it.     

Fluorescence polarization study of the alpha-ketoglutarate dehydrogenase complex from Escherichia coli

The lipoic acids of the alpha-ketoglutarate dehydrogenase multienzyme complex from Escherichia coli have been modified with two fluorescent probes, N-(1-pyrenyl)-maleimide and 5-[[[(iodoacetyl)amino]ethyl]amino]-naphthylene-1-sulfonic acid. Time-resolved fluorescence polarization of partially labeled complexes (18-77% inhibition of enzyme activity) reveals a complex depolarization process: one component of the anisotropy is characterized by a rotational correlation time much longer than the time scale of the measurements (less than or equal to 400 ns), reflecting the overall rotation of the complex, while a second component of the anisotropy decays with a rotational correlation time of 320 (+/- 50) ns. This decay is essentially independent of viscosity and is consistent with a model in which the depolarization is due to the dissociation from and rotation of lipoic acids between binding sites on the multienzyme complex. The sum of the rate constants characterizing the association and dissociation with the binding sites is approximately 3 x 10(6) s-1. In addition, approximately 5% of the anisotropy of the N-(1-pyrenyl)maleimide-labeled complex decays with a rotational correlation time of 25 ns; this can be attributed to local motion of the probe. At high extents of N-(1-pyrenyl)maleimide labeling (90-95% inhibition of enzyme activity), the anisotropy decay can be described by a constant term plus a rotational correlation time of about 1 microseconds. The increase in the correlation time probably reflects interactions between pyrene moieties. The N-(1-pyrenyl)maleimide-labeled dihydrolipoyl transsuccinylase core of the multienzyme complex has been isolated, and the anisotropy is constant over the observed time range of 300 ns. This suggests that the native structure is necessary for observation of lipoic acid movement within the complex. Fluorescent-labeled limited trypsin digestion fragments of the alpha-ketoglutarate dehydrogenase complex also have been isolated, and anisotropy measurements reveal substantial mobility of the label within the fragments. The time-resolved anisotropy of FAD in the native complex and in the isolated dihydrolipoyl dehydrogenase indicates some rapid local mobility of the FAD (rotational correlation time of 12 ns) that is viscosity independent, as well as a component of the anisotropy that is constant over the 35-ns time scale of the experiments.     

Application of a reference convolution method to tryptophan fluorescence in proteins. A refined description of rotational dynamics

A reference method for the deconvolution of polarized fluorescence decay data is described. Fluorescence lifetime determinations for p-terphenyl, p-bis[2-(5-phenyloxazolyl)]benzene and N-acetyltryptophanamide (AcTrpNH2) show that with this method more reliable fits of the decays can be made than with the scatterer method, which is most frequently used. Analysis of the AcTrpNH2 decay with p-terphenyl as the reference compound yields an excellent fit with lifetimes of 2.985 ns for AcTrpNH2 and 1.099 ns for p-terphenyl (20 degrees C), whereas the AcTrpNH2 decay cannot be satisfactorily fitted when the scatterer method is used. The frequency of the detected photons is varied to determine the conditions where pulse pile-up starts to affect the measured decays. At detection frequencies of 5 kHz and 15 kHz, which corresponds to 1.7% and 5% respectively of the rate of the excitation photons no effects are found. Decays measured at 30 kHz (10%) are distorted, indicating that pile-up effects play a role at this frequency. The fluorescence and fluorescence anisotropy decays of the tryptophan residues in the proteins human serum albumin, horse liver alcohol dehydrogenase and lysozyme have been reanalysed with the reference method. The single tryptophan residue of the albumin is shown to be characterized by a triple-exponential fluorescence decay. The anisotropy decay of albumin was found to be mono-exponential with a rotational correlation time of 26 ns (20 degrees C). The alcohol dehydrogenase has two different tryptophan residues to which single lifetimes are assigned. It is found that the rotational correlation time for the dehydrogenase changes with excitation wavelength (33 ns for lambda ex = 295 nm and 36 ns for lambda ex = 300 nm at 20 degrees C), indicating a nonspherical protein molecule. Lysozyme has six tryptophan residues, which give rise to a triple-exponential fluorescence decay. A single-exponential decay with a rotational correlation time of 3.8 ns is found for the anisotropy. This correlation time is significantly shorter than that arising from the overall rotation and probably originates from intramolecular, segmental motion.     

Measurement of rotational motion in membranes using fluorescence recovery after photobleaching.

A method has been developed for the measurement of the rotational motion of membrane components. In this method fluorescent molecules whose transition dipole moments lie in a given direction are preferentially destroyed with a short intense burst of polarized laser radiation. The fluorescence intensity, excited with a low intensity observation beam of polarized laser radiation, changes with time as the remaining fluorescent molecules rotate. The feasibility of the method has been demonstrated in a study of the rotation of the fluorescent lipid probe, dil ([bis,-2-(N-octadecyl-3,3-dimethyl-1-benzo[b]pyrrole]-trimethincyanine iodide) incorporated into membranes composed of distearoylphosphatidylcholine (DSPC) or dipalmitoylphosphatidylcholine (DPPC) and 0.20 mol% cholesterol, below the main chain-melting transition temperatures of the phosphatidylcholines. Rotation times in the 0.6-800 s range were observed. The fluorescence recovery (or decay) curves are in satisfactory agreement with theoretical calculations.     

Ligand interactions in the ArsA protein, the catalytic component of an anion-translocating adenosinetriphosphatase.

The ars operon of the conjugative R-factor R773 produces resistance to arsenicals in cells of Escherichia coli. The operon encodes an oxyanion pump which is composed of a membrane subunit, the 45.5-kDa ArsB protein, and a catalytic subunit, the 63-kDa ArsA protein. Purified ArsA protein is an arsenite(antimonite)-stimulated ATPase. From its amino acid sequence, as deduced from the nucleotide sequence, the ArsA protein has four tryptophanyl residues which could serve as intrinsic fluorescent probes for the study of substrate-induced conformational changes. Both static and dynamic measurements of tryptophan fluorescence were performed with the ArsA protein. Results from static anisotropy measurements indicated differences in molecular motion with addition of ATP, SbO2-, or Mg2+. These results were supported by time decay measurements of fluorescence anisotropy. The results of time decay measurements indicated a shorter correlation time, reflecting localized motion in the vicinity of the probe, and a longer correlation time, which could have arisen from rotation of the major portion of the molecule. The longer correlation time changed with addition of the various effectors, especially MgCl2, suggesting that binding of Mg2+ decreases probe mobility.     

Nanosecond spectroscopy of a dimeric enzyme: plasma amine oxidase.

The fluorescence dye 1-anilino-naphthalene-8-sulphonic acid (ANS) was used as a probe of non-polar binding sites in the enzyme plasma amine oxidase. Steady fluorescence measurements indicate that ANS binds to a single binding site of the dimeric enzyme with a dissociation constant of 5 microns. This binding site is different from the catalytic binding site. Nanosecond emission anisotropy measurements were performed on the ANS-enzyme in an effort to detect independent rotation of the subunits in the native enzyme. The observed rotational correlation time (phi = 105 ns) corresponds to the rotation of a rigid dimeric macromolecule. A rotational correlation time of 120 ns was obtained with the enzyme labelled with pyrenebutyric acid. It is concluded that the dimeric enzyme does not exhibit any modes of flexibility due to independent rotation of the subunits in the nanosecond range.     

Dipolar relaxation in a lipid bilayer detected by a fluorescent probe, 4'-dimethylaminochalcone

The dynamic behavior of polar molecules in egg phosphatidylcholine (PC) bilayers has been studied using a membrane fluorescent probe, 4'-dimethylaminochalcone (DMAC). Time and spectrally resolved fluorescence spectroscopy of DMAC incorporated in PC liposomes, as compared to studies of the probe in organic solvents, shows the existence of two independent populations, associated with different extent and speed of dipolar solvent relaxation. The first DMAC population represents approximately 69% of the fluorescence-emitting molecules, has a short fluorescence decay time (0.32 ns) and undergoes Stokes shift of 80 nm. The remaining 31% fraction of DMAC molecules has a decay time of 0.74 ns and undergoes a high (106 nm) Stokes shift. A fraction of the shift, ca. 24 nm for the first and 46 nm for the second population, is attributed to the fast (<0.1 ns) rotational relaxation of nearby dipolar molecules, which might be water. This two-state model accounts well for the detailed fluorescence properties of DMAC in egg PC, i.e. its broadened steady-state spectrum, its average fluorescence quantum yield and its complex wavelength-dependent fluorescence decays.     

Time-resolved tryptophan fluorescence anisotropy investigation of bacteriophage M13 coat protein in micelles and mixed bilayers

Coat protein of bacteriophage M13 is examined in micelles and vesicles by time-resolved tryptophan fluorescence and anisotropy decay measurements and circular dichroism experiments. Circular dichroism indicates that the coat protein has alpha-helix (60%) and beta-structure (28%) in 700 mM sodium dodecyl sulfate micelles and predominantly beta-structure (94%) in mixed dimyristoylphosphatidylcholine/dimyristoylphosphatidic acid (80/20 w/w) small unilamellar vesicles. The fluorescence decay at 344 nm of the single tryptophan in the coat protein after excitation at 295 or 300 nm is a triple exponential. In the micelles the anisotropy decay is a double exponential. A short, temperature-independent correlation time of 0.5 +/- 0.2 ns reflects a rapid depolarization process within the coat protein. The overall rotation of the coat protein-detergent complex is observed in the decay as a longer correlation time of 9.8 +/- 0.5 ns (at 20 degrees C) and has a temperature dependence that satisfies the Stokes-Einstein relation. In vesicles at all lipid to protein molar ratios in the range from 20 to 410, the calculated order parameter is constant with a value of 0.7 +/- 0.1 from 10 to 40 degrees C, although the lipids undergo the gel to liquid-crystalline phase transition. The longer correlation time decreases gradually on increasing temperature. This effect probably arises from an increasing segmental mobility within the coat protein. The results are consistent with a model in which the coat protein has a beta-structure and the tryptophan indole rings do not experience the motion of the lipids in the bilayer because of protein-protein aggregation.     

The dependence of the molecular dynamics of calmodulin upon pH and ionic strength

The mobilities of several fluorescent probes placed at different locations on calmodulin in the absence of Ca2+ have been found to depend upon the charge, ionic strength, and temperature. In general, the time decay of fluorescence anisotropy could be fitted with two rotational correlation times. The shorter of these reflects primarily the motion of the probe itself, while the longer corresponds to the motion of a major portion of the molecule. An increase in ionic strength or a decrease in net charge results in a decrease in the relative amplitude of the shorter correlation time, while an increase in temperature produces an increase in its amplitude. These results are consistent with, and suggest, that an increase in probe mobility accompanies an expansion of the calmodulin molecule under conditions of high electrostatic stress.     

Rotational modes of Ca2+-liganded calmodulin, as determined by time-domain fluorescence

The time decay of fluorescence anisotropy was monitored as a function of pH and temperature for complexes of 2,6-toluidinylnaphthalenesulfonate with calmodulin, with its proteolytic fragments, and with the 1:1 complex of calmodulin and melittin. For all the conditions examined the anisotropy decay of native calmodulin involved at least two rotational modes. These corresponded to a short correlation time of 2-3 ns, reflecting a localized motion in the vicinity of the binding site and a longer correlation time which arises from the rotation of a major portion of the molecule. The relative amplitudes of the two rotational modes were dependent upon temperature in the range 11-40 degrees C, the contribution of the more rapid mode increasing with temperature. The maximum immobilization of the probe occurred at pH 5.0 and 12 degrees C. While these results indicate the presence of internal rotations in Ca2+-liganded calmodulin, the magnitude of the longer correlation time is consistent with the crystallographic structure.     

A nanosecond fluorescence depolarization study on the segmental flexibility of receptor-bound immunoglobulin E

Time-resolved fluorescence anisotropy measurements have been used to examine the segmental flexibility of anti-dansyl immunoglobulin E (IgE) bound to its high-affinity receptor on membrane vesicles from rat basophilic leukemia cells. Although IgE in this complex exhibits only a restricted angular range of segmental motion, much of this restricted motion occurs on a relatively rapid time scale. A fast component of motion with a rotational correlation time of 15-35 ns may correspond to the twisting of Fab segments about their major axis. Intermolecular cross-linking by a short bivalent ligand, N,N'-didansylcadaverine, results in complete loss of this segmental motion. Solubilization of monomeric IgE-receptor complexes using a zwitterionic detergent results in a time-dependent anisotropy decay that exhibits both a fast component and a slower component that is intermediate between the decay for soluble and membrane-bound forms of IgE at long times after excitation. These results are discussed in terms of a model in which binding of IgE to its membrane-bound receptor restricts not only its global rotation but also its slower modes of segmental flexibility as well, while allowing its Fab segments to undergo rapid reorientation within a limited angular range.     

Nanosecond dynamics of horse heart apocytochrome c in aqueous solution as studied by time-resolved fluorescence of the single tryptophan residue (Trp-59)

The time-resolved fluorescence emission characteristics of the single tryptophan residue (Trp-59) of horse heart apocytochrome c--the precursor of the intramitochondrial cytochrome c--were studied in aqueous solution. The total fluorescence intensity decay measured over the whole emission spectrum was analyzed as a sum of three or four exponentials by the nonlinear least-squares method, the last model always providing a slight but significant decrease in the chi 2 values. Maximum entropy analysis, recently developed for time-resolved fluorometry (Livesey et al., 1987; Livesey & Brochon, 1987), strongly suggests the existence of a distribution including at least four separate classes of lifetimes. The center values were around 0.1-0.2, 1, 3, and 5 ns, in agreement with the lifetime values obtained by nonlinear least-squares regression analysis. As a function of the emission wavelength, these values remained constant within the experimental error, whereas a redistribution of the fractional amplitudes was observed: the contributions of the short components increased in the blue edge region of the emission spectrum. Temperature increase led essentially to a redistribution of the fractional amplitudes, affecting mostly that of the 5-ns component, which almost totally disappeared at high temperature (35-40 degrees C). The lifetime values were not significantly affected except for the 3-ns component, which decreased by about 15% in the temperature range studied. Such observations strongly suggest that the protein exists under different conformational substates in thermal equilibrium. Time-resolved fluorescence anisotropy measurements evidenced the existence of fast internal rotation of the Trp residue. An average maximum restricted angle of rotation of around 55 degrees was calculated. A second internal motion, slower by 1 order of magnitude, corresponding likely to a local motion of the peptide chain involving the Trp-59 residue, was detected on the anisotropy decay curve. Finally, the longest correlation time (5 ns) should correspond to the average rotation of the overall protein. Its value doubled as a function of the protein concentration, revealing an association process leading most likely to a dimer in the concentration range studied (2-139 microM). The flexibility of the peptide chain was more restrained in the associated than in the monomeric form, but the fast internal rotation of the Trp residue was not.     

Dynamics of benzo[a]pyrene diol epoxide adducts in poly(dG-dC).(dG-dC) studied by synchrotron excited fluorescence polarization anisotropy decay.

Time-resolved fluorescence studies have been performed on (+)-anti-7,8-dihydrodiol-9,10-epoxybenzo[a]pyrene adducts in double-stranded poly(dG-dC).(dG-dC). Part of the adduct population gives rise to excimer fluorescence. The heterogeneous fluorescence emission decay curves at 22 degrees C could be resolved into three components with lifetimes: 0.4 ns, 3 ns and 24 ns for the total fluorescence (monomer and excimer emission), and 0.5 ns, 5 ns and 24 ns, respectively, for excimer emission alone. The relative amplitudes for the longer lifetimes were larger for the pure excimer population than for the mixed population. The fluorescence polarization anisotropy decay curves were resolved into two components of rotational correlation times: 0.4 ns and 25 ns for the total fluorescence and 0.3 ns and 33 ns for the excimer fluorescence. We interpret the two rotational correlation times to correspond to local motion of the adduct and segmental motion of the polynucleotide, respectively.     

胡椒碱分子键合人血清白蛋白位点的表征

利用荧光光谱法、紫外光谱法并结合计算机模拟技术在分子水平上研究了胡椒碱与人血清白蛋白(human serum albumin HSA)的键合作用.同步荧光及紫外光谱图表明,胡椒碱对HSA微环境有影响.位点竞争试验证明,胡椒碱分子键合在HSA的位点Ⅱ区.通过荧光光谱滴定数据求得不同温度下(300K 310K和318 K)药物与蛋白相互作用的结合常数及结合位点数.分子模拟的结果显示了胡椒碱与HSA的键合区域和键合模式,表明药物与蛋白有较强的键合作用;维持药物与蛋白质的相互作用力主要是疏水用,兼有氢键(位于氨基酸残基Arg 257,Arg 222及Arg218位).通过实验数据计算得到的热力学参数(ΔH0与ΔS0的值分别为原33.11 kJ·mol-1和原18.90 J·mol原1·K-1)确定了胡椒碱与HSA分子的相互作用力类型主要为氢键兼范德华力.     

Conformational dynamics of bovine Cu, Zn superoxide dismutase revealed by time-resolved fluorescence spectroscopy of the single tyrosine residue.

The structural dynamics of bovine erythrocyte Cu, Zn superoxide dismutase (BSOD) was studied by time-resolved fluorescence spectroscopy. BSOD is a homodimer containing a single tyrosine residue (and no tryptophan) per subunit. Frequency-domain fluorometry revealed a heterogeneous fluorescence decay that could be described with a Lorentzian distribution of lifetimes. The lifetime distribution parameters (center and width) were markedly dependent on temperature. The distribution center (average lifetime) displayed Arrhenius behavior with an Ea of 4.2 kcal/mol, in contrast with an Ea of 7.4 kcal/mol for the single-exponential decay of L-tyrosine. This indicated that thermal quenching of tyrosine emission was not solely responsible for the effect of temperature on the lifetimes of BSOD. The distribution width was broad (1 ns at 8 degrees C) and decreased significantly at higher temperatures. Furthermore, the width of the lifetime distribution increased in parallel to increasing viscosity of the medium. The combined effects of temperature and viscosity on the fluorescence decay suggest the existence of multiple conformational substrates in BSOD that interconvert during the excited-state lifetime. Denaturation of BSOD by guanidine hydrochloride produced an increase in the lifetime distribution width, indicating a larger number of conformations probed by the tyrosine residue in the denatured state. The rotational mobility of the tyrosine in BSOD was also investigated. Analysis of fluorescence anisotropy decay data enabled resolution of two rotational correlation times. One correlation time corresponded to a fast (picosecond) rotation that contributed 62% of the anisotropy decay and likely reported local mobility of the tyrosine ring. The longer correlation time was 50% of the expected value for rotation of the whole (dimeric) BSOD molecule and appeared to reflect segmental motions in the protein in addition to overall tumbling. Comparison between rotational correlation times and fluorescence lifetimes of BSOD indicates that the heterogeneity in lifetimes does not arise from mobility of the tyrosine per se, but rather from dynamics of the protein matrix surrounding this residue which affect its fluorescence decay.     

Environmental modulation of M13 coat protein tryptophan fluorescence dynamics

The effects of detergent [deoxycholate (DOC) and phospholipid [dimyristoylphosphatidylcholine (DMPC)] environments on the rotational dynamics of the single tryptophan residue 26 of bacteriophage M13 coat protein have been investigated by using time-resolved single photon counting measurements of the fluorescence intensity and anisotropy decay. The total fluorescence decay of tryptophan-26 is complex but rather similar in DOC as compared to DMPC when analyzed in terms of a lifetime distribution (exponential series method). This similarity, in conjunction with the almost identical steady-state fluorescence spectra, indicates only minor differences between the tryptophan environments in DOC and DMPC. The reorientational dynamics of tryptophan-26 are dominated by slow rotation of the entire protein in both detergent and phospholipid environments. The resolved anisotropy decay in DOC can be approximated by a simple hydrodynamic model of protein/detergent micelle rotational diffusion, although the data indicative slightly greater complexity in the rotational motion. The tryptophan fluorescence anisotropy is not sensitive to protein conformational changes in DOC detected by nuclear magnetic resonance on the basis of pH independence in the range 7.5-9.1. In DMPC bilayers, restricted tryptophan motion with a correlation time of approximately 2 ns is observed together with a second very slow reorientational component. Resolution of the time constant for this slow rotation is obscured by the tryptophan fluorescence time window being too short to clearly locate its anisotropic limit. The possible contribution made by axial rotational diffusion of the protein to this slow rotational process is discussed.(ABSTRACT TRUNCATED AT 250 WORDS)     

Nanosecond spectroscopy of retinol.

Nanosecond fluorescence spectroscopy was used to study the unique binding site of the retinol-binding protein (RBP) from human serum. At pH 7.4, the binding of retinol to RBP caused the following spectroscopic changes in the ligand: (a) an enhancement of the fluorescence decay time (gamma = 8 ns); and (b) an increase in the emission anisotropy (A = 0.29). Retinol in hexane has a fluorescent decay time of 4.2 ns and a low emission anisotropy (A = 0.02). The increase in the fluorescence decay time of bound retinol is not due to dielectric relaxation effects of polar groups, since nanosecond time-resolved emission spectra of either retinol in glycerol or retinol bound to RBP, failed to show any time-dependent shifts in emission maxima during the time period investigated 0 to 30 ns. The degree of rotational mobility of bound retinol was investigated by time emission anisotropy measurements. The observed rotational correlation time (theta = 7.2 ns) is consistent with a rigid compact macromolecule of 21,000 molecular weight.