Conformational studies of polymers and copolymers of L-aspartate ester. II. Infrared studies and the factors involved in the formation of the omega-helix

It has already been show that the helix senses of poly(β-benzyl L -aspartate) and poly(β-methyl L -aspartate) are left-handed, while the poly esters of n-propyl, isopropyl, n-butyl, and phenethyl L -asparate are all right-handed. The effect of changes in helix sense from the left-handed to the right-handed α-helical form on the infrared spectra of copolymers of benzyl L -aspartate with ethyl, n-butyl, isopropyl, n-propyl, and phenethyl L -aspartate have been studied. Those show that for the right-handed helical form the amide band frequencies fall within the range given by Elliott,⁷ while for the left-handed form the frequencies are higher. The frequency ranges for the two helix senses are given and have been used to show that poly (β-n-propyl L -aspartate) in chloroform solution undergoes a transition from the right-handed to the left-handed helix form on heating. Polarized infrared studies of the different copolymers show that the disposition of the side chain ester groups is different for the two forms. Although methyl L -aspartate forms a left-handed α-helix similar to benzyl L -aspartate, the introduction of methyl L -aspartate residues into poly (β-benzyl L -aspartate) prevents the formation of the ω-helix. The factors involved in the formation of this helix form are discussed.     

Raman spectroscopic study of poly (β-benzyl-L-aspartate) and sequential polypeptides

Poly-β-benzyl-L -aspartate (poly[Asp(OBzl)]) forms either a lefthanded α-helix, β-sheet, ω-helix, or random coil under appropriate conditions. In this paper the Raman spectra of the above poly[Asp(OBzl)] conformations are compared. The Raman active amide I line shifts from 1663 cm^?1 to 1679 cm^?1 upon thermal conversion of poly[Asp(OBzl)] from the α-helical to β-sheet conformation while an intense line appearing at 890 cm^?1 in the spectrum of the α-helix decreases in intensity. The 890 cm^?1 line also displays weak intensity when the polymer is dissolved in chloroform–dichloroacetic acid solution and therefore is converted to the random coil. This line probably arises from a skeletal vibration and is expected to be conformationally sensitive. Similar behavior in the intensity of skeletal vibrations is discussed for other polypeptides undergoing conformational transitions. The Raman spectra of two cross-β-sheet copolypeptides, poly(Ala-Gly) and poly(Ser-Gly), are examined. These sequential polypeptides are model compounds for the crystalline regions of Bombyx mori silk fibroin which forms an extensive β-sheet structure. The amide I, III, and skeletal vibrations appeared in the Raman spectra of these polypeptides at the frequencies and intensities associated with β-sheet homopolypeptides. Since the sequential copolypeptides are intermediate in complexity between the homopolypeptides and the proteins, these results indicate that Raman structure–frequency correlations obtained from homopolypeptide studies can now be applied to protein spectra with greater confidence. The perturbation scheme developed by Krimm and Abe for explaining the frequency splitting of the amide I vibrations in β-sheet polyglycine is applied to poly(L -valine), poly-(Ala-Gly), poly(Ser-Gly), and poly[Asp(OBzl)]. The value of the “unperturbed” frequency, V₀, for poly[Asp(OBzl)] was significantly greater than the corresponding values for the other polypeptides. A structural origin for this difference may be displacement of adjacent hydrogen-bonded chains relative to the standard β-sheet conformation.     

Raman spectra of copoly(D,L-alanines)

Raman spectra in the region 1000–150 cm^?1 were measured for copoly(D ,L -alanines) with the D -residue contents, 3, 7, 10, and 20%, and compared with the spectrum of the α-helical poly-L -alanine. The 532- and 378-cm^?1 peaks were assigned to the L -residues with a right-handed α-helix-like local conformation or to the D -residues with a left-handed α-helix-like local conformation. From the intensity of the latter peak the contents of these local conformations were estimated as a function of the D -residue contents for the copolymers. The 264-cm^?1 peak, which has been assigned to the breathing vibration of the α-helical poly-L -alanine, shows a marked decrease in its intensity upon the introduction of the D residues. This result suggests that the overall deformation vibration of the α-helix arises from rather long sequences of the L - and D -alanine residues with the α-helical conformation and that the intensity of this vibration depends on the content of these sequences in the copolymers.     

Secondary structure of peptides. 4: 13C-Nmr CP/MAS investigation of solid oligo- and poly(L-alanines)

Primary and tertiary amine-initiated polymerizations of L -alanine-N-carboxyanhydride (L -Ala-NCA) were conducted at 20 or 100°C in a variety of solvents. The 75.5-MHz ¹³C-nmr CP/MAS spectra of the resulting poly(L -alanines) revealed that all samples contain both α-helix and pleated-sheet structures. Depending on the reaction conditions the α-helix content varied between ca. 1 and 99%. Reprecipitation from aprotic nonsolvents does not change the α-helix/β-sheet ratio, indicating that this ratio is thermodynamically controlled. Since relatively large amounts of oligopeptides of degree of polymerization (DP ) 4–6 can be extracted by means of acetic acid, it is concluded that (a) most poly(L -alanines) possess a bimodal molecular weight distribution, (b) the oligopeptide fraction with DP ? 11 is responsible for the β-sheet fraction of all samples, and (c) the two-stage crystal growth proposed by Komoto and Kawai is not correct. Solubilizing initiators such as poly(ethylene oxide) NH₂ prevent the precipitation of oligoalanine and, thus, the formation of a β-sheet structure. ¹³C-nmr CP/MAS measurements also show that tri- and tetra-L -alanines form insoluble β-sheet structures.     

NMR study of silk I structure of Bombyx mori silk fibroin with 15N- and 13C-NMR chemical shift contour plots

The metastable state silk I structures of Bombyx mori silk fibroin in the solid state were studied on the basis of ¹⁵N- and ¹³C-nmr chemical shifts of Ala, Ser, and Gly residues. The ¹⁵N cross-polarization magic angle spinning (CP/MAS) nmr spectra of the precipitated fraction after chymotrypsin hydrolysis of B. mori silk fibroin with the silk I and silk II forms were measured to determine the ¹⁵N chemical shifts of Gly, Ala, and Ser residues. For comparison, ¹⁵N CP/MAS nmr chemical shifts of Ala were measured for [¹⁵N] Ala Philosamia cynthia ricini silk fibroin with antiparallel β-sheet and α-helix forms. The ¹³C CP/MAS nmr chemical shifts of Ala, Ser, and Gly residues of B. mori silk fibroin with the silk I and silk II forms, as well as ¹³C CP/MAS nmr chemical shifts of Ala residue of P. c. ricini silk fibroin with β-sheet and α-helix forms, are used for the examination of the silk I structure. Both silk I and α-helix peaks are shifted to a lower field than silk II (β-sheet) for the Cα carbons of the Ala residues, while both Cβ carbon peaks are shifted to higher field. However, the silk I peak of the ¹⁵N nucleus of the Ala residue is shifted to lower field than the silk II peak, but the α-helix peak is shifted to high field. Thus, the difference in the structure between the silk I and α-helix is reflected in a different manner between the ¹³C and ¹⁵N chemical shifts. The Cα and Cβ chemical shift contour plots for Ala and Ser residues, and the Cα plot for the Gly residue, were prepared from the Protein Data Bank data obtained for 12 proteins and used for discussing the silk I structure quantitatively from the conformation-dependent chemical shifts. The plots reported by Le and Oldfield for ¹⁵N chemical shifts were also used for the purpose. All these chemical shift data support Fossey's model (Ala: ϕ = −80°, φ = 150°, Gly: ϕ = −150°, φ = 80°) and do not support Lotz and Keith's model (Ala: ϕ = −104.6°, φ = 112.2°, Gly: ϕ = 79.8°, φ = 49.7° or Ala: ϕ = −124.5°, φ = 88.2°, Gly: ϕ = −49.8°, φ = −76.1°) as the silk I structure. © 1997 John Wiley & Sons, Inc.     

Far-infrared spectra of polyalanines with alpha-helical and beta-form structures

Far-infrared spectra of poly-L -alanines having the α-helical conformation and the β-form structure were measured. The spectra of glycine–L -alanine copolymer, silk fibroin, and copoly-D ,L -alanines with different D :L compositions were also measured. In addition to the bands so far reported, four bands at 190, 107, 120, and 90 cm^?1were found for the α-helix conformation and the two bands at 442 and 247 cm^?1 were found for the β form. The 442 cm^?1 band consists of the parallel 432 cm^?1 and perpendicular 445 cm^?1 bands. The 247 cm^?1 band is well defined and has strong dichroism parallel to the direction of stretching. These two bands appear also for silk fibroin and glycine–L -alanine copolymer. All the far-infrared bands of copoly-D ,L -alanines can be interpreted as α-helix bands, the three peaks at 580, 478, and 420 cm^?1 being ascribed to the D -residue incorporated into the right-handed α-helix or to the L -residue in the left-handed α-helix.     

The random coil leads to beta transition of copolymers of L-lysine and L-valine: potentiometric titration and circular dichroism studies.

A series of copolymers of L -lysine and L -valine [poly(L -lysine^f L -valine^100-f)] containing 0–13% L -valine have been studied, in 0.10M KF solution, using potentiometric titration and circular dichroism spectroscopy. Incorporation of increasing amounts of valine into the copolymers favors β-sheet formation over α-helix formation at high pH and room temperature. The titrations were analyzed using the method of Zimm and Rice and the partial free energy (ΔG⁰_cβ) for the coil-to-β-sheet transition for valine is estimated at 900 cal/mole at 25°C. From the temperature dependence of the free energy, the partial enthalpy, ΔH⁰_cβ, and entropy, ΔS⁰_cβ, of the transition for valine is estimated to be 854 cal/mole and 6.0 e.u., respectively. The corresponding partial thermodynamic parameters for L -lysine are in agreement with published results. The fraction of β-sheet versus pH has been calculated for poly(L -lysine^86.8 L -valine^13.2) at 25.0°C using the titration data; data obtained from circular dichroism spectroscopy for the same copolymer are in good accord. It is concluded from these results that L -valine is a very strong β-sheet forming amino acid. Furthermore, these results indicate that the Zimm–Rice method is applicable to transitions between the coil and β-sheet states for a polypeptide containing two different residues.     

Conformational mapping of the N-terminal peptide of HIV-1 gp41 in membrane environments using C-enhanced Fourier transform infrared spectroscopy

The N-terminal domain of HIV-1 glycoprotein 41?000 (FP; residues 1-23; AVGIGALFLGFLGAAGSTMGARSCONH₂) participates in fusion processes underlying virus-cell infection. Here, we use physical techniques to study the secondary conformation of synthetic FP in aqueous, structure-promoting, lipid and biomembrane environments. Circular dichroism and conventional, ¹²C-Fourier transform infrared (FTIR) spectroscopy indicated the following α-helical levels for FP in 1-palmitoyl-2-oleoylphosphatidylglycerol (POPG) liposomes∼hexafluoroisopropanol (HFIP)>trifluoroethanol (TFE)>phosphate-buffered saline (PBS). ¹²C-FTIR spectra also showed disordered FP structures in these environments, along with substantial β-structures for FP in TFE or PBS. In further experiments designed to map secondary conformations to specific residues, isotope-enhanced FTIR spectroscopy was performed using a suite of FP peptides labeled with ¹³C-carbonyl at multiple sites. Combining these ¹³C-enhanced FTIR results with molecular simulations indicated the following model for FP in HFIP: α-helix (residues 3-16) and random and β-structures (residues 1-2 and residues 17-23). Additional ¹³C-FTIR analysis indicated a similar conformation for FP in POPG at low peptide loading, except that the α-helix extends over residues 1-16. At low peptide loading in either human erythrocyte ghosts or lipid extracts from ghosts, ¹³C-FTIR spectroscopy showed α-helical conformations for the central core of FP (residues 5-15); on the other hand, at high peptide loading in ghosts or lipid extracts, the central core of FP assumed an antiparallel β-structure. FP at low loading in ghosts probably inserts deeply as an α-helix into the hydrophobic membrane bilayer, while at higher loading FP primarily associates with ghosts as an aqueous-accessible, β-sheet. In future studies, ¹³C-FTIR spectroscopy may yield residue-specific conformations for other membrane-bound proteins or peptides, which have been difficult to analyze with more standard methodologies.     

Solid-state conformation of copolymers of β-benzyl-L-aspartate with L-alanine,L-leucine,L-valine, γ-benzyl-L-glutamate,or ϵ-carbobenzoxy-L-lysine

The solid-state conformation of copolymers of β-benzyl-L -aspartate [L -Asp(OBzl)] with L -leucine (L -Leu), L -alanine (L -Ala), L -valine (L -Val), γ-benzyl-L -glutamate [L -Glu(OBzl)], or ?-carbobenzoxy-L -lysine (Cbz-L -Lys) has been studied by ir spectroscopy and circular dichroism (CD). The ir spectra in the region of the amide I and II bands and in the region of 700–250 cm^?1 have been determined. The results from the ir studies are in good agreement with data obtained by CD experiments. Incorporation of the amino acid residues mentioned above into poly[L -Asp(OBzl)] induces a change from the left-handed into the right-handed α-helix. This conformational change for the poly[L -Asp(OBzl)] copolymers was observed in the following composition ranges: L -Leu, 0–15 mol %; L -Ala, 0–32 mol %; L -Val, 0–8 mol %; L -Glu(OBzl), 3–10 mol %; and Cbz-L -Lys, 0–9 mol %.     

Conformational constraints of amino acid side chains in alpha-helices

The conformational freedom of amino acid side chains is strongly reduced when the side chains occur on an α-helix. A quantitative evaluation of this freedom has been carried out by means of conformational energy computations for all naturally occurring amino acids and for α-aminobutyric acid when they are placed in the middle of a right-handed poly(L-alanine) α-helix. One of the three possible rotameric states for rotation around the C^α ? C^β bond (viz. g⁺) is excluded completely on the helix because of steric hindrance, and the relative populations of the other two rotamers (t and g^?) are altered because of steric interactions and the reduction of hydrogen-bonding possibilities. The computed tendencies of the changes in distributions of rotamers, on going from an ensemble of all backbone conformations to the α-helix, agree with the observed tendencies in proteins. Minimum-energy side-chain conformations in an α-helix have been tabulated for use in conformational energy computations on polypeptides.     

Simulation of the β- to α-sheet transition results in a twisted sheet for antiparallel and an α-nanotube for parallel strands: implications for amyloid formation

α-sheet has been proposed to be the main constituent of the toxic amyloid intermediate. Molecular dynamics simulations on proteins known to be involved in amyloid diseases have demonstrated that β-sheet can, under certain conditions, spontaneously convert to α-sheet via ββ→α(R)α(L) peptide-plane flipping. Using torsion-angle driving to simulate this flip the transition has been investigated for parallel and antiparallel sheets. Concerted and sequential flipping processes were simulated, the former allowing direct calculation of helical parameters. For antiparallel sheet, the strands tend to splay apart during the transition. This can be understood by consideration of the geometry of repeating dipeptide conformations. At the end of the transition antiparallel α-sheet is slightly twisted, comprising gently curving strands. In parallel sheet, the strands maintain identical conformations and stay hydrogen bonded during the transition as they curl up to suggest a hitherto unseen structure, the multi-helix α-nanotube. Intriguingly, the α-nanotube has some of the characteristics of the parallel β-helix, a single-helix structure also implicated in amyloid. Unlike the β-helix, α-nanotube formation could involve identical strands aligning with each other in register as in most amyloids.     

Far-infrared spectra of sequential polypeptides with -helical and polyglycine II structures

The sequential copolymers of glycine and L -alanine, L-valine and L -alanine, L-leucine and L -alanine, and L-phenylalanine and L -alanine and those containing the L-proline residues were synthesized. The infrared spectra in the region from 700 to 200 cm^-1 were measured for these polypeptides with the α-helical conformation or the polyglycine II structure and compared with the spectra of the β-form structures. The results showed that several infrared bands observed in the region from 600 to 200 cm^-1 clearly reflect not only the backbone conformations but also the local conformations of component amino acid residues of polypeptides with the α-helical, β-form and polyglycine II structures.     

Vibrational circular dichroism spectra of three conformationally distinct states and an unordered state of poly(L-lysine) in deuterated aqueous solution

Fourier transform ir vibrational circular dichroism (VCD) spectra in the amide I′ region of poly(L-lysine) in D₂O solutions have confirmed the existence of three distinct conformational states and an unordered conformational state in this homopolypeptide. Characteristic VCD spectra are presented for the right-handed α-helix, the antiparallel β-sheet, an extended helix conformation previously referred to as the so-called “random coil,” and a completely unordered conformation characterized by the absence of any amide I′ VCD. VCD for the antiparallel β-sheet in solution and the unordered chain conformation are presented for the first time. Each of the four different VCD spectra is unique in appearance and lends weight to the view that VCD has the potential to become a sensitive new probe of the secondary structure of proteins in solution.     

Analysis and prediction of the packing of α-helices against a β-sheet in the tertiary structure of globular proteins

The packing of α-helices and β-sheets in six $\text{α}\text{β}$ proteins (e.g. flavodoxin) has been analysed. The results provide the basis for a computer algorithm to predict the tertiary structure of an $\text{α}\text{β}$ protein from its amino acid sequence and actual assignment of secondary structure.The packing of an individual α-helix against a β-sheet generally involves two adjacent ± 4 rows of non-polar residues on the α-helix at the positions i, i + 4, i + 8, i + 1, i + 5, i + 9. The pattern of interacting β-sheet residues results from the twisted nature of the sheet surface and the attendant rotation of the side-chains. At a more detailed level, four of the α-helical residues (i + 1, i + 4, i + 5 and i + 8) form a diamond that surrounds one particular β-sheet residue, generally isoleucine, leucine or valine. In general, the α-helix sits 10 Å above the sheet and lies parallel to the strand direction.The prediction follows a combinational approach. First, a list of possible β-sheet structures (10⁶ to 10¹⁴) is constructed by the generation of all β-sheet topologies and β-strand alignments. This list is reduced by constraints on topology and the location of non-polar residues to mediate the sheet/helix packing, and then rank-ordered on the extent of hydrogen bonding. This algorithm was uniformly applied to 16 $\text{α}\text{β}$ domains in 13 proteins. For every structure, one member of the reduced list was close to the crystal structure; the root-mean-square deviation between equivalenced C^α atoms averaged 5.6 Å for 100 residues. For the $\text{α}\text{β}$ proteins with pure parallel β-sheets, the total number of structures comparable to or better than the native in terms of hydrogen bonds was between 1 and 148. For proteins with mixed β-sheets, the worst case is glyceraldehyde-3-phosphate dehydrogenase, where as many as 3800 structures would have to be sampled. The evolutionary significance of these results as well as the potential use of a combinatorial approach to the protein folding problem are discussed.     

The random coil → β transition of copolymers of L-lysine and L-isoleucine: Potentiometric titration and circular dichroism studies

Copolymers of L -lysine and L -isoleucine [poly(L -Lys^f,L -Val^{1 ? f})] containing 4–15% isoleucine were investigated using potentiometric titration and circular dichroism (CD) spectroscopy. With increasing isoleucine content, β-sheet formation is favored over α-helix formation at high pH and room temperature. The fraction of β-sheet present, as a function of pH, calculated from titrations of poly(L -Lys^85.2,L -Ile^14.8), agreed well with data obtained from CD studies for the same copolymer. Thermodynamic parameters were determined from titrations using the method of Zimm and Rice; the partial free energy (ΔG°_{C → β}) at 25° for the coil-to-β-sheet transition for isoleucine was estimated to be ?515 cal/mol; from the temperature dependence of free energy, the partial entropy (ΔS°_cβ), and the partial free enthalpy (ΔH°_{c → β}) of the coil → β transition for isoleucine is estimated to be 2.6 e.u. and 260 cal/mol, respectively. The partial thermodynamic parameters obtained for lysine are in good agreement with literature values. It is concluded from these studies that isoleucine has a very high potential for a β-sheet formation.     

Comparative analysis of 13C chemical shifts of β-sheet amyloid proteins and outer membrane proteins

Cross-β amyloid fibrils and membrane-bound β-barrels are two important classes of β-sheet proteins. To investigate whether there are systematic differences in the backbone and sidechain conformations of these two families of proteins, here we analyze the ¹³C chemical shifts of 17 amyloid proteins and 7 β-barrel membrane proteins whose high-resolution structures have been determined by NMR. These 24 proteins contain 373 β-sheet residues in amyloid fibrils and 521 β-sheet residues in β-barrel membrane proteins. The ¹³C chemical shifts are shown in 2D ¹³C–¹³C correlation maps, and the amino acid residues are categorized by two criteria: (1) whether they occur in β-strand segments or in loops and turns; (2) whether they are water-exposed or dry, facing other residues or lipids. We also examine the abundance of each amino acid in amyloid proteins and β-barrels and compare the sidechain rotameric populations. The ¹³C chemical shifts indicate that hydrophobic methyl-rich residues and aromatic residues exhibit larger static sidechain conformational disorder in amyloid fibrils than in β-barrels. In comparison, hydroxyl- and amide-containing polar residues have more ordered sidechains and more ordered backbones in amyloid fibrils than in β-barrels. These trends can be explained by steric zipper interactions between β-sheet planes in cross-β fibrils, and by the interactions of β-barrel residues with lipid and water in the membrane. These conformational trends should be useful for structural analysis of amyloid fibrils and β-barrels based principally on NMR chemical shifts.

     

Solution structure of P01, a natural scorpion peptide structurally analogous to scorpion toxins specific for apamin-sensitive potassium channel

The venom of the North African scorpion Androctonus mauretanicus mauretanicus possesses numerous highly active neurotoxins that specifically bind to various ion channels. One of these, P05, has been found to bind specifically to calcium-activated potassium channels and also to compete with apamin, a toxin extracted from bee venom. Besides the highly potent ones, several of these peptides (including that of P01) have been purified and been found to possess only a very weak, although significant, activity in competition with apamin. The amino acid sequence of P01 shows that it is shorter than P05 by two residues. This deletion occurs within an α-helix stretch (residues 5–12). This α-helix has been shown to be involved in the interaction of P05 with its receptor via two arginine residues. These two arginines are absent in the P01 sequence. Furthermore, a proline residue in position 7 of the P01 sequence may act as an α-helix breaker. We have determined the solution structure of P01 by conventional two-dimensional ¹H nuclear magnetic resonance and show that 1) the proline residue does not disturb the α-helix running from residues 5 to 12; 2) the two arginines are topologically replaced by two acidic residues, which explains the drop in activity; 3) the residual binding activity may be due to the histidine residue in position 9; and 4) the overall secondary structure is conserved, i.e., an α-helix running from residues 5 to 12, two antiparallel stretches of β-sheet (residues 15–20 and 23–27) connected by a type I′ β-turn, and three disulfide bridges connecting the α-helix to the β-sheet.     

Vibrational frequencies and modes of alpha-helix

Dichroic properties of the far-infrared absorption bands of the right-handed α-helix of poly-L -alanine were measured. The normal vibration frequencies of this structure were calculated. The assignments of bands were made and the vibrational modes dis-cussed. The frequencies of the α-helix vibrations with various phase differences were calculated. The frequencies of accordionlike vibrations and Young's modulus of the α-helix were estimated. The vibrational frequency for the right-handed α-helices of poly-D -alanine and poly(L -α-amino-n-butyric acid) were calculated, and the results were used for the interpretation of the spectra of copoly-D ,L -alanines and poly(L -α-amino-n-butyric acid). For the latter compound the existence of the rotational isomers in the side chain was strongly suggested. The vibrational modes of the bands characteristic of the α-helix were discussed with regard to the results of the normal coordinate treatment.     

Intramolecular steric effects and hydrogen bonding in regular conformations of polyamino acids

A survey has been made, by using computer methods, of the types of helices which polypeptide chains can form, taking into account steric requirements and intramolecular hydrogen-bonding interactions. The influence on these two requirements, of small variations in the bond angles of the peptide residues, or of small changes in the overall dimensions of the helix (pitch and residues per turn), have been assessed for the special case of the α-helix. Criteria for the formation of acceptable hydrogen bonds have also been applied to helices of other types, viz., the 3, γ^?, ω^?, and π-helices. It was shown that the N? H … O and H … O? C angles in hydrogen bonds are sensitive to changes in either the NC^αC′ bond angle or in the rotational angles about the N? C^α and C^α? C′ bonds. However, the variants of the α-helix observed experimentally in myoglobin can all be constructed without distortion of the hydrogen bonds. For α-helices, the steric and hydrogen bonding requirements are more easily fulfilled with an NC^αC′ bond angle of 111°, rather than 109.5°. The decreased stability observed for the left-handed α-helix relative to the right-handed one for L -amino acids is due essentially only to interactions of the C^β atom of the side chains with atoms in adjacent peptide units in the backbone, and interactions with atoms in adjacent turns of the helical backbone are not significantly different in the two helices. Restrictions in the freedom of rotation of bulky side chains may have significant kinetic effects during the formation of the α-helix from the “random coil” state.     

Secondary structure of peptides: 8. 13C n.m.r. CP/MAS investigation of solid poly(l-leucines) and poly(d-norvalines) prepared from N-carboxyanhydrides

¹³C n.m.r. CP/MAS spectra (50.3 and 75.4 MHz) of solid poly(l-lleucines) and poly(d-norvalines) measured with suitable acquisition parameters allow quantification of the composition of the secondary structure. The optimum acquisition parameters were found by systematic variation of the contact time by means of samples containing 5?0% α-helix structure. The polypeptides were prepared by primary or tertiary amine-initiated polymerizations of the corresponding amino acid NCAs and the average degrees of polymerization ($\text{DP}$) were determined by ¹H n.m.r. endgroup analysis. The mole fraction of α-helices increases with increasing $\text{DP}$; it depends on the nature of the solvent and to a lesser degree on the polymerization temperature. When prepared under identical conditions, poly(d-norvaline) samples contain more β-sheet structure than poly(l-leucine. Reprecipitation increases the α-helix content, demonstrating that a part of the original β-sheet structure is thermodynamically unstable. The presence of oligomers of $\text{DP}$ ?10 is mainly responsible for the thermodynamically stable part of the β-sheet structure. The chain growth mechanism is discussed.