Pressure-induced perturbation of ANS-apomyoglobin complex: frequency domain fluorescence studies on native and acidic compact states.

The pressure dependence of the flexibility of the 8-anilino-1-naphthalene sulfonate (ANS)-apomyoglobin complex was investigated in the range between atmospheric pressure and 2.4 kbar by frequency domain fluorometry. We examined two structural states: native and acidic compact. The conformational dynamics of the ANS-apomyoglobin complex were deduced by studying the emission decay of ANS, which can form a noncovalent complex with the apoprotein in both the native and the acidic compact forms. Because the free fluorophore has a very short lifetime (less than 75 ps), its contribution can be separated from the long-lived emission. The latter arises from ANS molecules bound to the protein and provides information on the structural and dynamic characteristics of the macromolecule. The fluorescence emission decay of the ANS-apomyoglobin complex at neutral pH has a broad fluorescence lifetime distribution (width at half-maximum = 4.1 ns). The small changes in the fluorescence distribution parameters that occur with changes in pressure indicate that the ANS-apomyoglobin complex at neutral pH holds its compactness even at 2.4 kbar. A small contraction of molecular volume has been detected at low pressure, followed by a slight swelling with an increase in flexibility at higher pressures. The heterogeneity of ANS fluorescence in the acidic compact state of apomyoglobin is even greater than that in the native form (distribution width = 10 ns); moreover, the acidic compact state appears more expanded and accessible to solvent molecules than the native state, as suggested by the distribution center, which is 11 ns for the former and 19 ns for the latter. The lifetime distribution center remains constant with increasing pressure, which suggests that no other binding site is formed at high pressure.     

Dynamics of ANS binding to tuna apomyoglobin measured with fluorescence correlation spectroscopy.

The dynamics of the binding reaction of ANS to native and partly folded (molten globule) tuna and horse apomyoglobins has been investigated by fluorescence correlation spectroscopy and frequency domain fluorometry. The reaction rate has been measured as a function of apomyoglobin and ANS concentrations, pH, and temperature. Examination of the autocorrelation functions shows that the reaction rate is fast enough to be observed in tuna apomyoglobin, whereas the reaction rate in horse apomyoglobin is on the same time scale as diffusion through the volume or longer. Specifically, for tuna apomyoglobin at pH 7 and room temperature the on rate is 2200 microM(-1) s(-1) and the off rate is 5900 s(-1), in comparison with k(on) = 640 microM(-1) s(-1) and k(off) = 560 s(-1) for horse myoglobin as measured previously. The independence of the reaction rate from the ANS concentration indicates that the reaction rate is dominated by the off rate. The temperature dependence of the on-rate shows that this rate is diffusion limited. The temperature dependence of the off rates analyzed by Arrhenius and Ferry models indicates that the off rate depends on the dynamics of the protein. The differences between horse and tuna apomyoglobins in the ANS binding rate can be explained in terms of the three-dimensional apoprotein structures obtained by energy minimization after heme removal starting from crystallographic coordinates. The comparison of the calculated apomyoglobin surfaces shows a 15% smaller cavity for tuna apomyoglobin. Furthermore, a negative charge (D44) is present in the heme cavity of tuna apomyoglobin that could decrease the strength of ANS binding. At pH 5 the fluorescence lifetime distribution of ANS-apomyoglobin is bimodal, suggesting the presence of an additional binding site in the protein. The binding rates determined by FCS under these conditions show that the protein is either in the open configuration or is more flexible, making it much easier to bind. At pH 3, the protein is in a partially denatured state with multiple potential binding sites for ANS molecule, and the interpretation of the autocorrelation function is not possible by simple models. This conclusion is consistent with the broad distribution of ANS fluorescence lifetimes observed in frequency domain measurements.     

ANS fluorescence: Potential to discriminate hydrophobic sites of proteins in solid states

In the current study, ANS fluorescence was established as a powerful tool to study proteins in solid-state. Silk fibroin from Bombyx mori cocoons was used as a paradigm protein. ANS incorporated into the films of silk fibroin exhibits fluorescence with two-lifetime components that can be assigned to the patches and/or cavities with distinct hydrophobicities. Decay associated spectra (DAS) of ANS fluorescence from both sites could be fit to the single log-normal component indicating their homogeneity. ANS binding sites in the protein film are specific and could be saturated by ANS titration. ANS located in the binding site that exhibits the long-lifetime fluorescence is not accessible to the water molecules and its DAS stays homogeneously broadened upon hydration of the protein film. In contrast, ANS from the sites demonstrating the short-lifetime fluorescence is accessible to water molecules. In the hydrated films, solvent-induced fluctuations produce an ensemble of binding sites with similar characters. Therefore, upon hydration, the short-lifetime DAS becomes significantly red-shifted and inhomogeneously broadened. The similar spectral features have previously been observed for ANS complexed with globular proteins in solution. The data reveal the origin of the short-lifetime fluorescence component of ANS bound to the globular proteins in aqueous solution. Findings from this study indicate that ANS is applicable to characterize dehydrated as well as hydrated protein aggregates, amyloids relevant to amyloid diseases, such as Alzheimer's, Parkinson, and prion diseases.     

Evidence of heterogeneous 1-anilinonaphthalene-8-sulfonate binding to beta-lactoglobulin from fluorescence spectroscopy.

Steady-state and dynamic fluorescence titrations show that: (a) the complex between beta-lactoglobulin (BLG) and 1-anilinonaphthalene-8-sulfonate (ANS) displays a heterogeneous equilibrium with large changes in the binding strength vs. pH and ion concentration; and (b) the fluorescence response of bound ANS reveals two separate lifetimes that suggest two different sites (or binding modes). While steady-state fluorescence titrations yield effective values of the binding constant and of the bound ANS quantum efficiency, it is shown that, by combining steady-state fluorescence and lifetime decay of ANS, it is possible to give quantitative estimates of the association constants for each site. When heading from the acid (pH approximately 2) to the native state (pH approximately 6) the main result is a very large reduction of the effective binding constant. This and the results of titrations vs. ionic strength suggest that electrostatic interactions are a major contribution to ANS binding to BLG.     

Similarity of fluorescence lifetime distributions for single tryptophan proteins in the random coil state.

The picosecond time-resolved fluorescence decay data of nine single-tryptophan (trp) proteins and two multi-trp proteins in their native and denatured states were analyzed by the maximum entropy method (MEM). In the denatured state (6 M guanidine hydrochloride) a majority of the single-trp proteins show bimodal (at 25 degrees C) and trimodal (at 85 degrees C) distributions with similar patterns and similar values for average lifetimes. In the native state of the proteins the lifetime distributions were bimodal or trimodal. These results (multimodal distributions) are contradictory to the unimodal Lorentzian distribution of lifetimes reported for some proteins in the native and denatured states. MEM analysis gives a unimodal distribution of lifetimes only when the signal-to-noise ratio is poor in the time-resolved fluorescence decay data. The unimodal distribution model is therefore not realistic for proteins in the native and denatured states. The fluorescence decay components of the bi- or trimodal distribution are associated with the rotamer structures of the indole moiety when the protein is in the random coil state.     

New insight on beta-lactoglobulin binding sites by 1-anilinonaphthalene-8-sulfonate fluorescence decay

The fluorescence time decay parameters of the beta-lactoglobulin-1-anilinonaphthalene-8-sulfonate complex have been investigated under physical and chemical perturbations (2 < pH < 8 and added electrolyte 0 < NaCl < 0.5 M) to obtain new insight on the nature of the protein binding interactions. A double exponential decay of the bound probe lifetime has been confirmed by the presence of a longer component, 11 to 14.5 ns, and a shorter component, 2.5 to 3.5 ns. The two lifetimes are ascribed to different binding modes associated also with different exposure to the solvent; in particular, the longer component is attributed to binding inside the hydrophobic beta barrel, while a "surface" site is suggested for the shorter component. A detailed analysis of the lifetime fractional intensities correlates the binding constants with ionic strength and supports the presence of electrostatic effects at both sites. A Debye-Hückel approach, applied to extrapolate the electrostatic free energy contribution vs. pH at vanishing ionic strength, gives interesting clues on the effective charge felt by the ANS ligands in the proximity of each site. In particular, binding is found to parallel the aspartate and glutamate titrations between pH 3 and pH 4.5; the "surface" site mainly responds to the presence of these local titrating charges while the "internal" site more closely follows the overall protein net charge.     

Insight into the conformational dynamics of specific regions of porcine pancreatic phospholipase A2 from a time-resolved fluorescence study of a genetically inserted single tryptophan residue

The effects of Ca2+ and substrate analogue binding on the conformational dynamics of porcine pancreas phospholipase A2 (PLA2) in different regions was explored by combining site-directed mutagenesis and time-resolved fluorescence measurements. The single tryptophan residue (Trp-3) of the wild-type protein (W3), in the alpha-helix A, was replaced by a phenylalanine residue (W3F), whereafter Trp was substituted either for leucine-31 (W31), located in the calcium binding loop, or for phenylalanine-94 (W94), located at the "back side" of the enzyme. Furthermore, mutants lacking the 62-66 sequence were constructed with the Trp at position 3 (delta W3) or 31 (delta W31). The total fluorescence intensity decays of Trp in each protein, in the protein-calcium and the protein-calcium-substrate analogue complexes, analyzed by the maximum entropy method (MEM) can be interpreted as distributions of separated lifetime classes. In the case of the W94 mutant, a major short-lived excited-state population (tau approximately 50 ps) is observed, probably deactivated by the interaction with two proximate disulfide bridges via a radiationless process. For the four other mutants, the respective barycenters of the four lifetime classes display comparable values, but the amplitude distributions are different for Trp-3 and Trp-31. The rotational mobility of the Trp residue varies along the peptide chain. Trp-3 experiences only a fast hindered motion. Trp-31 is sensitive to an additional local flexibility that is absent in the N-terminal part of the protein. The largest wobbling angle is observed at position 94. No effect of calcium binding occurs on the lifetime distribution of the Trp-3 and Trp-94 residues. Their mobilities are not affected. In contrast, calcium binding displays a strong influence on the excited-state population distribution of Trp-31. A major population decaying with the longest lifetime is selected in the W31 protein and contributes to approximately 50% of the decay. The local flexibility and the amplitude of motion of Trp-31 is wider in the protein-calcium complex than in the unliganded protein. Binding of the monomeric substrate analogue n-dodecylphosphocholine (C12PN) in the presence of calcium slightly affects the Trp-3 excited-state population distribution and its mobility. Trp-31 is more sensitive to this binding. In particular, a more restricted rotation of the Trp-31 residue and a decrease of the peptide local flexibility as protein-calcium complexes are observed in both the W31 and delta W31 mutants.(ABSTRACT TRUNCATED AT 400 WORDS)     

Conformational substates of myoglobin detected by extrinsic dynamic fluorescence studies

The extent of conformational substates of two apomyoglobins, i.e., sperm whale and tuna apomyoglobin, was investigated by examining the fluorescence decay in the frequency domain of the extrinsic fluorophore TNS [6-(p-toluidino)-2-naphthalenesulfonic acid] bound to the heme binding site. Data analysis was performed in terms of a continuous, unimodal lifetime distribution having a Lorentzian shape. The results were compared with those for the free fluorophore in an isotropic nonviscous solvent. The incorporation of TNS into the protein matrix resulted in a broadening of the lifetime distribution due to the microenvironmental heterogeneity generated by structural fluctuations. The larger width of lifetime distribution observed for TNS bound to tuna apomyoglobin was related to a more extended conformational space accessible to the fluorophore in this protein compared to sperm whale myoglobin. A temperature increase from 15 to 40 degrees C produced a further broadening of the lifetime distributions of TNS bound to both proteins. This result can be explained by assuming the existence of conformational substates at high energy content or separated by high energy barriers, which are not populated at low temperature. The overall picture emerging from the reported data is that the lifetime distributions of TNS bound to apomyoglobins are determined largely by the number of conformational substates accessible to the protein matrix and, to a lesser extent, by the interconversion rates among these states.     

Structural analysis of the operator binding domain of Tn10-encoded Tet repressor: a time-resolved fluorescence and anisotropy study.

An engineered Tn10-encoded Tet repressor, bearing a single Trp residue at position 43, in the putative alpha-helix-turn-alpha-helix motif of the operator binding domain, was studied by time-resolved fluorescence and anisotropy. Fluorescence intensity decay data suggested the existence of two classes of Trp-43, defined by different lifetimes. Analysis of anisotropy data were consistent with a model in which each class was defined by a different lifetime, rotational correlation time, and fluorescence emission maximum. The long-lifetime class had a red-shifted spectrum, similar to that of tryptophan zwitterion in water, and a short rotational correlation time. In contrast, the spectrum of the short-lifetime class was blue-shifted 10 nm compared to that of the long-lifetime class. Its correlation time was similar to that of the protein, which showed that Trp in this class was entirely constrained. Trp in this latter class could not be quenched by iodide, whereas most of the long-lifetime class was easily accessible. Presence of disruptive agents, such as 1 M GuCl or 3 M KCl, did not alter markedly the lifetimes but increased the weight of the short-lifetime component. In the same time, the rotational correlation time of this component was dramatically reduced. Taken together, our data suggest that the long-lifetime class could correspond to the tryptophan residues exposed to solvent whereas the short-lifetime class would correspond to the tryptophan residues embedded inside the hydrophobic core holding the helix-turn-helix motif. Destabilization of hydrophobic interactions would lead to an increase in the weight of the latter class for entropic reasons. Analysis of the fluorescence parameters of Trp-43 could provide structural information on the operator binding domain of Tet repressor.     

Demonstration of an associated anisotropy decay by frequency-domain fluorometry

We used frequency-domain fluorometry to demonstrate the presence of an associated decay of fluorescence anisotropy. In such systems the individual correlation times are associated with distinct emitting species, each with its own characteristic lifetime and rotational correlation times. We obtained an associated system using 1-anilino-8-naphthalenesulfonic acid (ANS) in the presence of increasing amounts of apomyoglobin. When both free and apomyoglobin-bound ANS contributed to the emission the differential polarized phase angles become negative at particular frequencies, even though the fundamental anisotropy (r0) is greater than zero. Additionally, the modulated anisotropy decreases at high frequencies. Both observations appear to be the unique consequence of an associated anisotropy decay, and are not possible for a multiexponential anisotropy decay of a single species.     

Anion-binding centers of human serum albumin during an N-F conformation transition and in isolated albumin and blood serum

Fluorescent probe N-phenyl-1-amino-8-sulfonaphthalene (ANS) was used for studying pH-dependent structural N-F-transition in human serum albumin of two kinds: in commercial albumin and in natural blood serum. The kinetics of ANS fluorescence decay in albumin solutions was measured. There were found two types of the sites occupied by ANS in albumin under physiological conditions (pH 7.4). In the first binding site ANS fluorescence decay time was 16.6 +/- 0.3 nsec and it was not significantly changed at N-F transition (pH 4.0). In the second binding site the decay time was dependent on pH in commercial albumin and was not significantly changed in serum. In the second binding site there were individual differences of ANS decay time (4.3 +/- 0.6 nsec). The observed ANS fluorescence intensity enhancing (about 40-50%) in N-F transition may be explained by an increase of albumin binding sites capacity for ANS.     

Steady-state and time-resolved fluorescence studies on wild type and mutant chromatium vinosum high potential iron proteins: holo- and apo-forms

Detailed circular dichroism (CD), steady-state and time-resolved tryptophan fluorescence studies on the holo- and apo- forms of high potential iron protein (HiPIP) from Chromatium vinosum and its mutant protein have been carried out to investigate conformational properties of the protein. CD studies showed that the protein does not have any significant secondary structure elements in the holo- or apo- HiPIP, indicating that the metal cluster does not have any effect on formation of secondary structure in the protein. Steady-state fluorescence quenching studies however, suggested that removal of the iron-sulfur ([Fe(4)S(4)](3+)) cluster from the protein leads to an increase in the solvent accessibility of tryptophans, indicating change in the tertiary structure of the protein. CD studies on the holo- and apo- HiPIP also showed that removal of the metal prosthetic group drastically affects the tertiary structure of the protein. Time-resolved fluorescence decay of the wild type protein was fitted to a four-exponentials model and that of the W80N mutant was fitted to a three-exponentials model. The time-resolved fluorescence decay was also analyzed by maximum entropy method (MEM). The results of the MEM analysis agreed with those obtained from discrete exponentials model analysis. Studies on the wild type and mutants helped to assign the fast picosecond lifetime component to the W80 residue, which exhibits fast fluorescence energy transfer to the [Fe(4)S(4)](3+) cluster of the protein. Decay-associated fluorescence spectra of each tryptophan residues were calculated from the time-resolved fluorescence results at different emission wavelengths. The results suggested that W80 is in the hydrophobic core of the protein, but W60 and W76 are partially or completely exposed to the solvent.     

Interpretation of fluorescence decays using a power-like model

A power-like decay function, characterized by the mean excited-state lifetime and relative variance of lifetime fluctuation around the mean value, was applied in analysis of fluorescence decays measured with the aid of time-correlated single photon counting. We have examined the fluorescence decay, in neutral aqueous medium, of tyrosine (L-tyrosine and N-acetyl-L-tyrosinamide), and of the tyrosine residues in a tryptophan-free protein, the enzyme purine nucleoside phosphorylase from Escherichia coli in a complex with formycin A (an inhibitor), and orthophosphate (a co-substrate). Tryptophan fluorescence decay was examined in neutral aqueous medium for L-tryptophan, N-acetyl-L-tryptophanamide, and for two tryptophan residues in horse liver alcohol dehydrogenase. To detect solvent effect, fluorescence decay of Nz-acetyl-L-tryptophanamide in aqueous medium was compared with that in dioxan. Hitherto, complex fluorescence decays have usually been analyzed with the aid of a multiexponential model, but interpretation of the individual exponential terms (i.e., pre-exponential amplitudes and fluorescence lifetimes), has not been adequately characterized. In such cases the intensity decays were also analyzed in terms of the lifetime distribution as a consequence of an interaction of fluorophore with environment. We show that the power-like decay function, which can be directly obtained from the gamma distribution of fluorescence lifetimes, is simpler and provides good fits to highly complex fluorescence decays as well as to a purely single-exponential decay. Possible interpretation of the power-like model is discussed.     

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.     

Time-resolved fluorescence studies of tryptophan mutants of Escherichia coli glutamine synthetase: conformational analysis of intermediates and transition-state complexes.

Single tryptophan-containing mutants of low adenylylation state Escherichia coli glutamine synthetase have been studied by frequency-domain fluorescence spectroscopy in the presence of various substrates and inhibitors. At pH 6.5, the Mn-bound wild-type enzyme (wild type has two tryptophans/subunit) and the mutant enzymes exhibit heterogeneous fluorescence decay kinetics; the individual tryptophans are adequately described by a triple exponential decay scheme. The recovered lifetime values are 5.9 ns, 2.6 ns, and 0.4 ns for Trp-57 and 5.8 ns, 2.3 ns, and 0.4 ns for Trp-158. These values are nearly identical to the previously reported results at pH 7.5 (Atkins, W.M., Stayton, P.S., & Villafranca, J.J., 1991, Biochemistry 30, 3406-3416). In addition, Trp-57 and Trp-158 both exhibit an ATP-induced increase in the relative fraction of the long lifetime component, whereas only Trp-57 is affected by this ligand at pH 7.5. The transition-state analogue L-methionine-(R,S)-sulfoximine (MSOX) causes a dramatic increase in the fractional intensity of the long lifetime component of Trp-158. This ligand has no effect on the W158S mutant protein and causes a small increase in the fractional intensity of the long lifetime component of the W158F mutant protein. Addition of glutamate to the ATP complex, which affords the gamma-glutamylphosphate-ADP complex, results in the presence of new lifetime components at 7, 3.2, and 0.5 ns for Trp-158, but has no effect on Trp-57. Similar results were obtained when ATP was added to the MSOX complex; Trp-57 exhibits heterogeneous fluorescence decay with lifetimes of 7, 3.5, and 0.8 ns. Decay kinetics of Trp-158 are best fit to a nearly homogeneous decay with a lifetime of 5.5 ns in the MSOX-ATP inactivated complex. These results provide a model for the sequence of structural and dynamic changes that take place at the Trp-57 loop and the central loop (Trp-158) during several intermediate stages of catalysis.     

Lifetime and quenching of tryptophan fluorescence in whiting parvalbumin

The fluorescence lifetime of the single tryptophan in whiting parvalbumin has been measured by time-correlated single-photon counting. In the presence of saturating calcium, greater than 2 mol/mol of protein, the decay of fluorescence is accurately single exponential with a lifetime of 4.6 ns (0.1 M KCl, 20 mM borate, 1 mM dithiothreitol, 20 degrees C, pH 9). Upon complete removal of calcium from parvalbumin with ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid the emission decay becomes biphasic, and a second more rapid decay process with a lifetime of 1.3 ns comprising approximately 18% of the fluorescence emission at 350 nm is observed. The fluorescence emission of the calcium-saturated form is not measurably quenched by iodide. In contrast, upon complete removal of calcium, the fluorescence is completely quenchable as shown by extrapolation of the data to infinite iodide concentration. These results indicate that there is a large increase in the accessibility of the tryptophan residue in the protein to solvent upon removal of calcium. Stern-Volmer plots of the quenching data are nonlinear and indicate that there is more than one quenchable conformation of the calcium-free protein. The lifetime and quenching results are consistent with the presence of significant concentrations of only two stoichiometric species, apoparvalbumin and parvalbumin--Ca2, at partial occupancy of the calcium binding sites.     

The intrinsic tyrosine fluorescence of histone H1. Steady state and fluorescence decay studies reveal heterogeneous emission

In wavelength-resolved steady state spectra we observe three different kinds of emission from histone H1, a class A protein with only a single tyrosine residue. Unfolded H1 emissions that peak at approximately 300 and 340 nm can both be excited maximally at approximately 280 nm. Another, peaking much further to the red at approximately 400 nm, can be excited maximally at approximately 320 nm. The 300-nm fluorescence can be resolved by lifetime measurements into three components with decay times of approximately 1, 2, and 4 ns. On sodium-chloride-induced refolding of H1, simplification of the emission properties occurs. The 340 and 400-nm components disappear while the two shorter lifetime components of the 300-nm band diminish in amplitude and are replaced by the 4-ns decay. We believe that the 340-nm emission is tyrosinate fluorescence resulting from excited-state proton transfer. The origin of the 400-nm emission remains uncertain. We assign the 1 and 2-ns components of the 300-nm emission to two states of tyrosine in denatured H1 and the 4-ns decay to fluorescence of the single tyrosine residue in the globular region of refolded H1. Our results support the contention that salt induced folding of H1 is a cooperative two state process, and permit us to better understand the previously reported increases in fluorescence intensity and anisotropy on salt-induced folding.     

1-Anilino-8-Naphthalene Sulfonate (ANS) Is Not a Desirable Probe for Determining the Molten Globule State of Chymopapain

The molten globule (MG) state of proteins is widely detected through binding with 1-anilino-8-naphthalene sulphonate (ANS), a fluorescent dye. This strategy is based upon the assumption that when in molten globule state, the exposed hydrophobic clusters of protein are readily bound by the nonpolar anilino-naphthalene moiety of ANS molecules which then produce brilliant fluorescence. In this work, we explored the acid-induced unfolding pathway of chymopapain, a cysteine proteases from Carica papaya, by monitoring the conformational changes over a pH range 1.0–7.4 by circular dichroism, intrinsic fluorescence, ANS binding, acrylamide quenching, isothermal titration calorimetry (ITC) and dynamic light scattering (DLS). The spectroscopic measurements showed that although maximum ANS fluorescence intensity was observed at pH 1.0, however protein exhibited ∼80% loss of secondary structure which does not comply with the characteristics of a typical MG-state. In contrast at pH 1.5, chymopapain retains substantial amount of secondary structure, disrupted side chain interactions, increased hydrodynamic radii and nearly 30-fold increase in ANS fluorescence with respect to the native state, indicating that MG-state exists at pH 1.5 and not at pH 1.0. ITC measurements revealed that ANS molecules bound to chymopapain via hydrophobic interaction were more at pH 1.5 than at pH 1.0. However, a large number of ANS molecules were also involved in electrostatic interaction with protein at pH 1.0 which, together with hydrophobically interacted molecules, may be responsible for maximum ANS fluorescence. We conclude that maximum ANS-fluorescence alone may not be the criteria for determining the MG of chymopapain. Hence a comprehensive structural analysis of the intermediate is essentially required.     

A fluorescence lifetime study of virginiamycin S using multifrequency phase fluorometry

Using multifrequency phase fluorometry, fluorescence lifetimes have been assigned to the different protolytic forms of the antibiotic virginiamycin S. These lifetimes are 0.476 +/- 0.005 ns for the uncharged form, 1.28 +/- 0.2 and 7.4 +/- 0.2 ns for the zwitterionic form, 1.19 +/- 0.01 ns for the negatively charged form, and 1.9 +/- 0.1 ns for the double negatively charged form. The assignments are based on lifetime measurements as a function of pH, volume percent ethanol, and excitation wavelength. Excited-state proton transfer is taken into account. It is complete at pH values lower than 1, and no fluorescence of the fully protonated charged form is observed. At pH 8, an excited-state pK* increase is calculated, but proton association is too slow to cause excited-state proton transfer. The addition of divalent cations, at pH 9.4, increases the lifetime of the negatively charged form to a value dependent upon the specific nature of the cation (7.58 +/- 0.06 ns for Mg2+, 6.54 +/- 0.02 ns for Ca2+, and 3.74 +/- 0.05 ns for Ba2+). Monovalent cations do not influence the lifetimes, indicating that their binding to the macrocycle does not influence the fluorescent moiety. The model compound 3-hydroxypicolinamide shows an analogous behavior, but the retrieved lifetime can differ significantly.     

Tyrosine and tyrosinate fluorescence of S-100b. A time-resolved nanosecond fluorescence study. The effect of pH, Ca(II), and Zn(II)

The properties of the tyrosine and tyrosinate emissions from brain S-100b have been studied by nanosecond time-resolved fluorescence at emission wavelengths in the range 305 to 365 nm. The effect of pH on the fluorescence has been studied at pH 6.5, 7.5, and 8.5 for the Ca(II) apo and holo forms of the protein, and for the apo and holo forms in the presence and absence of Zn(II) at pH 7.5. The fluorescence decay is biexponential at pH 8.5 and triexponential at pH 6.5 and 7.5. The three components of the decay have wavelength and metal ion dependent lifetimes in the ranges 0.06 to 1.05 ns, 0.49 to 3.76 ns, and 3.60 to 14.5 ns. The observation of a long lifetime component at wavelengths characteristic of emission from tyrosinate suggests that in class A proteins this may be a useful diagnostic of the environment of tyrosine in their native structures. The time-resolved emission spectra provide evidence for efficient, subnanosecond protolysis of the excited state of the single tyrosine (Tyr17) under all conditions studied except in 6 M guanidium chloride in which the protein shows only emission from tyrosine (lambda em 305 nm), suggesting that the tyrosinate emission is a property of the tertiary structure of the native protein. The Zn(II)-dependence of the fluorescence is fully consistent with the earlier suggestion that Tyr17 is near the Zn(II) binding site and remote from the high affinity Ca(II) binding site.