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SynopsisThe set of synthetic polytripeptides and polyhexapeptides which can adopt a triplehelical form constitute a good model system for investigating collagen structure. Here we consider previous and new infrared spectroscopic studies on collagen and present the infrared spectra of a number of polymers with collagen-like features.The amide A band position for all triple-helical polypeptides is higher than that observed for most proteins and polypeptides, and this high frequency appears to be related to the degree of supercoiling of the triple helix. It is possible that with increased supercoiling of the three chains the angles between the groups involved in the intramolecular hydrogen bonds become less favorable, or these bonds may become unusually long.The frequency of the amide I band varies considerably for triple-helical polypeptides with different amino acid sequences, and often minor bands are observed. This finding contrasts with the observations for polypeptides in a pleated sheet or a-helical form, where the same amide I frequency is observed regardless of the amino acid composition.An explanation for this variation is proposed in terms of the hydrogen bonding propert,ies of imino acids.Significant spectral changes in the amide I region are observed on hydration in the spectra of some triple-helical polypept.ides, but corresponding changes have not been found in the collagens examined.

Gramicidin A is thought to form a dimer channel through which alkali cations and hydrogen ions can passively permeate lipid bilayer membranes. The present work describes four conformational species which have been isolated from a single organic solvent system and individually characterized by circular dichroism, proton nuclear magacetate, and dioxane were Spectrograde (Matheson Coleman and Bell). Ethanol was "Rossville Gold Shield" (Commercial Solvents Corporation); dimethyl sulfoxide was "spectroanalyzed" (Fisher); and the chloroform was analytical grade (Baker).Thin-Layer Chromatography. All silica thin-layer chromatography (tic) plates were from Quantum Industries. The plates were developed in dioxane-water (100:1) (v/v) and visualized with the tryptophan reagent, A(A-dimethylaminobenzaldehyde (Greenstein and Winitz, 1961). Absolute spot mobilities relative to the solvent front were approximately 0.4 for species 4, 0.3 for 3, 0.15 for 2, and 0.04

The polymerization of 7-benzyl-N-carboxy-L-glutamate anhydride with initiators such as sodium hydroxide, sodium methoxide, sodium borohydride and triethylamine can lead to polypeptides of molecular weight over 500,000 (degree of polymerization > 2000). In the cases of the first three initiators the molecular weight is related to the anhydride-initiator ratio (A/I) but is always higher than calculated for a given A/I. With triethylamine initiation the molecular weight is essentially independent of the amount of initator over a twenty-fold range of A/I. Data are presented that show that increasing the anhydride concentration, or raising the temperature of polymerization, yields polymers of much lower molecular weight. The solvent in which the polymerization takes place also affects the molecular weight. Evidence is presented that the N-carboxy anhydride ring opens in two different ways in the presence of sodium methoxide. This may be an explanation of the obtaining of molecular weights higher than calculated from the A/I's.Since the original synthesis of ce-amino acid-Ncarboxy anhydrides3 these compounds have been used extensively for the preparation of poly-'amino acids.4 Water, alcohols and amines can serve as initiators or catalysts for the polymerization of N-carboxy-a-amino acid anhydrides, and most previous work has been performed using such materials as initiators. There are only a few studies on the molecular weights of the polypeptides obtained and few data, except the work of Stahmann,6 on the effect of varying initiator concentration upon the molecular weight. Furthermore, there is no evidence that very high molecular weight polypeptides (molecular weight >100,000) could be prepared by the aforementioned means.

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