Conformational aspects and rotational dynamics of synthetic adrenocorticotropin-(1-24) and glucagon in reverse micelles

Organization and dynamics of the N-terminal domain of chemokine receptor CXCR1 in reverse micelles: effect of graded hydration

Water plays a fundamental role in the folding, structure, dynamics, and function of proteins and peptides. The extracellular N-terminal domain of chemokine receptors is crucial in mediating binding affinity, receptor selectivity, and regulating function. The flexible N-terminal domain becomes ordered in membranes and membrane-mimetic assemblies, thereby indicating that the membrane could play an important role in regulating CXC chemokine receptor 1 (CXCR1) function. In view of the role of hydration in lipid-protein interactions in membranes, we explored the organization and dynamics of a 34-mer peptide of the CXCR1 N-terminal domain in reverse micelles by utilizing a combination of fluorescence-based approaches and circular dichroism spectroscopy. Our results show that the secondary structure adopted by the CXCR1 N-domain is critically dependent on hydration. The tryptophan residues of the CXCR1 N-domain experience motional restriction and exhibit red edge excitation shift (REES) upon incorporation in reverse micelles. REES and fluorescence lifetime exhibit reduction with increasing reverse micellar hydration. Time-resolved fluorescence anisotropy measurements reveal the effect of hydration on peptide rotational dynamics. Taken together, these results constitute the first report demonstrating modulation in the organization and dynamics of the N-terminal domain of a chemokine receptor in a membrane-like environment of varying hydration. We envisage that these results are relevant in the context of hydration in the function of G protein-coupled receptors.

Water gradient in the membrane-water interface: a time-resolved study of the series of n-(9-anthroyloxy) stearic acids incorporated in AOT/water/iso-octane reverse micelles

The water radial distribution in AOT/iso-octane/water reverse micelles (RM), used to mimic the membrane-water interface, was examined by excited-state lifetime and transient spectral measurements of the series of n-(9-anthroyloxy) stearic acids (n-AS), with n = 2, 3, 6, 7, 9, 10, and 12. A water gradient in the RM extended from the polar head group region up to the middle of the surfactant carbon chains. A fast intramolecular excited-state relaxation, involving the rotation of the carboxylic group of the ester bond with respect to the anthracene ring, gave rise to a nanosecond time-dependent fluorescence Stokes shifts (TDFSS). In water-filled RMs, we only observed a water-induced TDFSS occurring over subnano- and nanosecond time scales with decreasing amplitudes and rates as a function of depth, according to the decreasing water gradient and the slowing down of the anthroyloxy moiety rotational motion. This water-induced TDFSS is most likely the result of both H-bond formation and general dipolar relaxation, as indirectly showed by measurements with DMF (a nonprotic polar solvent) instead of water in RMs.

Site-specific hydration status of an amphipathic peptide in AOT reverse micelles

Reverse micelles formed by sodium bis(2-ethylhexyl) sulfosuccinate (AOT) in isooctane (IO) and water have long been used as a means to provide a confined aqueous environment for various applications. In particular, AOT reverse micelles have often been used as a template to mimic membrane-water interfaces. While earlier studies have shown that membrane-binding peptides can indeed be incorporated into the polar cavity of AOT reverse micelles where they mostly fold into an alpha-helical structure, the underlying interactions leading to the ordered conformation are however not well understood. Herein, we have used circular dichroism (CD) and infrared (IR) spectroscopies in conjunction with a local IR marker (i.e., the CN group of a non-natural amino acid, p-cyano-phenylalanine) and a global IR reporter (i.e., the amide I' band of the peptide backbone) to probe the conformation as well as the hydration status of an antimicrobial peptide, mastoparan x (MPx), in AOT reverse micelles of different water contents. Our results show that at, w0=6, MPx adopts an alpha-helical conformation with both the backbone and hydrophobic side chains mostly dehydrated, whereas its backbone becomes partially hydrated at w0=20. In addition, our results suggest that the amphipathic alpha-helix so formed orients itself in such a manner that its positively charged, lysine-rich, hydrophilic face points toward the negatively charged AOT head groups, while its hydrophobic face is directed toward the polar interior of the water pool. This picture is in marked contrast to that observed for the binding of MPx to phospholipid bilayers wherein the hydrophobic surface of the bound alpha-helix is buried deeper into the membrane interior.

On the behavior of indole-containing species sequestered within reverse micelles at sub-zero temperatures

We report on the effects of temperature (+30 to -100 degrees C) on the fluorescence from N-acetyl tryptophanamide (NATA) and human serum albumin (HSA) sequestered within Aerosol-OT (AOT) reversed micelles. NATA reports simultaneously from the polar and non-polar side of the reverse micelle interface. As the sample temperature decreases, the relative fraction of NATA molecules associated with the polar side increases. This redistribution process is characterized by DeltaH = -14.8 +/- 0.6 kJ/mol and DeltaS = -54 +/- 2 J/(K mol). The activation energy for thermal quenching (E(a,TQ)) associated with the polar side NATA molecules is 6.7 kJ/mol before the micelles have shed water and 1.0 kJ/mol after water shedding (below approximately -20 degrees C). The time-resolved fluorescence intensity decay for tryptophan-214 in HSA is triple exponential. We suggest that these lifetimes arise from three indole residue conformations in equilibrium. Cooling the sample causes a freezing-in of the least quenched conformer; the other conformers are frozen out. The E(a,TQ) value for the shortest lifetime component is 6 kJ/mol. The E(a,TQ) for the long and intermediate lifetime components are equivalent (approximately 1.5 kJ/mol).

Structure, Stability, and Hydration of a Polypeptide in AOT Reverse Micelles

In this communication, we provide theoretical evidence that the folded structure of a simple peptide, alanine zwitterionic octapeptide, or A8, unstable in solution, becomes stable in a reverse micelle (RM) of appropriate size. Our molecular dynamics simulations were carried out for realistic models of sodium 2-ethylhexylsulfosuccinate RM in isooctane, simulated for an extended period of time. For the RM of the smaller size, we find that a helical structure is stable for the whole length of the simulation. On the contrary, the peptide very quickly takes an extended structure in larger micelles.

Nanosecond dynamics of a mimicked membrane-water interface observed by time-resolved stokes shift of LAURDAN

We studied the dipolar relaxation of the surfactant-water interface in reverse micelles of AOT-water in isooctane in the nanosecond and subnanosecond time ranges by incorporating the amphipathic solvatochromic fluorescent probes LAURDAN and TOE. A negative component was observed in the fluorescence decays in the red edge of the emission spectrum-the signature of an excited state reaction-with LAURDAN but not for TOE. The deconvolution of the transient reconstructed spectra of LAURDAN based on a model constructed by adding together three log-normal Gaussian equations made it possible to separate the specific dynamic solvent response from the intramolecular excited state reactions of the probe. The deconvoluted spectrum of lowest energy displayed the largest Stokes shift. This spectral shift was described by unimodal kinetics on the nanosecond timescale, whereas the relaxation kinetics of water-soluble probes have been reported to be biphasic (on the subnanosecond and nanosecond timescales) due to the heterogeneous distribution of these probes in the water pool. Most of this spectral shift probably resulted from water relaxation as it was highly sensitive to the water to surfactant molar ratio (w(0)) (60-65 nm at w(0) = 20-30). A small part of this spectral shift (9 nm at w(0) = 0) probably resulted from dipolar interaction with the AOT polar headgroup. The measured relaxation time values were in the range of the rotational motion of the AOT polar headgroup region as assessed by LAURDAN and TOE fluorescence anisotropy decays.

The structure and dynamics of ACTH (1-10) on the surface of a sodium dodecylsulfate (SDS) micelle: a molecular dynamics simulation study

ACTH (1-10), an adrenocorticotropin hormone fragment, was studied by molecular dynamics (MD) simulation in the NPT ensemble in an explicit sodium dodecylsulfate (SDS) micelle. Initially, distance restraints derived from NMR nuclear Overhauser enhancements were incorporated during the equilibration stage of the simulation. The analyses of the trajectories from the subsequent unrestrained MD showed that ACTH (1-10) does not conform to a helical structure at the micelle-water interface; however, the structure is amphipathic. The loss of the helical structure is due to decreased intramolecular hydrogen bonding accompanied by an increase of hydrogen bonding between the amide hydrogens of the peptide and the micelle head-groups. ACTH (1-10) was found to lie on the surface of the SDS micelle. Most of the hydrophobic interactions came from the side-chains of Met-4, Phe-7 and Trp-9. The peptide bonds were either hydrated or involved in intramolecular hydrogen bonding. Decreased hydration for the backbone of His-6 and Phe-7 was due to intermolecular hydrogen bonding with the SDS head-groups. The time correlation functions of the N-H bonds of the peptide in water and in the micelle showed that the motions of the peptide, except for the N- and C-termini, are significantly reduced when partitioned in the micelle.

Conformation induction in melanotropic peptides by trifluoroethanol: fluorescence and circular dichroism study

Conformation induction in the two related peptides, alpha-melanocyte stimulating hormone (alpha-MSH) and delta-melanocyte stimulating hormone (delta-MSH), have been studied in solvent media containing varying percentages of the membrane-mimetic solvent 2,2,2-trifluoroethanol (TFE) using fluorescence and circular dichroism (CD) spectroscopy. Singular value decomposition (SVD) analysis of the CD spectra at different TFE concentrations showed that these spectra can be described as linear combinations of only two distinct basis spectra, corresponding to the peptides in the random-coil and 'folded' conformations. For alpha-MSH the spectrum of the folded state is very similar to the standard spectrum of the alpha-helix, while that for delta-MSH has partial resemblance to the helical spectrum. Fitting the data on ellipticity (at 222 nm) as a function of TFE volume fraction to an equation based on a two-state model describing TFE-induced conformation induction in the peptides gave values of (1.1 +/- 0.4) and (4.2 +/- 0.5) kcal mol-1 for alpha-MSH and delta-MSH, respectively, for the free energy of equilibrium between the helix and coil forms in water. Measurement of fluorescence emission parameters (emission maximum, quantum yield, steady-state anisotropy and mean excited-state lifetime) indicated that the microenvironment around the single tryptophan residues of both peptides changes in like manner with increasing concentration of TFE in the solvent. The similarity of fluorescence behaviour of the peptides suggests that their Trp fluorophores do not participate in secondary structure formation in TFE.

Enzymes in low water systems

Water is fundamental for enzyme action and for formation of the three-dimensional structure of proteins. Hence, it may be assumed that studies on the interplay between water and enzymes can yield insight into enzyme function and formation. This has proven correct, because the numerous studies that have been made on the behavior of water-soluble and membrane enzymes in systems with a low water content (reverse micelles or enzymes suspended in nonpolar organic solvents) have revealed properties of enzymes that are not easily appreciated in aqueous solutions. In the low water systems, it has been possible to probe the relation between solvent and enzyme kinetics, as well as some of the factors that affect enzyme thermostability and catalysis. Furthermore, the studies show that low water environments can be used to stabilize conformers that exhibit unsuspected catalytic properties, as well as intermediates of enzyme function and formation that in aqueous media have relatively short life-times. The structure of enzymes in these unnatural conditions is actively being explored.

Cell type and spatial location dependence of cytoplasmic viscosity measured by time-resolved fluorescence microscopy

Information on the cell type and spatial location dependence of cytoplasmic viscosity would be very useful in understanding some of the processes occurring in the cell. For this purpose, fluorescent dye kiton red (sulforhodamine B) was loaded into a variety of cells such as Swiss 3T3 fibroblasts, human mononuclear cells, Sarcoma-180 tumor cells, Chinese hamster ovary cells, plant cells from Digitalis lanata, stamen hair cells of Tradescantia, and guard mother cells of Allium cepa. Space-resolved measurements of cytoplasmic viscosity were carried out by using an experimental set-up wherein a picosecond laser system was coupled with an epifluorescence microscope. The spatial resolution of this set-up was approximately 1.0 micron, and reliable dynamic fluorescence measurements could be obtained from 10(2) to 10(3) fluorescent molecules. Fluorescence lifetime measurements showed that a large fraction (approximately 70%) of kiton red was in the free form. Fluorescence anisotropy decay of kiton red in cells was analyzed by a two population (free and bound) model. The microviscosity of cytoplasm was estimated from the anisotropy decay kinetics of the free probe. It was found that the cytoplasmic viscosity is dependent on both the cell type and spatial location within a cell. Furthermore, both the average value of viscosity and spatial variation within a cell were larger in the plant cells when compared to the animal cells. Model studies in various simpler systems have shown that the higher viscosity observed in some part of the cell could be due to either physical restriction and/or the presence of high concentrations of small solutes and macromolecules.

Time-resolved fluorescence studies of ribonuclease T1 in reversed micelles

Time-resolved fluorescence intensity and anisotropy decay data were obtained for ribonuclease T1 entrapped in bis(2-ethylhexyl) sodium sulfosuccinate/heptane reverse micelles, as a function of the size of the inner water pool at neutral pH. Data have been presented previously to show that this protein retains its native structure and undergoes reversible thermal unfolding in these reverse micelles (Shastry and Eftink,Biochemistry 36, in press). The fluorescence decay of entrapped protein is similar to that for the protein in buffer. The rotational correlation time of entrapped ribonuclease T1 is found to be longer than that in buffer; this rotational correlation time decreases with increasing size of the water pool but is still over twice the value for the protein in buffer for the largest size of water pool investigated, indicating an increased microviscosity within the reverse micelle. Thermal unfolding of the protein results in a significant decrease in the rotational correlation time of the entrapped proteins, consistent with the protein being unfolded but not interacting with the inner surfactant wall of the reverse micelle.

The fluorescence and circular dichroism of proteins in reverse micelles: application to the photophysics of human serum albumin and N-acetyl-L-tryptophanamide

Evidence is presented that a compartmentalised protein exists in its native state only within a particular size of aqueous cavity. This behaviour is shown to exist in AOT reverse micelles using fluorescence quenching and circular dichroism (CD) studies of human serum albumin (HSA). In particular, far ultraviolet CD measurements show that a reduction in quencher accessibility to the fluorophore is consistent with the protein being nearest to its native conformation at a waterpool size of around 80 A diameter. We also show that the biexponential fluorescence decay of N-acetyl-L-tryptophanamide (NATA) in AOT reverse micelles arises from the probe being located in two distinct sites within the interfacial region. The more viscous of these two sites is located on the waterpool side of the interface and the other is located on the oil side of the interface.

Somatostatin in a water-restricted environment: fluorescence and circular dichroism study in AOT reverse micelles

Steady-state and time-resolved fluorescence emission from the single tryptophan residue of somatostatin, and the kinetics of quenching of this emission, were studied in aqueous solution and in reverse micelles of sodium bis (2-ethylhexyl) sulfosuccinate (AOT)/water/isooctane, a system that mimics the water-membrane interface well. Incorporation into micelles caused blue shifts and reduced band-widths of the emission peaks and altered the quantum yields with respect to emission from bulk water. Steady-state anisotropy values also increased considerably on micellization. These observations point to reduced polarity of the environment around the Trp residue of the peptide, as well as restricted freedom of its rotational motions, due to transfer from the aqueous to the micellar phase. Fluorescence emission kinetics of the Trp moiety followed biexponential decay laws in both aqueous and micellar media. Static and dynamic quenching constants were measured for acrylamide and CCI4 quenchers localized in the micellar and organic pseudophases, respectively, using both steady-state and time-resolved experiments. The efficiency of dynamic quenching by acrylamide became vanishingly small in going from water to reverse micelles, in sharp contrast to the comparable quenching efficiencies exhibited by CCI4 in micelles and acrylamide in water. The circular dichroic (CD) spectrum of the native peptide in water indicated the possibility of some amount of beta-type secondary structure being present. Conformational analysis of CD spectra in micelles showed that the relative amount of this structural feature was enhanced for the micellized peptide but was insensitive to the water content of micelles.(ABSTRACT TRUNCATED AT 250 WORDS)

Fluorescence dynamics of staphylococcal nuclease in aqueous solution and reversed micelles

The dynamical fluorescence properties of the sole tryptophan residue (Trp-140) in Staphylococcus aureus nuclease (EC 3.1.31.1) have been investigated in aqueous solution and reversed micelles composed of either sodium bis(2-ethylhexyl)sulfosuccinate (AOT) in isooctane or cetyltrimethylammonium chloride (CTAC) in isooctane/hexanol (12:1 by volume). The fluorescence decay of nuclease in the different environments can be described by a trimodal distribution of fluorescence lifetimes at approx. 0.5, 1.5 and 5.0 ns. The relative amplitudes depend on the environment. For pH 9.0 solutions the contribution of the two shortest lifetime components in the distribution is largest for AOT and smallest for CTAC reversed micelles. There is reasonable agreement between the average fluorescence lifetime and the fluorescence quantum efficiency confirming a significant fluorescence quenching in AOT reversed micelles. Fluorescence anisotropy decay revealed that the tryptophan environment in aqueous nuclease solutions is rigid on a nanosecond timescale. When nuclease was entrapped into reversed micelles the tryptophan gained some internal flexibility as judged from the distinct presence of a shorter correlation time. The longer correlation time reflected the rotational properties of the protein-micellar system. Modulation of the overall charge of nuclease (isoelectric point pH 9.6) by using buffer of pH 9.0 and pH 10.4, respectively, and of the size of empty micelles by selecting two values of the water to surfactant molar ratio, had only a minor effect on the rotational properties of nuclease in the positively charged reversed micelles. Encapsulation of nuclease in anionic reversed micelles resulted in the development of protein bound to aggregated structures which are immobilised on a nanosecond timescale. According to far UV circular dichroism results the secondary structure of nuclease only followed the already published pH-dependent changes. Encapsulation had no major effect on the overall secondary structure.

Fluorescence study of melanocyte stimulating hormones in AOT reverse micelles

Fluorescence emission from the single tryptophan residues of two melanocyte stimulating hormones, alpha-MSH and delta-MSH, and their quenching kinetics were studied in aqueous solution and in reverse micelles of AOT/water/isooctane. Incorporation into micelles caused blue shifted and narrower emission peaks, altered quantum yields and considerably enhanced anisotropies for both peptides when compared to emission from bulk water. The variation of emission parameters with micellar water content was interpreted to suggest that while the tryptophan in alpha-MSH lies in close vicinity of the water-AOT molecular interface, that in delta-MSH is solubilized in the central water pool. Total emission intensity decays followed complex (biexponential) kinetics in both aqueous and micellar media. Although the mean lifetimes for both peptides were always nearly the same, the average rotational correlation times in micelles for alpha-MSH were three times as much as those for delta-MSH. Stern-Volmer plots obtained using acrylamide and CCl4 as quenchers localized in the micellar and organic pseudophases, respectively, were non-linear and dependent on emission wavelength. Quenching by acrylamide was more efficient for delta-MSH than for alpha-MSH, while the opposite was true for quenching by CCl4. The implication of this result for localization of the peptides in micelles was consistent with the earlier one emerging from these studies.

Protein flexibility and aggregation state of human epidermal growth factor. A time-resolved fluorescence study of the native protein and engineered single-tryptophan mutants

A time-resolved fluorescence spectroscopic study of the recombinant human epidermal growth factor (hEGF), a bis(tryptophan)-containing protein (Trp49-Trp50), and of the two single-tryptophan-containing engineered mutants with Trp49 or Trp50 replaced by Phe ([W49F]hEGF, [W50F]hEGF), was undertaken in order to gain insight into the conformational dynamics of the C-terminal region. Quite different position-dependent microenvironments for the two Trp residues are shown by comparing the fluorescence intensity decay of both mutants. Trp50 in the single-tryptophan mutant [W49F]EGF probably undergoes a dominant interaction with the solvent. A more heterogeneous environment of Trp49 in the [W50F]hEGF mutant is found. Moreover, the fluorescence decay of the native hEGF is not simply the additive result of the decays of both mutants: the Trp2 sequence confers a conformation of the C-terminal sequence which is more in contact with the rest of the protein molecule. By contrast, the fluorescence anisotropy decay of the native protein is quite similar to that of the single-tryptophan mutants. A high degree of rotational freedom in the C-terminal region of the protein is demonstrated. The resonance energy transfer, which could contribute to the anisotropy decay, appears therefore not to be highly efficient with respect to the depolarization motions. In addition to these local conformational and dynamic aspects of the hEGF C-terminal sequence, the fluorescence anisotropy decay data demonstrate the existence of a dimerization process of the native protein which is dependent on pH and protein concentration. This phenomenon influences the excited-state lifetime profiles and, therefore, the local conformational equilibrium of the C-terminal region.

Ligand binding at membrane mimetic interfaces. Human serum albumin in reverse micelles

The behaviour of human serum albumin in the presence of three chemically distinct ligands: oxyphenylbutazone, dansylsarcosine and hemin, has been compared in buffer and in reverse micelles of isooctane, water, and either sodium bis(2-ethylhexyl)sulfosuccinate or hexadecyl trimethylammonium bromide, systems selected to mimic the membrane-water interface. Upon micellar incorporation, the dansylsarcosine-albumin complex dissociated, as evidenced by fluorescence emission spectroscopy (red shift from 485 nm to 570 nm) and by fluorescence polarization measurements. In contrast, the hemin-albumin complex remained stable in reverse micelles, as judged from the Soret absorption band at 408 nm and the molar absorption coefficient of 8.4 x 10(4) M-1 cm-1. The oxyphenylbutazone to albumin binding curves reveal that while the association constant remained unchanged (Ka approximately 1.0 x 10(5) M-1), only a fraction of the albumin molecules present reacted with the ligand. The results were unaffected by the nature and the concentration of the surfactant. These findings can be interpreted in the light of conformational changes induced in human serum albumin by the large micellar inner surface area. The blue shift of the fluorescence emission maximum from 344 nm in buffer to 327 nm in sodium bis(2-ethylhexyl)sulfosuccinate micelles and the lesser reactivity/accessibility of the fluorophore to oxidation by N-bromosuccinimide, indicate perturbations of the sole tryptophan-214 microenvironment. However, the distance between the indole residue and tyrosine-411 does not seem substantially modified by the 15% decrease affecting the alpha helices of the albumin molecule. It is proposed that the results reported herein reflect the interactions of albumin with a membrane-like interface which generates two protein subpopulations differing in their membrane-surface and ligand affinities. Overall and local conformational changes, originating from this surface-induced effect, may thus constitute a ligand-release facilitating mechanism acting at cellular membrane levels.

The interactions of horse heart apocytochrome c with phospholipid vesicles and surfactant micelles: time-resolved fluorescence study of the single tryptophan residue (Trp-59)

The interactions of horse heart apocytochrome c with membrane interfaces were studied on membrane models including micelles of the anionic surfactant sodium dodecyl sulfate (SDS), the micelle forming lipid analogs dodecylphosphoglycol (C12PG), tetradecylphosphoglycol (C14PG), and dodecylphosphocholine (C12PN), and the negatively charged phospholipid 1-palmitoyl-2-oleoylsn-glycero phosphocholine (POPS) forming small unilamellar vesicles (SUV). The time-resolved fluorescence of the single tryptophan residue (Trp-59) emission was monitored to characterize the modifications of the conformational equilibrium and of the internal dynamics of the protein, which can be brought about by its binding to these model membranes. In most of the cases, as for the protein in solution, the excited state lifetime distribution of the Trp emission was described by four discrete classes, whose relative proportions and barycenters vary significantly in the different complexes formed. In the complex with POPS, however, the decay analysis showed only 3 lifetime classes: the long lifetime class displayed a barycenter value smaller than that observed for the protein in aqueous solution but with a much higher proportion, indicating a stabilization of this conformer in the membrane-bound form of the protein. A similar sensitivity of the Trp-59 excited state to deactivation by thermal collisions in water and in the protein/POPS complex was observed, indicating a probable location of Trp-59 at the membrane/water interface. The effects of protein binding to C12PN, C12PG and C14PG micelles on the long lifetime class proportion were similar to that of POPS but, in addition, there was a large contribution of a short lifetime component which was absent in POPS vesicles.(ABSTRACT TRUNCATED AT 250 WORDS)

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.