Approximate ab initio molecular orbital calculations are presented for the di-, tri-, tetra-, and pentapeptides of glycine. Six key conformations of polyglycine were considered: fully extended, parallel chain-pleated sheet, antiparallel chain-pleated sheet, α helix, 3-10 helix, and polyglycine II. Beyond the pentapeptide level the α-helix and 3-10-helix conformations were predicted to be the most stable. The stability of the latter pair of conformations is attributed to hydrogen bonding, an effect which is confirmed by a detailed population analysis of the wave function. Additional evidence for hydrogen bond formation is provided by the localized molecular orbitals which are reported for several conformations studied here. An analysis of the eigenvalue spectrum of these molecules reveals a band structure that is characteristic of periodic systems. All calculations were performed using the partial retention of diatomic differential overlap (PRDDO) approximation. Comparisons of the PRDDO results with experiment and with other molecular orbital techniques are also made.

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