Conformation of sequential polypeptides of (Lysi-Leuj), (Lysi-Serj), and (Lys-Gly) in sodium dodecyl sulfate solution

A series of sequential polypeptides (Lys_iR_j)_n (R is Leu, Ser, or Gly) and random copolypeptides, (Lys^x, Leu^y)_n, were synthesized. Their conformation in NaDodSO₄ solution was determined by CD. Only (Lys-Leu)_n, (Lys-Ser)_n, and (Lys₃-Ser)_n adopt a stable β-form in the surfactant solution; (Lys-Ser₂)_n, (Lys-Ser₃)_n, (Lys₂-Ser₂)_n, and (Lys₂-Ser)_n have an unstable β-form, which reverts to an unordered form in high NaDodSO₄ concentrations, even though both Ser and DodSO-bound Lys⁺ are β-formers. In contrast, (Lys-Gly)_n remains unordered in NaDodSO₄ solution. On the other hand, Lys-rich (Lys₂-Leu)_n forms an unstable helix and (Lys₂-Leu₂)_n a stable helix in NaDodSO₄ solution. In 25 mM NaDodSO₄ (Lys^x, Leu^y)_n also forms a helix up to x = 75 and reverts to the β-form at x = 90. This compares with the helical conformation of (Lys^x, Ala^y)_n up to x = 65 and its β-form at x = 90, suggesting that Leu is an even stronger helix-former than Ala. Our results may provide a plausible explanation for the increase in helicity and disruption of the β-form for many proteins in NaDodSO₄ solution, that is, the polypeptide chain of a protein usually favors a helical conformation over a β-form in the presence of excess surfactant.     

Basic polypeptides as histone models. Synthesis, conformation, and interaction with DNA of statistical copolymers (Lys x,Ala y)n.

Statistical copolymers (Lys^x,Ala^y)_n were synthesized by copolymerization of N-carboxyanhydrides of L -amino acids. The conformation of copolymers in aqueous solutions was investigated using circular dichroism (CD). Calculations based on the CD data showed that polymers (Lys^x,Ala^y)_n can exhibit a random conformation, an α-helix, and a β-structure in various ratios. CD spectra of complexes of copolymers with DNA prepared by gradual dialysis from a high ionic strength to 0.15 M NaCl can be correlated with the copolymer conformation in medium and high ionic strength. For copolymers forming an α-helix and β-structure, these spectra show resemblance with similar spectra of complexes of those histones that are able to exhibit ordered conformations.     

Conformation of sequential and random lysine–phenylalanine copolypeptides

The sequential copolypeptides (Lys-Phe-Lys)_n and (Lys₂-Phe-Lys)_n and a series of related random copolypeptides were investigated with respect of their ability to adopt the α-helix or β-conformation. Conformational transitions were induced by increasing the pH or by addition of NaClO₄ or methanol and were observed by recording the CD spectra. In contrast to the respective alternating copolypeptide (Phe-Lys)_n with its strong tendency for the β-structure reported previously, (Lys-Phe-Lys)_n can adopt either secondary structure, whereas (Lys₂-Phe-Lys)_n strongly favors the α-helix. Together with the random copolypeptides, whose composition varied from 20 to 50 mol % phenylalanine and whose average molecular weights ranged from 10,000 to 90,000, the influence of the phenylalanine content and of the chain length on conformational stability and the rotatory strength of the respective secondary structures were elaborated.     

Molecular modeling and design of regioselectively addressable functionalized templates with rigidified three‐dimensional structures

Extensive conformational analysis of a series of β‐alkyl substituted cyclopeptides—cyclo(Pro¹–Xaa²–Nle³–Ala⁴–Nle⁵–Pro⁶–Xaa⁷–Nle⁸–Ala⁹–Nle¹⁰) and cyclo[Pro¹–Xaa²–Nle³–(Cys⁴– Nle⁵–Pro⁶–Xaa⁷–Nle⁸–Cys⁹)–Nle¹⁰] as well as their corresponding unsubstituted core structures cyclo(Pro¹–Xaa²–Ala³–Ala⁴–Ala⁵–Pro⁶–Xaa⁷–Ala⁸–Ala⁹–Ala¹⁰) and cyclo(Pro¹–Xaa²–Ala³–Cys⁴– Ala⁵–Pro⁶–Xaa⁷–Ala⁸–Cys⁹–Ala¹⁰) has been performed employing both the ECEPP/2 and the MAB force fields (Xaa = Gly, L ‐Ala, D ‐Ala, Aib, and D ‐Pro). Results show that (a) possible three‐dimensional structures of the cyclo(Pro¹–Gly²–Lys³–Ala⁴–Lys⁵–Pro⁶–Gly⁷–Lys⁸–Ala⁹–Lys¹⁰) molecule are not limited to a single extended “rectangular” conformation with all Lys side chains oriented at the same side of the molecule; (b) conformational equilibrium in monocyclic analogues obtained by replacements of conformationally flexible Gly residues for L ‐Ala, D ‐Ala, Aib, or D ‐Pro is not significantly shifted towards the target “rectangular” conformational type; and (c) introduction of disulfide bridges between positions 4 and 9 is a very powerful way to stabilize the target conformations in the resulting bicyclic molecules. These findings form the basis for further design of rigidified regioselectively addressable functionalized templates with many application areas ranging from biostructural to diagnostic purposes. © 1999 John Wiley & Sons, Inc. Biopoly 50: 361–372, 1999     

Kinetics of coil to α-helix to β-structure transitions of poly(L-ornithine) in low concentrations of sodium dodecyl sulfate

Conformational changes of poly(L-ornithine) [(Orn)_n] were studied in a sodium dodecyl sulfate (NaDodSO₄) solution by CD. (Orn)_n adopted an unstable and a stable helical structure below and above the NaDodSO₄ concentration range where β-structure was favored, respectively. CD stopped-flow was used to monitor the transitions from coil to the unstable helix, from the helix to β-structure, and from coil to β-structure. Only the rate of the helix to β-structure transition was accelerated by an increase in NaDodSO₄ concentration, whereas the rates of the others were independent of NaDodSO₄ concentration. The fractions of coil, α-helix, and β-structure in each conformation of (Orn)_n caused by NaDodSO₄ were computed by simulating a mixed spectrum of typical CD spectra for these structures to the experimentally obtained spectrum. The contents of the unstable and stable helical structures were less than 50 and 73%, respectively.     

Preferred solution and calculated conformations of dermorphin and analysis of structure–conformation–activity relationships in the series [A1an]-dermorphin

We describe the solution (¹H-nmr) and calculated conformations of the opiatelike peptide dermorphin and the analysis of structure–conformation–activity relationships in the series [Alaⁿ]-dermorphin. We used ¹H-nmr spectroscopy to study dermorphin and its analogs [Alaⁿ]-dermorphin (with n = 1, 2…7) dissolved in dimethylsufoxide. Conformational energy calculations using semiempirical partitioned energy function methods were then carried out on dermorphin and its [L -Ala²]-analog. Agreement between calculation and experiment is satisfying, both suggesting predominance of a type I β-turn around Pro⁶-Ser⁷ at the C-terminus and of an extended structure in the central sequence Phe³-Gly⁴-Tyr⁵. Detailed analysis by step-by-step substitutions with Ala indicates that intraresidue interactions dominate over medium-range interactions (between adjacent residues), although the latter may also have a noticeable influence in shaping conformations. As a general feature, the effects of substitutions on the arrangement of side chains are always larger on the succeeding residue than on the preceding residue. Almost all the variations of activity observed in the analogs can be explained from conformational changes occurring in the aromatic side chains of the biologically important Tyr¹, Phe³, and Tyr⁵ on substitutions effected on adjacent residues (fluctuations via medium-range interactions).     

Unusual conformational preferences of β-alanine containing cyclic peptides. VII

In the present paper we describe the synthesis, purification, and single crystal x-ray analysis of the cyclic pentapeptide cyclo-(Pro-Phe-Phe-β-Ala-β-Ala). This compound crystallizes in the orthorhombic space group P2₁2₁2₁ from methanol and adopts in the solid state an unusual conformation characterized by a cis β-Ala⁵-Pro¹ peptide bond and by an intramolecular hydrogen bond stabilizing a C₁₁- and a C₁₂-ring structure. The C₁₁, structure contains the Phe³ and the β-Ala⁴ at the corner position of the turn; it is the first observation of a type II β-turn enlargement due to the insertion of an extra methylene group of the β-alanine residue. The rest of the molecule participates in a newly characterized C₁₂-ring structure, which incorporates a β-Ala residue at position i of the turn. © 1996 John Wiley & Sons, Inc.     

The pH dependence of the equilibrium constant KHyd for the hydrolysis of the Lys15-Ala16 reactive-site peptide bond in bovine pancreatic trypsin inhibitor (aprotinin)

ThepH dependence of the equilibrium constant K_Hyd for the hydrolysis of the Lys¹⁵-Ala¹⁶ reactive-site peptide bond of the bovine pancreatic trypsin inhibitor (aprotinin) was investigated over thepH range 2.3–6.5. Solutions of aprotinin, modified aprotinin with the Lys¹⁵-Ala¹⁶ peptide bond cleaved and mixtures of both species were incubated with 10 mol% porcine -trypsin. The state of equilibrium was determined by analytical cation-exchange HPLC. The K_Hyd values obtained did not exactly obey the simple equation of Dobry et al. (1952), which had to be used in an extended form with two additional parameters for a satisfactory fit. ThepH-independent equilibrium constant is 0.90 and thepK values of the Lys¹⁵ carboxyl group and of the Ala¹⁶ amino group are 3.10 and 8.22, respectively. ThepK of an additional group is apparently perturbed by the peptide-bond hydrolysis. It is 4.60 in the native and 4.40 in the modified aprotinin.     

Phosphorylation of cMyBP-C Affects Contractile Mechanisms in a Site-specific Manner

Cardiac myosin binding protein-C (cMyBP-C) is a cardiac-specific, thick-filament regulatory protein that is differentially phosphorylated at Ser²⁷³, Ser²⁸², and Ser³⁰² by various kinases and modulates contraction. In this study, phosphorylation-site-specific effects of cMyBP-C on myocardial contractility and cross-bridge kinetics were studied by sinusoidal analysis in papillary and trabecular muscle fibers isolated from t/t (cMyBP-C-null) mice and in their counterparts in which cMyBP-C contains the ADA (Ala²⁷³-Asp²⁸²-Ala³⁰²), DAD (Asp²⁷³-Ala²⁸²-Asp³⁰²), and SAS (Ser²⁷³-Ala²⁸²-Ser³⁰²) mutations; the results were compared to those from mice expressing the wild-type (WT) transgene on the t/t background. Under standard activating conditions, DAD fibers showed significant decreases in tension (∼50%), stiffness, the fast apparent rate constant 2πc, and its magnitude C, as well as its magnitude H, but an increase in the medium rate constant 2πb, with respect to WT. The t/t fibers showed a smaller drop in stiffness and a significant decrease in 2πc that can be explained by isoform shift of myosin heavy chain. In the pCa-tension study using the 8 mM phosphate (Pi) solution, there was hardly any difference in Ca²⁺ sensitivity (pCa₅₀) and cooperativity (n_H) between the mutant and WT samples. However, in the solutions without Pi, DAD showed increased n_H and slightly decreased pCa₅₀. We infer from these observations that the nonphosphorylatable residue 282 combined with phosphomimetic residues Asp²⁷³ and/or Asp³⁰² (in DAD) is detrimental to cardiomyocytes by lowering isometric tension and altering cross-bridge kinetics with decreased 2πc and increased 2πb. In contrast, a single change of residue 282 to nonphosphorylatable Ala (SAS), or to phosphomimetic Asps together with the changes of residues 273 and 302 to nonphosphorylatable Ala (ADA) causes minute changes in fiber mechanics.     

A high-resolution 13C-nmr study of collagenlike polypeptides and collagen fibrils in solid state studied by the cross-polarization–magic angle-spinning method. Manifestation of conformation-dependent 13C chemical shifts and application to conformational characterization

We have recorded high-resolution ¹³C-nmr spectra of collagen fibrils in the solid state by the cross-polarization–magic-angle-spinning(CP–MAS)method and analyzed the spectra with reference to those of collagenlike polypeptides. We used two kinds of model polypeptides to obtain reference ¹³C chemical shifts of major amino acid residues of collagen (Gly, Pro, Ala, and Hyp): the 3₁-helical polypeptides [(Gly)_nII, (Pro)_nII, (Hyp)_n, and (Ala? Gly? Gly)_nII], and the triple-helical polypeptides [(Pro? Gly? Pro)_n and (Pro? Ala? Gly)_n]. Examination of the ¹³C chemical shifts of these polypeptides, together with our previous data, showed that the ¹³C chemical shifts of individual amino acid residues are the same, within experimental error (±0.5 ppm), among different polypeptides with different primary sequences, if the conformations are the same. We found that the ¹³C chemical shifts of Ala residues of the 3₁-helical (Ala? Gly? Gly)_n and triple-helical (Pro? Ala? Gly)_n are significantly displaced, compared with those of the α-helix, β-sheet, and silk I form, and can be utilized as excellent probes to examine conformational features of collagen-like polypeptides. Further, the ¹³C chemical shifts of Gly and Pro residues in the triple-helical polypeptides are substantially displaced from those found in (Gly)_nII and (Pro)_nII of the 3₁-helix, reflecting further conformational change from the 3₁-helix to the supercoiled triple helix. In particular, the ¹³C chemical shifts of Gly C ? O carbons of the triple-helical polypeptides are substantially displaced upfield (4.1–5.1 ppm), with respect to those of the 3₁-helical polypeptides. These displacements are interpreted by that Gly C ? O of the former is not involved in NH …? O ? C hydrogen bonds, while this carbon of the latter is linked by these kinds of hydrogen bonds. On the basis of these ¹³C chemical shifts, as reference data for the collagenlike structure, we were able to assign the ¹³C-nmr peaks of Gly, Ala, Pro, and Hyp residues of collagen fibrils, which are in good agreement with the values expected from the model polypeptides mentioned above. We also discuss a plausible conformational change of collagen fibrils during denaturation.     

Semisynthesis and properties of some insulin analogs

The trypsin-catalyzed coupling of bovine (Boc)₂-desoctapeptide (B23-B30)-insulin with synthetic octapeptides, H-Gly-X₂-X₃-X₄-Thr-Pro-Lys(Boc)-Thr-OH (X₂ = Phe or Ala, X₃ = Phe or Ala, X₄ = Tyr or Ala), followed by deprotection and purification produced the [Ala^B24, Thr^B30]-, [Ala^B25, Thr^B30]-, and [Ala^B26, Thr^B30]-analogs of bovine insulin in yields of 32, 35, and 32%, respectively. The biological activity of these analogs decreased in the order, normal insulin ([Thr^B30]-bovine insulin) = Ala^B26-insulin > Ala^B25-insulin > Ala^B24-insulin, as assayed for receptor binding and some other biological effects, in contrast with the corresponding Leu-analogs of human insulin, in which the activity decreased in the order, normal insulin > Leu^B24-insulin > Leu^B25-insulin. The affinity to insulin antibodies greatly diminished in both Ala^B24-insulin and Leu^B24-insulin but not in the B25-substituted analogs. The CD spectra of the Leu- and the Ala-analogs were compared with those of normal insulins to show that no apparent correlation seems to exist between the decrease in biological activity and the conformational changes observed in solution. The effects of organic solvents on the peptide-bond equilibrium and on the stability of trypsin are also discussed.     

Conformation and aggregation of melittin: effect of pH and concentration of sodium dodecyl sulfate

The conformation of melittin, a surface-active polypeptide, in solution was studied by CD spectra between 190 and 240 nm. The molecule was essentially unordered (possibly with a trace of helix) in water without salt at neutral pH. Upon deprotonation of four of the six cationic groups at pH 12 the polypeptide became partially helical (about 35%). The addition of NaDodSO₄ to an aqueous melittin solution first caused the solution to become turbid but it became clear again in excess surfactant solution. The conformational changes depended on the molar NaDodSO₄/melittin ratio, R. With R from 2.34 to 23.4, the melittin solution was turbid and the polypeptide conformation was probably a mixture of α-helix and β-sheets. This was supported by the ir spectrum of the turbid solution, which indicated the presence of both conformations. With R = 46.8 or 468 (1 or 10 mM NaDodSO₄) the polypeptide conformation was characteristic of an α-helix, about 70–80% of the molecule, regardless of whether the surfactant was above or below its critical micelle concentration. This compared well with the x-ray results of 92% helix in crystals. The lower helicity of melittin in NaDodSO₄ solution might be attributed to the end effects that destabilize the first and last turn of an helix at its N- and C-terminus, respectively.     

Phosphorylation of cMyBP-C Affects Contractile Mechanisms in a Site-specific Manner

Cardiac myosin binding protein-C (cMyBP-C) is a cardiac-specific, thick-filament regulatory protein that is differentially phosphorylated at Ser²⁷³, Ser²⁸², and Ser³⁰² by various kinases and modulates contraction. In this study, phosphorylation-site-specific effects of cMyBP-C on myocardial contractility and cross-bridge kinetics were studied by sinusoidal analysis in papillary and trabecular muscle fibers isolated from t/t (cMyBP-C-null) mice and in their counterparts in which cMyBP-C contains the ADA (Ala²⁷³-Asp²⁸²-Ala³⁰²), DAD (Asp²⁷³-Ala²⁸²-Asp³⁰²), and SAS (Ser²⁷³-Ala²⁸²-Ser³⁰²) mutations; the results were compared to those from mice expressing the wild-type (WT) transgene on the t/t background. Under standard activating conditions, DAD fibers showed significant decreases in tension (∼50%), stiffness, the fast apparent rate constant 2πc, and its magnitude C, as well as its magnitude H, but an increase in the medium rate constant 2πb, with respect to WT. The t/t fibers showed a smaller drop in stiffness and a significant decrease in 2πc that can be explained by isoform shift of myosin heavy chain. In the pCa-tension study using the 8 mM phosphate (Pi) solution, there was hardly any difference in Ca²⁺ sensitivity (pCa₅₀) and cooperativity (n_H) between the mutant and WT samples. However, in the solutions without Pi, DAD showed increased n_H and slightly decreased pCa₅₀. We infer from these observations that the nonphosphorylatable residue 282 combined with phosphomimetic residues Asp²⁷³ and/or Asp³⁰² (in DAD) is detrimental to cardiomyocytes by lowering isometric tension and altering cross-bridge kinetics with decreased 2πc and increased 2πb. In contrast, a single change of residue 282 to nonphosphorylatable Ala (SAS), or to phosphomimetic Asps together with the changes of residues 273 and 302 to nonphosphorylatable Ala (ADA) causes minute changes in fiber mechanics.     

Carrier design: Conformational studies of amino acid (X) and oligopeptide (X-DL-Alam) substituted poly(L-lysine)

The present study was undertaken to examine the influence of the reversal of the sidechain sequential order on the conformation of branched polypeptides. At the same time, the influence of the optically active amino acid joined directly to the poly (L -Lys) backbone and the DL -Ala oligomer grafted as chain-terminating fragment were separately analyzed. Therefore two sets of polypeptides were synthesized corresponding to the general formula poly [Lys-(X_i,)] (XK) and poly[Lys-(DL -Ala_m-X_i)] (AXK) when X = Ala, D -Ala, Leu, D -Leu, Phe, D -Phe, Ile, Pro, Glu.,D -Glu, or His. For coupling amino acid X to polylysine, three types of active ester methods were compared: the use of pentafluorophenyl or pentachlorophenyl ester, and the effect of the addition of an equimolar amount of 1-hydroxybenzotriazole. After cleavage of protecting groups, AXK polypeptides were synthesized by grafting short oligo (DL -Ala) chains to XK by using N-carboxy-DL -Ala anhydride. The CD measurements performed in water solutions of various pH values and ionic strengths were used for classification of the polypeptide conformations as either ordered (helical) or unordered. Different from what was observed with the unsubstituted poly (L -Lys), poly[Lys-(X_i)] type polypeptides can adopt ordered structure even under nearly physiological conditions (pH 7.3, 0.2M NaCl). These data suggest that the introduction of amino acid residue with either (ar) alkyl side chain (Ala, Leu, Phe) or negatively charged side chain (Glu) promotes markedly the formation of ordered structure. Comparison of chiroptical properties of poly [Lys- (DL -Ala_m-X_i)] and of poly [Lys- (X_i)] reveals that side-chain interactions play an important role in the stabilization of ordered solution conformation of AXK type branched polypeptides. The results give rather conclusive evidence that not only hydrophobic interactions, but also ionic attraction, can be involved in the formation and stabilization of helical conformation of branched polypeptides. © 1993 John Wiley & Sons, Inc.     

Interaction of amphipathic polypeptides with phospholipids: characterization of conformations and the CD of oriented beta-sheets

The effects of interaction with phospholipids on the conformation of the alternating copolymer, poly(Leu-Lys), and the random copolymer poly(Leu⁵⁰, Lys⁵⁰) have been investigated by CD and ir spectroscopy. Poly(Leu-Lys) undergoes a partial unordered → β-sheet transition in solution in the presence of lysolecithin. On addition of lysolecithin plus cholate, an unordered → α -helix transition is observed. In films deposited from these solutions, poly(Leu-Lys) adopts the anti-parallel β-sheet conformation, as in aqueous solutions at moderate ionic strength. Polarized ir spectra showed that the plane of the β-sheet in such films deviates from the plane of the film by no more than 14°. The random copolymer, poly(Leu⁵⁰, Lys⁵⁰), is α-helical in the presence of lysolecithin and lysolecithin plus cholate, regardless of whether the sample is a solution or a film. CD measurements on the poly(Leu-Lys) films provide information about the component of the CD tensor for light propagating normal to the plane of the β-sheet. These measurements show (1) a negative n → π* CD band (214 nm maximum) with higher intensity than the average CD for isotropic solution; and (2) a positive band in the π → π* region (195 nm maximum), which is weaker than that in the isotropic spectrum.     

ThepH dependence of the equilibrium constantK Hyd for the hydrolysis of the Lys15-Ala16 reactive-site peptide bond in bovine pancreatic trypsin inhibitor (aprotinin)

ThepH dependence of the equilibrium constant K_Hyd for the hydrolysis of the Lys¹⁵-Ala¹⁶ reactive-site peptide bond of the bovine pancreatic trypsin inhibitor (aprotinin) was investigated over thepH range 2.3–6.5. Solutions of aprotinin, modified aprotinin with the Lys¹⁵-Ala¹⁶ peptide bond cleaved and mixtures of both species were incubated with 10 mol% porcine β-trypsin. The state of equilibrium was determined by analytical cation-exchange HPLC. The K_Hyd values obtained did not exactly obey the simple equation of Dobry et al. (1952), which had to be used in an extended form with two additional parameters for a satisfactory fit. ThepH-independent equilibrium constant is 0.90 and thepK values of the Lys¹⁵ carboxyl group and of the Ala¹⁶ amino group are 3.10 and 8.22, respectively. ThepK of an additional group is apparently perturbed by the peptide-bond hydrolysis. It is 4.60 in the native and 4.40 in the modified aprotinin.     

Polypeptides. LVI. Effect of lithium bromide and of temperature on the conformation of a copolymer of glutamic acid and lysine

By using optical rotatory dispersion measurements, the helix content of poly Glu⁵⁰Lys⁵⁰ has been investigated and compared with that of poly Glu²⁰Lys²⁰Ala⁶⁰ in aqueous solutions. Measurements were made at pH 3 and at pH 8 in various concentrations of lithium bromide. Various factors affecting helix stabilization are considered and their perturbation by lithium bromide is related to the shape of the observed transition curves. A residual helix content of 12% in 8M LiBr, based upon a b₀ of +100 for a fully random conformation, was observed for poly Glu⁵⁰Lys⁵⁰ at pH 3 and 8. The loss of helix content of poly Glu⁵⁰Lys⁵⁰ as a function of temperature is also reported. ΔH is approximately ?6.9 kcal./mole for the overall transition, compared to ?6.5 kcal./mole for poly Glu²⁰Lys²⁰Ala⁶⁰. The midpoint of the broad transition is near 40°C. at pH 3, but much lower, at ?10 to 0°C., at pH 8. These results are discussed in terms of the stabilizing factors for the partial helix content of the polypeptides.     

Differential scanning calorimetry studies of NaCl effect on the inverse temperature transition of some elastin-based polytetra-, polypenta-, and polynonapeptides

Differential scanning calorimetry studies of the effect of NaCl on protein-based polymer self-assembly has been carried out on six elastin-based synthetic sequential polypeptides- i.e., the polypentapeptide (L -Val¹-L -Pro²-Gly³-L -Val⁴-Gly⁵)_n and its more hydrophobic analogues (L -Leu¹-L -Pro²-Gly³-L -Val⁴-Gly⁵)_n and (L -Val¹-L -Pro²-L -Ala³-L -Val⁴-Gly⁵)_n; the polytetrapeptide (L -Val¹-L -Pro²-Gly³-Gly⁴)_n and its more hydrophobic analogue (L -IIe¹-L -Pro²-Gly³-Gly⁴)_n; and the polynonapeptide (a pentatetra hybrid), (L -Val¹-L -Pro²-Gly³-L -Val⁴-Gly⁵-L -Val⁶-L -Pro⁷-Gly⁸-Gly⁹)_n. Previous physical characterizations of the polypentapeptides have demonstrated the occurrence of an inverse temperature transition since increase in order of the polypentapeptide, as the temperature is raised from below to above that of the transition, has been repeatedly observed using different physical characterizations. In the present experiments, it is observed that the transition temperatures of the polypeptides studied are linearly dependent on NaCl concentration. The molar effectiveness of NaCl in shifting the transition temperature ΔT_m/[N], is about 14°C/[N], with the dependence on peptide hydrophobicity being fairly small. Interestingly, however, the δΔQ/ [N] does depend on the hydrophobicity of a polypeptide.     

Salt-induced conformational change of random copolymers (Lys50Tyr50)n and (Lys50Phe50)n studied by 1H- and 13C-nuclear magnetic resonances. Aggregation behavior of helical segments

¹H- and ¹³C-nmr studies of conformational transitions of random amino acid copolymers containing aromatic residues (Lys⁵⁰Tyr⁵⁰)_n and (Lys⁵⁰Phe⁵⁰)_n in the presence of neutral salts were performed to serve as models of the aggregation behavior of polypeptides of biological significance. The ¹H and ¹³C signal intensities of Tyr and Phe residues decreased preferentially with increasing concentration of neutral salts such as NaCl and NaClO₄. This behavior contrasts with that of (Lys)_n in the presence of similar neutral salts, where the displacement of the ¹³C signal is clearly seen on transition from the random-coil to the helical conformation. On the basis of the previous conformational studies, the loss of the peak areas is ascribed to the presence of immobilized helical segments by hydrophobic interaction between aromatic side chains. The remaining resonances are due to the residual random-coil regions, since the values of nuclear Overhauser enhancements and chemical shifts are unchanged in the presence and absence of the neutral salts.     

Molecular dynamics simulations of conformation and chain length dependent terahertz spectra of alanine polypeptides

Terahertz absorption spectra of alanine polypeptides in water were simulated with classical molecular dynamics at 310 K. Vibrational modes and oscillator strengths were calculated based on a quasi-harmonic approximation. Absorption spectra of Ala_n (n = 5, 15, 30) with different chain lengths and Ala₁₅ in coiled and helical conformations were studied in 10–40 cm^? 1 bandwidth. Simulation results indicated both the chain length and the conformation have significant influences on THz spectra of alanine polypeptides. With the increase of chain length, the average THz absorption intensity increases. Compared with the helical Ala₁₅ polypeptide, the THz spectra of coiled one shows stronger absorption peaks. These results were explained from different numbers of hydrogen bonds formed between polypeptides and the surrounding water molecules.