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.     

The effect of SDS on protein zone dispersion in polyacrylamide gel electrophoresis

The zone dispersions of the reduced subunit of β-lactoglobulin B and its derivative with sodium dodecyl sulfate (SDS) were measured during polyacrylamide gel electrophoresis (PAGE) using the apparatus for continuous optical scanning at 280 nm. The ratio of apparent diffusion coefficients (D′) of the reduced subunit of β-lactoglobulin B (1.71 × 10^?6 cm²/s) and of its SDS-derivative (7.1 × 10^?7 cm²/s) was found to be 2.4 under the conditions of PAGE (pH 10.4, 0.015 ionic strength, 1°C, 4 mA/cm² current density, 50 μg protein load, 10% T gel) used. This is nearly twice the value of 1.3 predicted, under the assumption of sphericity for these protein molecules, on the basis of the binding of 1.4 g of SDS per gram of protein. It is postulated that the increment in zone sharpness (decrease in apparent diffusion coefficient) over that predicted by SDS binding alone is a general property of SDS-proteins providing gel electrophoresis in SDS-containing buffers with a resolving power larger than that obtained in the absence of the detergent.     

Conformational changes of α-lactalbumin and its fragment,Phe31-Ile59, induced by sodium dodecyl sulfate

Conformational changes of bovine α-lactalbumin in sodium dodecyl sulfate (SDS) solution were studied with the circular dichroism (CD) method using a dilute phosphate buffer ofpH 7.0 and ionic strength 0.014. The proportions of α-helix and β-structure in α-lactalbumin were 34% and 12%, respectively, in the absence of SDS. In the SDS solution, the helicity increased to 44%, while the β-structure disappeared. In order to verify the structural change from β-structure to α-helix, the moiety, assuming the β-structure in the α-lactalbumin, was isolated by a chymotryptic digestion. The structure of this α-lactalbumin fragment, Phe³¹-Ile⁵⁹, was almost disordered. However, the fragment adopted a considerable amount of α-helical structure in the SDS solution. On the other hand, the tertiary structure of α-lactalbumin, detected by changes of CD in the near-ultraviolet region, began to be disrupted before the secondary structural change in the surfactant solution. Dodecyl sulfate ions of 80 mol were cooperatively bound to α-lactalbumin. Although the removal of the bound dodecyl sulfate ions was tried by the dialysis against the phosphate buffer for 5 days, 4 mol dodecyl sulfates remained per mole of the protein. The remaining amount agreed with the number of stoichiometric binding site, determined by the Scatchard plot, indicating that the stoichiometric binding was so tight.     

Structural and conformational changes of β-lactoglobulin B: an infrared spectroscopic study of the effect of pH and temperature

Infrared spectra of 2.5 mM solutions of β-lactoglobulin B were recorded as a function of pH (from pH 2 to pH 13) and as a function of temperature (from −100°C to +90°C). An analysis of the pH- and temperature-induced changes in the secondary structures was performed based on changes in the conformation-sensitive amide I bands of β-lactoglobulin. Whereas the total of β-structure remains constant (56–59%) between pH22 and pH 10, the proportions of the various β-components do change. In particular, the dimerization of the monomeric protein, induced by raising the pH from 20 to 3, leads to an increase in the intensity of the 1636 cm⁻¹ band (associated with antiparallel β-sheet), at the expense of the 1626 cm⁻¹ band (associated with exposed β-strands). Both the thermal and alkaline denaturation of β-lactoglobulin occur in two distinct stages. Although the spectra (i.e., the structures) after complete thermal or alkaline denaturation are clearly different, the spectrum of the protein after the first stage of thermal denaturation (at about 60°C) is the same as that after the first stage of alkaline denaturation (at pH 11), suggesting a common denaturation intermediate, which probably represents a crossover point in a complex potential hypersurface.     

Secondary structural changes in the intact and the disulfide Bridges cleaved β-lactoglobulin A and B in solutions of urea,guanidine hydrochloride,and sodium dodecyl sulfate

The relative proportions of α-helix, β-sheet, and unordered form in β-lactoglobulin A and B were examined in solutions of urea, guanidine, and sodium dodecyl sulfate (SDS). In the curve-fitting method of circular dichroism (CD) spectra, the reference spectra of the corresponding structures determined by Chen et al. (1974) were modified essentially according to the secondary structure of β-lactoglobulin B predicted by Creamer et al. (1983), i.e., that the protein has 17% α-helix and 41% β-sheet. The two variants showed no appreciable difference in structural changes. The reduction of disulfide bridges in the proteins increased β-sheet up to 48% but did not affect the α-helical proportion. The α-helical proportions of nonreduced β-lactoglobulin A and B were not affected below 2 M guanidine or below 3 M urea, but those of the reduced proteins began to decrease in much lower concentrations of these denaturants. By contrast, the α-helical proportions of the nonreduced and reduced proteins increased to 40–44% in SDS. The β-sheet proportions of both nonreduced and reduced proteins, which remained unaffected even in 6 M guanidine and 9 M urea, decreased to 24–25% in SDS.     

Raman studies of the crystalline, solution, and alkaline-denatured states of beta-lactoglobulin.

The Raman spectra of β-lactoglobulin in the crystalline, freeze-dried, and solution states are compared. The spectra of the freeze-dried and crystalline proteins were practically identical. The conformationally sensitive amide III line appearing at 1242 cm^?1 increased in intensity 30% upon dissolution of the protein in water which is interpreted as a conformational change in the disordered chains of the protein. This result appears to be a phenomenon for globular proteins containing a large disordered chain fraction. The alkaline denaturation of β-lactoglobulin was studied. When the pH was increased from 6.0 to 11.0, the amide III line shifted from 1242 to 1246 cm^?1, broadened, and decreased in intensity. This is consistent with the conversion of β-sheet regions in β-lactoglobulin to the disordered conformation, as has been proposed by other investigators. At pH 13.5 the amide III shifts to 1257 cm^?1 characteristic of a completely disordered protein, indicating that any remaining “core” of β-sheet has been randomized. Several changes in the intensities of the tyrosine and tryptophan vibrations accompany the denaturation. As the pH is increased from 6.0 (native state) to 11.0 (denatured state) the intensity ratio of two tyrosine ring vibrations, I₈₅₅ cm^?1/I₈₃₀ cm^?1, decreases from 1.0:0.9 to 1.0:1.3. The same ratio for a copolymer consisting of 95% glutamic acid and 5% tyrosine at pH 7.0, where the polymer forms a random coil exposing the tyrosine to the aqueous environment, is 1.0:0.62. This ratio more closely resembles that corresponding to β-lactoglobulin at pH 6.0 (native state) than pH 11.0 (denatured state) suggesting that the average tyrosine in the denatured state may be in a more hydrophobic environment than in the native state. A time-dependent polymerization of the denatured protein reported by other investigators and observed by us may account for the change in the tyrosine environment. A tryptophan vibration appearing at 833 cm^?1 in the spectrum of the native state becomes weak as the pH is increased to 11.0. The intensity of this line may also reflect the local environment of the tryptophan residue.     

Characterization of partially folded intermediates of papain in presence of cationic, anionic, and nonionic detergents at low pH

A systematic investigation of the effects of detergents [Sodium dodecyl sulphate (SDS), hexa decyltrimethyl ammonium bromide (CTAB) and Tween-20] on the structure of acid-unfolded papain (EC.3.4.22.2) was made using circular dichroism (CD), intrinsic tryptophan fluorescence, and 1-anilino 8-sulfonic acid (ANS) binding. At pH 2, papain exhibits a substantial amount of secondary structure and is relatively less denatured compared with 6 M GdnHCl (guanidine hydrochloride) but loses the persistent tertiary contacts of the native state. Addition of detergents caused an induction of alpha-helical structure as evident from the increase in the mean residue ellipticity value at 208 and 222 nm. Near-UV CD spectra also showed the regain of native-like spectral features in the presence of 8 mM SDS and 3.5 mM CTAB. Induction of structure in acid-unfolded papain was greater in the presence SDS followed by CTAB and Tween-20. Intrinsic tryptophan fluorescence studies indicate the change in the environment of tryptophan residues upon addition of detergents to acid-unfolded papain. Addition of 8 mM SDS resulted in the loss of ANS binding sites exhibited by a decrease in ANS fluorescence intensity, suggesting the burial of hydrophobic patches. Maximum ANS binding was obtained in the presence of 0.1 mM Tween-20 followed by CTAB, indicating a compact "molten-globule"-like conformation with enhanced exposure of hydrophobic surface area. Acid-unfolded papain in the presence of detergents showed the partial recovery of enzymatic activity. These results suggest that papain at low pH and in the presence of SDS exists in a partially folded state characterized by native-like secondary structure and tertiary folds. While in the presence of Tween, acid-unfolded papain exists as a compact intermediate with molten-globule-like characteristics, viz. enhanced hydrophobic surface area and retention of secondary structure. While in the presence of CTAB it exists as a compact intermediate with regain of native-like secondary and partial tertiary structure as well as high ANS binding with the partially recovered enzymatic activity, i.e., a molten globule state with tertiary folds.     

Probing the fatty acid binding site of β-lactoglobulins

The interactions of fatty acids with porcine and bovine β-lactoglobulins were measured using tryptophan fluorescence enhancement. In the case of bovine β-lactoglobulin, the apparent binding constants for most of the saturated and unsaturated fatty acids were in the range of 10^?7 M at neutralpH. Bovine β-lactoglobulin displays only one high affinity binding site for palmitate with an apparent dissociation constant of 1·10^?7 M. The strength of the binding was decreasing in the following way: palmitate > stearate > myristate > arachidate > laurate. Caprylic and capric acids are not bound at all. The affinity of β-lactoglobulin for palmitate decreased as thepH of the incubation medium was lowered and BLG/palmitate complex was not observed atpH's lower than 4.5. Surprisingly, chemically modified bovine β-lactoglobulin and porcine β-lactoglobulin did not bind fatty acids in the applied conditions.     

Kerr effect study of the aqueous solutions of three globular proteins

A Kerr effect study is reported in which measurements have been made on the magnitudes of both the steady maxima and the decays of the birefringence of solutions of ovalbumin, bovine γ-globulin, and β-lactoglobulin. For each protein, results are presented on solutions covering the concentration range of 0.3–1.7 g./100 ml. in order to obtain by extrapolation, values of the specific Kerr constant K_sp, and the birefringence relaxation time τ_{25, w} at zero concentration. The relaxation times thus obtained for ovalbumin (18.3 nsec.) and γ-globulin (157 nsec.) have been shown to be compatible with molecular models and dimensions presented in the literature. All experiments showed the need for careful extrapolation to zero concentration if reliable parameters are to be obtained: for example a 1% solution of ovalbumin or l.5% solution of γ-globulin, would give values for τ which are 50% too high when compared with the true value at infinite dilution. The gradual fall in τ for γ-globulin as the pH was lowered from 6.7 to 3.0 was also studied for three solvents. Fisher's generalized model for the arrangement of the polar residues around the outside of a globular protein has been developed to account for ellipsoidal particles and has been used to demonstrate the suitability and usefulness of this treatment in predicting the conformation and dimensions of these proteins. Rather unusual birefringence traces for β-lactoglobulin were obtained, which may indicate the dissociation of aggregates, or of the parent molecule into its subunits, under the influence of strong electric fields.     

Conformation similarities of the globular and tailed forms of acetylcholinesterase from Torpedo california

The conformation of the globular dimer (G₂), the tailed asymmetric dodecamer (A₁₂, also containing some tailed octamer A₈) and the globular tetramer (G₄, prepared by removing the collagen-like tail from A₁₂) of acetylcholinesterase (acetylcholine acetylhydrolase, EC 3.1.1.7) was studied by circular dichroism (CD) in the ultraviolet region. The G₂ and G₄ forms had similar conformation with about 40% α-helix, 35% β-sheets and 4% β-turns; the tailed form had a lower helicity (about 34%) and β-form (about 25%) content probably because of the presence of the tail whose CD spectrum resembles that of an unordered form, but it had about the same amount of β-turns as the other two forms. All three forms also had similar CD spectra in the near-ultraviolet region due to their non-peptide chromophores. The pH, thermal and urea denaturation of the three acetylcholinesterase forms was also similar to each other. The pH-dependency of both the enzymatic activity and CD intensity of the three forms showed bell-shaped curves with a plateau at pH 7–8. The activity was completely lost at pH below 5 or above 10, but the corresponding CD spectra retained 70–80% of the original magnitudes. Thermal denaturation of the three forms at pH 7.5 showed a conformational transition and loss of activity between 30 and 40°C, but the CD intensity of the helical band at 222 nm was reduced by only 20–30%. Urea denaturation of the three form began at 1 M urea; it was protein concentration- and time-dependent. Again, the activity disappeared faster than the decreasing CD intensity. Thus, the overall conformation of the three acetylcholinesterase forms appears to be relatively stable, but their active site is easily perturbed by changing the environment. The loss of activity correlated well with the disapperance of the CD band of tryptophan(s) in the near-ultraviolet region, suggesting that the Trp residue(s) might be at or near the active center of the enzyme.     

Tyrosine optical activity as a probe of the conformation and interactions of fibronectin

A very intense negative band is observed at ～ 183 nm in the CD spectrum of fibronectin from bovine plasma. This transition has not previously been reported, probably because it occurs in a spectral region that has not been readily accessible in earlier studies. At longer wavelength, the observed CD is very similar to spectra reported for human and chick material, having positive bands at ～230 and ～200 nm, and a negative band at ～215nm. The low molar ellipticity of the negative band ([θ] ≈ ?2.5 × 10³ deg cm² dmol^?1) suggests little α-helix or β-sheet structure. The new transition, and the two positive bands at higher wavelength, do not correspond to known transitions of the peptide backbone, but all three are present in the CD of N-acetyltyrosineamide. It is therefore suggested that the observed CD behavior of fibronectin arises predominantly from the optical activity of tyrosine side chains. The contribution of this side-chain optical activity to the CD of other proteins is discussed. On raising pH to ionize tyrosine residues, the positive CD band at ～230 nm is lost in both N-acetyltyrosineamide and in fibronectin. The spectral change is fully reversible in the model compound, but only partially reversible in fibronectin. From this evidence, and the magnitude of the 183-nm band, it is suggested that some or all of the tyrosine residues in fibronectin may be present within ordered domains. The possible role of S? S bonds in maintaining tertiary structure is discussed. The interaction of fibronectin with heparin is accompanied by a large increase in the 183-nm band and by slight enhancement of the negative band at 215 nm, consistent with some limited formation of β-sheet. Present results indicate that CD may be of considerable value in characterization of the molecular organization and biologically relevant interactions of fibronectins and of related glycoproteins of the extracellular matrix.     

Conformation and interactions of uteroglobin fragments 4–14 and 49–65 in aqueous solution containing surfactant micelles

The conformation of two fragments of rabbit uteroglobin is described. The peptides are PRFAHVIENLL and PQTTRENIMKLTEKIVK, corresponding to helices I and IV in the crystal structure. CD shows that both peptides interact with sodium dodecyl sulfate (SDS) micelles and change their conformation to an α-helix. The helical content estimated from the CD band at 222 nm is about 40% in each peptide. Surface tension measurements show that both peptides lower the critical micellar concentration (cmc) of SDS, with a more dramatic effect in the case of helix I. This peptide by itself acts as a surfactant, and is able to interact with SDS even below the observed cmc, forming β aggregates. Proton magnetic resonance (¹H-nmr) suggests that flexible helices are present. The longest helical stretches compatible with ¹H-nmr data extend from Phe⁶ to Leu¹⁴ for helix I and from Arg⁵³ to Ile⁶³ for helix IV. © 1993 John Wiley & Sons, Inc.     

Interaction of beta-lactoglobulin and cytochrome c: complex formation and iron reduction.

β-Lactoglobulin forms a soluble complex with cytochrome c in mildly alkaline solutions of low ionic strength. Sedimentation velocity experiments suggest that the complex (maximum s₂₀ = 3.7) consists of one cytochrome c molecule per β-lactoglobulin monomer unit. At pH 8 or higher, the presence of β-lactoglobulin causes reduction of ferri- to ferrocytochrome c. The initial rate of reduction at a single temperature depends primarily on the concentration of β-lactoglobulin, although the final percentage ferrocytochrome c obtained is constant at molar ratios of three or more β-lactoglobulin monomers to one cytochrome c molecule. The temperature dependence of the initial rate of iron reduction resembles that for alkaline denaturation of β-lactoglobulin. The displacement of N-dansylaziridine, a sulfhydryl specific dye, from bovine β-lactoglobulin during iron reduction, and the formation of nonreducing complexes between the analogous swine protein (no sulfhydryls) and cytochrome c suggest that the sulfhydryl group of β-lactoglobulin is the electron donor.     

Cloning,purification and biochemical characterisation of a GH35 beta-1,3/beta-1,6-galactosidase from the mucin-degrading gut bacterium <Emphasis Type="Italic">Akkermansia muciniphila</Emphasis>

A putative GH35 β-galactosidase gene from the mucin-degrading bacterium Akkermansia muciniphila was successfully cloned and further investigated. The recombinant enzyme with the molecular mass of 74 kDa was purified to homogeneity and biochemically characterised. The optimum temperature of the enzyme was 42 °C, and the optimum pH was determined to be pH 3.5. The addition of sodium dodecyl sulphate (SDS) reduced the enzyme’s activity significantly. The addition of Mg²⁺-ions decreased the activity of the β-galactosidase, whereas other metal ions or EDTA showed no inhibitory effect. The enzyme catalysed the hydrolysis of β1,3- and β1,6- linked galactose residues from various substrates, whereas only negligible amounts of β1,4-galactose were hydrolysed. The present study describes the first functional characterisation of a β-galactosidase from this human gut symbiont.     

Microenvironment of tryptophan residues in β-lactoglobulin derivative polypeptide-sodium dodecyl sulfate complexes

The changes of microenvironment of tryptophan residues in β-lactoglobulin A and its cyanogen bromide (CNBr) fragments with the binding of sodium dodecyl sulfate (SDS) were studied with measurements of the rates of N-bromosuccinimide (NBS) modification reactions by stopped-flow photometry. Two tryptophan residues of carboxyamidomethylated (RCM) β-lactoglobulin A in the states of their complexes with SDS were clearly distinguishable by their differences in NBS modification rates. We confirmed by experiments with CNBr fragments containing tryptophan residue. The modification rates of Trp 19 in RCM β-lactoglobulin A-SDS complexes were about 10-fold smaller than those expected for tryptophan residues exposed entirely to the aqueous solvent. The Trp 61 was hardly changed. The change of rate constants for Trp 19 was virtually consistent with those observed when N-acetyl-l-tryptophan ethylester was dissolved in SDS micelles. For various species of polypeptide-SDS complexes, all tryptophan residues were reactive to NBS and also, for some of them, the differences in NBS modification rates were observed between tryptophan residues on a common polypeptide chain. These results suggest micellar and heterogeneous bindings of SDS to polypeptides.     

Proteins from melanosomes of mouse and chick pigment cells

Purified subcellular fractions containing melanosomes from B-16 mouse melanoma were treated with 2% sodium dodecyl sulfate or 0.5 M sodium hydroxide to dissolve protein. Quantitative measurements indicate that each melanosome contains 0.065×10^?10 of protein or about 19% by weight. SDS acrylamide gel electrophoresis of proteins from purified melanosomes resolved six polypeptide bands of major density and about 15 minor bands. These results indicate that the melanosome may be more complex than previous genetic, biochemical or morphological evidence had suggested.     

PHYSICOCHEMICAL PROPERTIES OF THE PROTEOLYTIC ENZYME FROM THE LATEX OF THE MILKWEED,ASCLEPIAS SPECIOSA TORR. SOME COMPARISONS WITH OTHER PROTEASES : I. CHEMICAL PROPERTIES,ACTIVATION-INHIBITION,pH-ACTIVITY,AND TEMPERATURE-ACTIVITY CURVES

1. A study has been made of the properties of a hitherto unreported proteolytic enzyme from the latex of the milkweed, Asclepias speciosa. The new protease has been named asclepain by the authors. 2. The results of chemical, diffusion, and denaturation tests indicate that asclepain is a protein. 3. Like papain, asclepain dots milk and digests most proteins, particularly if they are dissolved in concentrated urea solution. Unlike papain, asclepain did not clot blood. 4. The activation and inhibition phenomena of asclepain resemble those of papain, and seem best explained on the assumption that free sulfhydryl in the enzyme is necessary for proteolytic activity. The sulfhydryl of asclepain appears more labile than that of papain. 5. The measurement of pH-activity curves of asclepain on casein, ovalbumin, hemoglobin, edestin, and ovovitellin showed no definite digestion maxima for most of the undenatured proteins, while in urea solution there were well defined maxima near pH 7.0. Native hemoglobin and ovovitellin were especially undigestible, while native casein was rapidly attacked. 6. Temperature-activity curves were determined for asclepain on hemoglobin, casein, and milk solutions. The optimum temperature was shown to increase with decreasing time of digestion.     

Amine boranes as alternative reducing agents for reductive alkylation of proteins

The use of several commercially available amine-borane complexes was investigated for the reductive methylation of amino groups of several proteins. An earlier study in our laboratory, using turkey ovomucoid as the model protein, showed that dimethylamine borane is a slightly weaker reducing agent, but a good, less toxic substitute for sodium cyanoborohydride (K. F. Geoghegan, J. C. Cabacungan, H. B. F. Dixon, and R. E. Feeney, 1981, Int. J. Peptide Protein Res.17, 345–352). N-α-Acetyl-l-lysine, poly-l-lysine, turkey ovomucoid, bovine serum albumin, chicken ovalbumin, β-lactoglobulin, casein, and soybean protein were reductively methylated with dimethylamine borane and trimethylamine borane. The latter produced a consistently lower degree of modification even in the presence of sodium dodecyl sulfate. In a comparison that included the boranes triethylamine, t-butylamine, morpholine, and pyridine, pyridine borane was found to be slightly stronger than sodium cyanoborohydride. In a pH 7 solution containing 2 mmN-α-acetyl-l-lysine and 20 mm formaldehyde, complete dimethylation was achieved with about 10 mm pyridine borane after 2 h incubation at 22°C, while more than 15 mm was necessary with sodium cyanoborohydride. Like dimethylamine borane, both pyridine borane and triethylamine borane showed a reducing capacity at pH 7 which was as high as that at pH 9. Reductive alkylation under neutral to mild acid conditions allows modification of alkaline labile proteins and also limits the side reactions between proteins and formaldehyde.     

The measurement of transmembrane helices by the deconvolution of CD spectra of membrane proteins: A review

The interpretation of the CD spectra of proteins to date requires additional secondary structural information of the proteins to be analyzed, such as x-ray or nmr data. Therefore, these methods are inappropriate for a CD data base whose secondary structures are unknown, as in the case of the membrane proteins. The Convex Constraint Analysis algorithm [A. Perczel, M. Hollósi, G. Tusnády, and G. D. Fasman (1991) Protein Engineering, Vol. 4, 669–679], on the other hand, operates only on a collection of spectral data to extract the common spectral components with their spectral weights. The linear combinations of these derived “pure” CD curves can reconstruct the original data set with great accuracy. For a membrane protein data set, the five-component spectra so obtained from the deconvolution consisted of two different types of α-helices (the α-helix in the soluble domain and the α_T-helix, for the transmembrane α-helix), a β-pleated sheet, a class C-like spectrum related to β-turns, and a spectrum correlated with the unordered conformation. The deconvoluted CD spectrum for the α_T-helix was characterized by a positive red-shifted band in the range 195–200 nm (+95,000 deg cm² dmol^?l), with the intensity of the negative band at 208 nm being slightly less negative than that of the 222 nm band (?50,000 and ?60,000 deg cm² dmol^?1, respectively) in comparison with the regular α-helix, with a positive band at 190 nm and two negative bands at 208 and 222 nm with magnitudes of + 70,000, ?30,000, and ?30,000 deg cm² dmol^?1, respectively. © 1994 John Wiley & Sons, Inc.     

Studies on interaction of oligopeptides with sodium dodecyl sulfate: Stopped-flow kinetics of chemical modification of tryptophan residues withN-bromosuccinimide

The stopped-flow kinetics of the reaction between oligopeptides containing tryptophan residues andN-bromosuccinimide (NBS) were studied in 50 mM sodium phosphate buffer (pH 7.0) containing sodium dodecyl sulfate (SDS). Decreases in the reaction rates attributable to the interaction between oligopeptides and SDS were observed, and oligopeptides studied were classified into types I and II on the basis of the interaction modes. Type I oligopeptides were dissolved in SDS micelles; type II oligopeptides interacted cooperatively with SDS monomers. The manner of interaction between SDS and oligopeptides of type II could be interpreted by a simple equilibrium relation: oligopeptide+n·(SDS)=oligopeptide·(SDS) _n .