pH-dependent aggregate forms and conformation of Alzheimer amyloid beta-peptide (12-24)

The conformational transition to a beta-structure and the aggregation process of Alzheimer amyloid beta-peptide (12-24) [abbreviated as A beta(12-24)] were studied. The influence of sample dissolution methods for the aggregate structure was examined by electron microscopy (EM). The difference in the width of the aggregate of A beta(12-24) depended on the pH immediately after sample dissolution. Two types of sample dissolution methods, F and R, were employed. For dissolution method F, the peptide sample was immediately dissolved in water and then adjusted to pH 2.2 by adding buffer, while for dissolution method R, the peptide was directly dissolved in the buffer solution. In the latter case, the starting pH was 3.0. Slight fibrils (10-12 nm in diameter) were observed with method F, and wider ribbon-like aggregates (17-20 nm in diameter) with method R, despite the same pH range. A difference between methods F and R was also detected in the CD spectra, especially at pHs near 5.0. The CD intensity of the 214 nm band with method R changed with pH, with the highest value at pH 3.7, whereas that with method F was unchanged at pHs below 5.0. The temperature-dependent CD results showed that a thermostable aggregate of A beta(12-24) occurs at higher pHs than 3.0. NMR analysis showed that deprotonation of the C-terminal carboxylate group in A beta(12-24) triggered the aggregate formation, and the transition from a random coil to a beta-conformation in the C-terminal region of V18-V24 was detected on analysis of the (3)Ja(N) coupling constant in the pH range of 2.2 to 3.0.     

Phage T4 lysozyme. Physical properties and reversible unfolding.

Phage T4 lysozyme has been used extensively in studies of the genetic code. However, little work has been done on the characterization of the purified enzyme. Therefore, we determined the spectral properties of native T4 lysozyme and used these properties to follow the unfolding transition. The ultraviolet absorption spectrum and solvent perturbation difference spectrum indicate that the aromatic amino acids are extensively exposed to solvent. The CD and ORD spectra are characteristic of a high fraction of helix. Guanidine hydrochloride denaturation results show that over a T4 lysozyme concentration range of 0.07-1 g/l the c-m equals 2.7 M guanidine hydrochloride at pH 5 and that the transition is 100% reversible as judged by enzymatic assay and four different spectrophotometric criteria: CD at 295 nm, CD at 223 nm, fluorescence intensity at 350 nm and wavelength of maximum fluorescence. Guanidine hydrochloride denaturation at pH 2.5 was followed using fluorescence emission and has a c-m equals 1.7 M guanidine hydrochloride, indicating a strong pH dependence of chemical unfolding. Reversible thermal denaturation conditions were located at acid pH, 0.2 M NaCl, 10-4 M dithiothreitol and 10-6 M T4 lysozyme. The CD signal at 223 nm was used to measure the unfolding. Thermodynamic analysis of the thermal data showed an increase in T-m, increment H-unf and increment S-unf with increasing pH.     

Optical properties and viscosity of hyaluronic acid in mixed solvents: Evidence of conformational transition

Circular dichroism, optical rotatory dispersion, and viscosity of hyaluronic acid at various solvents compositions, concentrations, and pH values have been studied. The data show a large change in the molecular properties in organic/water solvents such as ethanol, p-dioxane, or acetonitrile/water at pH ? pK_a. At this pH range of aqueous solution, hyaluronic acid shows a CD minimum near 210 nm whereas in the presence of organic solvent it exhibits a strong negative dichroism (below 200 nm) and a positive band near 226 nm. It undergoes a sharp, cooperative transition with respect to pH and solvent. The observed CD features are assigned to the π-π* and n-π* transitions of the amide and carboxyl chromophores. The ORD results show a gradual blue shift of trough at 220 nm with increasing magnitude of rotation when the organic solvents and hydrogen ion concentrations are increased. A one-term Drude's equation was used to analyze the ORD data, and the result show a variation of dispersion parameters with different solvents in accordance with the observed CD changes. The intrinsic viscosity of hyaluronic acid in mixed solvent at pH 2.6 is lower than that of aqueous solution. All the observed property changes of hyaluronic acid are reversed on addition of foramide in mixed solvents indicating that the hydrogen bonds are involved in this transition. The observed spectroscopic and hydrodynamic features are attributed to a conformational change of hyaluronic acid in a mixed solvent involving intramolecular hydrogen bonding between the acetamido and carboxyl groups. The possible conformational state of hyaluronic acid in solution under various conditions is discussed in terms of the reported helical structure of hyaluronic acid from x-ray diffraction studies.     

Spectroscopic analysis of thermal denaturation of Cajanus cajan proteinase inhibitor at neutral and acidic pH by circular dichroism

The conformational changes accompanying thermal denaturation under neutral, acidic and reducing conditions of Cajanus cajan proteinase inhibitor were investigated using near- and far-ultraviolet circular dichroism (CD) spectroscopy. The protein inhibitor shows a reversible N<-->D transition at neutral pH with a Tm approximately equal to 63 degrees C. The negative CD band intensities at 200 nm (far-UV) and near about 280 nm (near-UV) decrease as a result of thermal stress. The effect is more pronounced at low pH and in the presence of dithiothreitol. Only partial reversibility is observed under acidic conditions. Significant changes in the near- as well as far-ultraviolet CD spectrum are observed in the presence of dithiothreitol suggestive of the importance of disulfide linkages in maintaining the structure of C. cajan proteinase inhibitor.     

Circular dichroism and the pH-induced β-coil transition of poly(S-carboxymethyl-L-cysteine) and its side-chain homolog

The CD of aqueous solutions of poly(S-carboxymethyl-L -cysteine) and poly(S-carboxyethyl-L -cysteine) has been measured at different pH, and the pH-induced β-coil transition is observed by changes in residue ellipticity of dichroic bands around 200 and 225 nm. The residue ellipticity at 200 nm of the former polypeptide is twice as large as that of the latter, when the β-conformation is formed in solution. However, the β-conformation of the latter polypeptide is more stable against electrostatic repulsion than that of the former. The transition curve of poly(S-carboxymethyl-L -cysteine) has also been determined for different molecular weights. The curves were found to be completely coincident with one another if the degree of polymerization were higher than about 100. Such a transition curve is generally divided into three steps: initiation, cooperative formation, and rearrangement of hydrogen bonds. The cooperative step is very sharp, occurring at a constant pH. These steps become agglomerated into two or one when the polypeptide concentration or added salt concentration is increased.     

Cadmium-thiolate clusters in metallothionein: spectrophotometric and spectropolarimetric features

Cd-thiolate cluster formation in rabbit liver metallothionein 1 (MT) has been followed at pH 8.4 by monitoring spectroscopic features below 300 nm as a function of increasing Cd-to-apometallothionein (apoMT) ratio. The emerging absorption profiles form a family of closely similar spectra attributable to tetrahedral Cd-tetrathiolate coordination previously established for Cd7-MT [Vasák, M., K?gi, J.H.R., & Hill, H.A.O. (1981) Biochemistry, 20, 2852-2856]. However, there is a 6-nm red shift of the unresolved lowest energy absorption band when greater than 3 equiv of Cd(II) is incorporated. This shift is paralleled by a changeover in the circular dichroism (CD) features of MT from a broad monophasic positive CD profile with ellipticity bands near 240 and 220 nm to a biphasic CD spectrum characterized by positive ellipticity bands at 260 and 224 nm and an interposed negative band at 240 nm. Both features can be attributed to a changeover from separate Cd-tetrathiolate units formed at low metal-to-apoMT ratio to Cd-thiolate clusters when the supply of cysteine ligands becomes limiting. A comparable red shift signaling the transition from the mononuclear to a trinuclear tetrahedral Cd-tetrathiolate complex is also observed upon titration of the synthetic tetrathiol dodecapeptide N-Ac-Pro-Cys-Orn-Cys-Pro-Glu-Cys-Glu-Cys-Arg-Arg-Val with Cd(II). The latter studies also provide evidence for the predominantly ligand (sulfur) character of the lowest energy Cd-tetrathiolate ligand-metal charge-transfer transition. As a corollary it is inferred that the biphasic CD profile arises from excitonic coupling of these sulfur-centered transition dipole moments dissymmetrically oriented within the Cd(II)-thiolate clusters.     

Equilibrium and transient intermediates in folding of human macrophage migration inhibitory factor.

Acid, guanidinium-Cl and urea denaturations of recombinant human macrophage migration inhibitory factor (MIF) were measured using CD and fluorimetry. The acid-induced denaturation was followed by CD at 200, 222, and 278 nm and by tryptophan fluorescence. All four probes revealed an acid-denatured state below pH 3 which resembled a typical molten globule. The pH transition is not two-state as the CD data at 222 nm deviated from all other probes. Urea and guanidinium-Cl denaturations (pH 7, 25 degrees C) both gave an apparent DeltaGU app H2O of 31 +/- 3 kJ.mol-1 when extrapolated to zero denaturant concentration. However, denaturation transitions recorded by fluorescence (at the same protein concentration) occurred at lower urea or guanidinium-Cl concentrations, consistent with an intermediate in the course of MIF denaturation. CD at 222 nm was not very sensitive to protein concentration (in 10-fold range) even though size-exclusion chromatogryphy (SEC) revealed a dimer-monomer dissociation prior to MIF unfolding. Refolding experiments were performed starting from acid, guanidinium-Cl and urea-denatured states. The kinetics were multiphasic with at least two folding intermediates. The intrinsic rate constant of the main folding phase was 5.0 +/- 0.5 s-1 (36.6 degrees C, pH 7) and its energy of activation 155 +/- 12 kJ.mol-1.     

Effect of metal ions and EGTA on the optical properties of concanavalin A at alkaline pH

In our earlier communications, we reported the effect of salts and alcohols on alpha-chymotrypsinogen [1] and the existence of stable intermediates at low pH in bromelain [2] and glucose oxidase [3]. In the present study, the role of metal ions and EGTA on the conformation of concanavalin A at alkaline pH was studied by near- and far-UV circular dichroism, fluorescence emission spectroscopy and binding of a hydrophobic dye, 1-anilino-8-naphthalene sulfonate (ANS). Far-UV CD spectra showed the transition from an ordered secondary structure at pH 7 with a trough at 223 nm to a relatively unordered state at pH 12. Near-UV CD spectra showed the loss of signal at 290 nm, thereby indicating the disruption of native three dimensional structure. Maximum ANS binding occurred at pH 12 suggesting the presence of an intermediate or molten globule-like state at alkaline pH.     

Circular dichroism studies of copper(II)- hyaluronic acid complexes in relation to conformation of the polymer

The study of the Cu(II)-hyaluronate complexes by absorption and CD spectra, as well as by acid–base titration and viscosity, provides information about the nature of ligands and the conformation of the polymer. Three different complexes have been identified. The first (complex I), which is formed between pH 3 and 6, involves mainly the carboxyl groups of the polymer as ligands and is characterized by a strong absorption band at 238 nm. In this complex formation, the CD properties of hyaluronate do not charge appreciably. The second (complex II) forms between pH 6 and 8 bad shows a major change in CD properties. The changes include (1) a new positive CD band at 250 nm and a strong negative on in the π → π* amide transition region and (2) the disappearance of the negative n → π* amide CD band near 210 nm. A sharp increase in absorbance at 238 nm from complex I to II has been attributed to a conformational transition which is also manifested in the CD features of hyaluronate. Complex II involves, in addition to the carboxyl group, the nitrogen atom of the deprotonated acetamido group coordinated to Cu(II). The absorption at 230–280 nm is associated with the optically active charge-transfer transitions involving ligands to metal ion. At higher concentrations of the polymer or at higher pH, complex II aggregates to a gel, complex III. Chondroitin, differing from hyaluronic acid in the C-4 hydroxyl group configuration of the glucosamine moiety, does not show any CD change in the presence of Cu(II).The results provide further support to our fourfold helical structure of Cu(II)–hyaluronate complex at pH between 6 and 8. Intrinsic viscosities of hyaluronate in the presence of the cupric ion is lower than in the presence of other monovalent or bivalent cations, indicating a compact conformation of the polymer when it is complexed with Cu(II).     

Circular dichroism and conformational properties of modeccin,a toxic protein

According to its circular dichroism (CD) spectrum, modeccin, a toxic lectin from the roots of the South African plantModecca digitata, is structurally similar to the ricins and abrins. In nearly neutral and weakly alkaline solutions (pH 7.6–9.0) the CD spectra of modeccin displayed a positive CD band at 190–195 nm and a negative band at 210–220 nm, indicating the presence of some α-helix and β-sheet structures. In the near-ultraviolet zone, we observed positive CD bands at 232 and 245 nm and weak negative bands at 285 and 293 nm. In more strongly alkaline solutions of pH 9.5–10.2 the CD bands in the farultraviolet zone were not affected, but the CD band at 232 nm diminished and the CD band at 245 nm was enhanced. These transitions were reversible. At pH 11.2–11.5 the CD band at 232 nm disappeared completely, and the CD bands in the far-ultraviolet diminished. The CD bands at 285 and 293 nm were affected very little by the alkali, and these bands were assigned to buried tryptophan side chains. Sodium dodecyl sulfate and 2,2,2-trifluoroethanol disorganized the tertiary structure of modeccin and reconstructed the secondary structure into a new form with a higher helix content than in the native protein.     

Unusual CD couplet pattern observed for the pi*<--n transition of enantiopure (Z)-8-methoxy-4-cyclooctenone: an experimental and theoretical study by electronic and vibrational circular dichroism spectroscopy and density functional theory calculation

Ultraviolet absorption (UV) and electronic circular dichroism (ECD) spectra of enantiopure (Z)-8-methoxy-4-cyclooctenone (MCO) were measured in hexane to give a normal single UV absorption band at 298 nm, which is assigned to the carbonyl's pi*<--n transition. Unexpectedly, the ECD spectrum exhibited an apparent couplet pattern with vibrational fine structures. Obviously, the conventional CD exciton coupling mechanism cannot be applied to this bisignate CD signal observed for single-chromophoric MCO. Variable temperature-ECD and vibrational circular dichroism (VCD) spectral measurements, simultaneous UV and ECD spectral band resolution, and density functional theory (DFT) calculations of energy and structure revealed that this apparent CD couplet originates from a rather complicated spectral overlap of more than three conformers of MCO, two of which exhibit mirror-imaged ECD spectra at appreciably deviated wavelengths. In the simultaneous band-resolution analysis, the observed UV and ECD spectra were best fitted to four overlapping bands. Two major conformers were identified by comparing the experimental IR and VCD spectra with the simulated ones, and the other two by comparing the observed UV and ECD spectra with the theoretical ones obtained by time-dependent DFT calculations. It was shown that the combined use of experimental ECD and VCD spectra and theoretical DFT calculations can give a reasonable interpretation for the Cotton effects of the conformationally flexible molecule MCO.     

Acid denaturation of recombinant porcine growth hormone: formation and self-association of folding intermediates

We have investigated the conformational changes incurred during the acid-induced unfolding and self-association of recombinant porcine growth hormone (pGH). Acidification (pH 8 to pH 2) of pGH resulted in intrinsic fluorescence, UV absorbance, and near-UV CD transitions centered at pH 4.10. At pH 2.0, a red shift in the fluorescence emission maximum of approximately 3 nm and a 15% loss of the far-UV CD signal at 222 nm imply that the protein did not become extensively unfolded. Acidification in the presence of 4 M urea resulted in similar pH-dependent transitions. However, these occurred at a higher pH (approximately 5.2). At pH 2.0 + 4 M urea, an 8 nm red shift in the fluorescence emission maximum suggests that unfolding was greater than in the absence of urea. The presence of a prominent peak centered at 298 nm in the near-UV CD spectrum, which is absent without urea, signifies further differences in the intermediates generated at pH 2. Sedimentation equilibrium experiments in the analytical ultracentrifuge showed that native pGH and the partially unfolded intermediates reversibly self-associate. Self-association was strongly promoted at pH 2 while urea reduced self-association at both pH 8 and pH 2. These results demonstrate that acidification of pGH in the absence or presence of 4 M urea induced the formation of molten globule-like states with measurable differences in conformation. Similarities and differences in these structural conformations with respect to other growth hormones are discussed.     

Unique,pH-dependent biphasic band shape of the visible circular dichroism of curcumin-serum albumin complex

Interaction between the plant derived polyphenolic type curcumin molecule having anticarcinogenic, antiinflammatory, and antioxidant activities, and human serum albumin was studied at different pH values by circular dichroism (CD) and electronic absorption spectroscopy. The weak, induced CD spectrum of curcumin-HSA complex measured at pH 7.4 in the visible spectral region shows striking changes upon alkalization; CD spectra collected between pH 7.7 and 9.3 exhibit characteristic, oppositely signed CD band pair according to the visible absorption band of HSA-bound curcumin. At 0.3 curcumin/HSA molar ratio, typical molar CD values are Delta epsilon (496.6nm)+40M(-1)cm(-1) and Delta epsilon (426.8nm)-40M(-1)cm(-1), respectively (pH 9.0, t=37 degrees C). The induced optical activity is attributed to a bent, right-handed chiral conformation of the HSA-bound curcumin molecule within which intramolecular exciton coupling occurs between the electric dipole transition moments of the dissymmetrically juxtaposed feruloyl chromophores. Deprotonation of phenolic OH group(s) of curcumin seems to be the reason leading to the conformational alteration of HSA-bound curcumin.     

Effects of acid pH and urea on the spectral properties of the LHII antenna complex from the photosynthetic bacterium Ectothiorhodospira sp.

The aim of this study was to investigate the spectral modifications of the LHII antenna complex from the purple bacterium Ectothiorhodospira sp. upon acid pH titration both in the presence and absence of urea. A blue shift specifically and reversibly affected the B850 band around pH 5.5-6.0 suggesting that a histidine residue most probably participated in the in vivo absorption red shifting mechanism. This transition was observed in the presence and absence of urea. Under strong chaotropic conditions, a second transition occurred around pH 2.0, affecting the B800 band irreversibly and the B850 reversibly. Under these conditions a blue shift from 856 to 842 nm occurred and a new and strong circular dichroism signal from the new 842 nm band was observed. Reverting to the original experimental conditions induced a red shift of the B850 band up to 856 nm but the circular dichroism signal remained mostly unaffected. Under the same experimental conditions, i.e. pH 2.1 in the presence of urea, part of the B800 band was irreversibly destroyed with concomitant appearance of a band around 770 nm due to monomeric bacteriochlorophyll from the disrupted B800. Furthermore, Gaussian deconvolution and second derivative of the reverted spectra at pH 8.0 after strong-acid treatment indicated that the new B850 band was actually composed of two bands centered at 843 and 858 nm. We ascribed the 858 nm band to bacteriochlorophylls that underwent reversible spectral shift and the 843 nm band to oligomeric bacteriopheophytin formed from a part of the B850 bacteriochlorophyll. This new oligomer would be responsible for the observed strong and mostly conservative circular dichroism signal. The presence of bacteriopheophytin in the reverted samples was definitively demonstrated by HPLC pigment analysis. The pheophytinization process progressed as the pH decreased below 2.1, and at a certain point (i.e. pH 1.5) all bacteriochlorophylls, including those from the B800 band, became converted to oligomeric bacteriopheophytin, as shown by the presence of a single absorption band around 843 nm and by the appearance of a single main elution peak in the HPLC chromatogram which corresponded to bacteriopheophytin.     

Thermodynamic characterization of an intermediate state of human growth hormone.

The thermal denaturation of recombinant human growth hormone (rhGH) was studied by differential scanning calorimetry and circular dichroism spectroscopy (CD). The thermal unfolding is reversible only below pH 3.5, and under these conditions a single two-state transition was observed between 0 and 100 degrees C. The magnitudes of the deltaH and deltaCp of this transition indicate that it corresponds to a partial unfolding of rhGH. This is also supported by CD data, which show that significant secondary structure remains after the unfolding. Above pH 3.5 the thermal denaturation is irreversible due to the aggregation of rhGH upon unfolding. This aggregation is prevented in aqueous solutions of alcohols such as n-propanol, 2-propanol, or 1,2-propanediol (propylene glycol), which suggests that the self-association of rhGH is caused by hydrophobic interactions. In addition, it was found that the native state of rhGH is stable in relatively high concentrations of propylene glycol (up to 45% v/v at pH 7-8 or 30% at pH 3) and that under these conditions the thermal unfolding is cooperative and corresponds to a transition from the native state to a partially folded state, as observed at acidic pH in the absence of alcohols. In higher concentrations of propylene glycol, the tertiary structure of rhGH is disrupted and the cooperativity of the unfolding decreases. Moreover, the CD and DSC data indicate that a partially folded intermediate with essentially native secondary structure and disordered tertiary structure becomes significantly populated in 70-80% propylene glycol.     

Differences in the effects of TFE on the folding pathways of human stefins A and B.

Trifluoroethanol (TFE) has been used to probe differences in the stability of the native state and in the folding pathways of the homologous cysteine protein inhibitors, human stefin A and B. After complete unfolding in 4.5 mol/L GuHCl, stefin A refolded in 11% (vol/vol) TFE, 0.75 mol/L GuHCl, at pH 6.0 and 20 degrees C, with almost identical first-order rate constants of 4.1 s-1 and 5.5 s-1 for acquisition of the CD signal at 230 and 280 nm, respectively, rates that were markedly greater than the value of 0.11 s-1 observed by the same two probes when TFE was absent. The acceleration of the rates of refolding, monitored by tyrosine fluorescence, was maximal at 10% (vol/vol) TFE. Similar rates of refolding (6.2s-1 and 7.2 s-1 for ellipticity at 230 and 280 nm, respectively) were observed for stefin A denatured in 66% (vol/vol) TFE, pH 3.3, when refolding to the same final conditions. After complete unfolding in 3.45 mol/L GuHCl, stefin B refolded in 7% (vol/vol) TFE, 0.57 mol/L GuHCl, at pH 6.0 and 20 degrees C, with a rate constant for the change in ellipticity at 280 nm of 32.8 s-1; this rate was only twice that observed when TFE was absent. As a major point of distinction from stefin A, the refolding of stefin B in the presence of TFE showed an overshoot in the ellipticity at 230 nm to a value 10% greater than that in the native protein; this signal relaxed slowly (0.01 s-1) to the final native value, with little concomitant change in the near-ultraviolet CD signal; the majority of this changes in two faster phases. After denaturation in 42% (vol/vol) TFE, pH 3.3, the kinetics of refolding to the same final conditions exhibited the same rate-limiting step (0.01 s-1) but were faster initially. The results show that similarly to stefin A, stefin B forms its hydrophobic core and predominant part of the tertiary structure faster in the presence of TFE. The results imply that the alpha-helical intermediate of stefin B is highly structured. Proteins 1999;36:205-216.     

Circular dichroism microscopy of compact forms of DNA and chromatin in vivo and in vitro: cholesteric liquid-crystalline phases of DNA and single dinoflagellate nuclei

Two highly condensed structures of DNA have been analyzed in the circular dichroism (CD) microscope: the cholesteric liquid-crystalline phase of DNA and the nucleus of a dinoflagellate (Prorocentrum micans). In both cases, the DNA shows a helical cholesteric organization, but the helical pitch equals about 2500 nm in the first case and 250 nm in the second one. Since the absorption band of DNA is located at 260 nm, the reflection and absorption bands are well separated in the cholesteric phase of DNA and are overlapping in the dinoflagellate nucleus. However, both structures give a very strong negative CD signal at 265 nm. We show that this very strong signal cannot correspond to a Borrmann effect, i.e., to a superposition of the absorption and reflection bands, but is a differential absorption of left versus right circularly polarized light. This anomalous differential absorption is probably due to a significant scattering of light, inside of the structure, which produces a resonance phenomenon in the absorption band of the chromophore. Therefore, for any helical structure containing a chromophore, the apparent CD can be expressed as CD = [(epsilon L - epsilon R)cl] + (psi L - psi R) + (SL - SR) The first term is true absorption and is located in the absorption band of the chromophore, and the last term is true scattering and is observed at the wavelength corresponding to the helical pitch of the structure. The second term (psi L - psi R) corresponds to the anomalous differential absorption observed in dense superhelical structures of DNA. It superimposes to the first term in the absorption band of the chromophore. psi L - psi R is a measure of the perfection of the helical structure and of the density of chromophores in the material. Intercalative dyes [ethidium bromide and meso-tetrakis(4-N-methylpyridyl)porphine (H2TMpyP-4) and its nickel(II) derivative (NiIITMpyP-4)] were inserted in the dinoflagellate chromatin. The CD signal recorded in their absorption band mimics the signal observed in the absorption band of DNA. In both structures, the negative sign of the CD at 265 nm indicates that the twist occurring between DNA. In both structures, the negative sign of the CD at 265 nm indicates that the twist occurring between DNA molecules is left-handed, and we show that this situation is the most frequently encountered in vivo and vitro.     

Detection of an equilibrium intermediate in the folding of a monomeric insulin analog.

To determine the conformational properties of the C-terminal region of the insulin B-chain relative to the helical core of the molecule, we have investigated the fluorescence properties of an insulin analog in which amino acids B28 and B29 have been substituted with a tryptophan and proline residue respectively, ([WB28,PB29]insulin). The biological properties and far-UV circular dichroism (CD) spectrum of the molecule indicate that the conformation is similar to that of native human insulin. Guanidine hydrochloride (GdnHCl)-induced equilibrium denaturation of the analog as monitored by CD intensity at 224 nm indicates a single cooperative transition with a midpoint of 4.9 M GdnHCl. In contrast, when the equilibrium denaturation is observed by steady-state fluorescence emission intensity at 350 nm, two distinct transitions are observed. The first transition accounts for 60% of the observed signal and has a midpoint of 1.5 M GdnHCl. The second transition roughly parallels that observed by CD measurements with an approximate midpoint of 4.5 M GdnHCl. The near-UV CD spectrum, size-exclusion, and ultracentrifugation properties of [WB28,PB29]insulin indicate that this analog does not self-associate in a concentration-dependent manner as does human insulin. Thus, the observed fluorescence changes must be due to specific conformational transitions which occur upon unfolding of the insulin monomer with the product of the first transition representing a stable folding intermediate of this molecule.     

Molten-globule like partially folded states of human serum albumin induced by fluoro and alkyl alcohols at low pH

Human serum albumin (HSA) exists in a molten-globule like state at low pH (pH 2.0). We studied the effects of trifluoroethanol (TFE) and hexafluoroisopropanol (HFIP) on the acid-denatured state of HSA by far-UV circular dichroism (CD), near-UV CD, tryptophan fluorescence, and 1-anilinonaphthalene-8-sulfonic acid (ANS) binding. At pH 2.0, these alcohols induced the formation of alpha-helical structure as evident from the increase in mean residue ellipticity (MRE) value at 222 nm. On addition of different alcohols, HSA exhibited a transition from the acid-denatured state to the alpha-helical state and loss of ANS-binding sites reflected by the decrease in ANS fluorescence at 480 nm. However, the concentration range required to bring about the transition varied greatly among different alcohols. HFIP was found to have highest potential whereas methanol was least effective in inducing the transition. The order of effectiveness of alcohols was shown to be: HFIP > TFE > 2-chloroethanol > tert-butanol > isopropanol > ethanol > methanol as evident from the Cm values. The near-UV CD spectra and tryptophan fluorescence showed the differential effects of halogenated alcohols with those of alkanols. A comparison of the m values, showing the dependence of Delta GH on alcohol concentration, suggests that the helix stabilizing potential of different alcohols is due to the additive effect of different constituent groups present whereas remarkably higher potential of HFIP involves some other factor in addition to the contribution of constituent groups.     

Formation of the M(N) (M(open)) intermediate in the wild-type bacteriorhodopsin photocycle is accompanied by an absorption spectrum shift to shorter wavelength, like that in the mutant D96N bacteriorhodopsin photocycle

Maximum of the M intermediate difference spectrum in the wild-type Halobacterium salinarium purple membrane is localized at 405-406 nm under conditions favoring accumulation of the M(N) intermediate (6 M guanidine chloride, pH 9.6), whereas immediately after laser flash the maximum is localized at 412 nm. The maximum is also localized at 412 nm 0.1 msec after the flash in the absence of guanidine chloride at pH 11.3. Within several milliseconds the maximum is shifted to short-wavelength region by 5-6 nm. This shift is similar to that in the D96N mutant which accompanies the M(N) (M(open)) intermediate formation. The main two differences are: 1) the rate of the shift is slower in the wild-type bacteriorhodopsin, and is similar to the rate of the M to N intermediate transition (t1/2 approximately 2 msec); 2) the shift in the wild-type bacteriorhodopsin is observed at alkaline pH values which are higher than pK of the Schiff base (approximately 10.8 at 1 M NaCl) in the N intermediate with the deprotonated Asp-96. Thus, the M(N) (M(open)) intermediate with open water-permeable inward proton channel is observed only at high pH, when the Schiff base and Asp-96 are deprotonated. The data confirmed our earlier conclusion that the M intermediate observed at lower pH has the closed inward proton channel.