SynopsisThe general conformations of a system of three linked peptide units are studied, and it is found that there are three types of conformations which contain NH. . .O hydrogen bonding between the first and the third units. One of them is part of a 3lrhelix, while the other two are noiihelical. The two iionhelical conformations are very similar, and in both the cases the peptide chain turns around, reversing the direction of progress. Such a conformation can therefore occur in the region where a polypeptide chain folds back on itself, as in the cross-p structure. The method of representing these interesting tripeptide conformations in a (+,+) map is described. Examples of such hydrogen-bonded, nonhelical conformations which occur in peptides and proteins are discussed-e.g., in cyclohexaglycyl, an open tetrapeptide Gly-L-Pro-L-Leu-Gly, and in parts of the lysozyme chain.

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Figure 10. Comparison of the measured [tj] for schizophyllan in 0.01 N NaOH with Yamakawa and Yoshizaki's theoretical values for wormlike cylinders calculated for q = 150,200,300 nm, and » with ML and d fixed at 2190 nm"1 and 2.2 nm, respectively. schizophyllan in water,4 the model triple helix of schizophyllan,3 and triple helices of other polysaccharides17"19 in the crystalline state. Our pitches from light scattering and viscosity in 0.01 N NaOH agree not only with Yanaki et al.'s values4 from sedimentation velocity and viscosity but also with the value estimated from the molecular model for schizophyllan. They are also close to the values for a /3-1,3-d-xylan,17 lentinan (/3-1,3-D-glucan),18 and curdlan (/3-1,3-D-glucan),19 leading to the conclusion that the triple-helical structure of schizophyllan in 0.01 N NaOH and water is very similar to that of these other polysaccharides in the crystalline state.