O Hn

C (Figure B.2.1). It is assumed the amide unit "always" prefers a planar arrangement with the NH and CO orientated in an s-trans fashion.

Plotting the dihedral angle C against F produces a diagram in which more or less preferred conformations can be found. Regions which represent favored conformations of the amino acids in the diagram are shown in dark whereas less favored regions are shown in gray and unfavored regions in white (Figure B.2.2). Two major regions of favored conformation are apparent; these represent the areas which are observed in a ¿S-sheet (C = 125°, F = —120°, approx.) or a right handed a-helical structure (C = -50°, F = —90°, approx.) [2].

Fig. B.2.2.

In contrast with the right-handed a-helical structure of proteins a similar left-handed folding is theoretically possible in which the dihedral angles would be approximately C = 50° and F = 90°. Because of the substituent and the S configuration of ''naturally dominating'' amino acids, however, the left-handed form is less favored. Steric interaction occurs between the substituent and the peptide backbone if a left-handed helical structure is adopted.

Only the achiral glycine can readily adopt conformations different from those in the favored regions; proline, because of, the ring system, is highly constrained in its conformation.

References

SAsiskhArAn, Adv. Protein Chem. 1968, 23, 283.

2 A. L. Morris, M. W. MAcArthur, E. G. Hutchinson, J. M. Thornton, Proteins, 1992, 12, 345.

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