Ab initio normal mode calculations have been performed on isolated alanine di- through octa-(i.e., blocked) peptides in uniform alpha(R), beta, and polyproline II conformations to determine how the (CD alpha)-D-alpha stretch mode, which has been proposed as a possible determinant of the phi,psi conformation at the C-alpha atom (Mirkin, N. G.; Krimm, S. J. Phys. Chem. A 2004, 108, 10923), depends on conformation and sequence length. This set of frequencies, including results on some kinked structures, demonstrates that such a discrimination is likely to be possible through experimental observations of peptides synthesized with successive deuteration at the H-alpha sites, on the basis of at least three properties: the values of the frequency at the first residue, the pattern of successive frequency differences, and the frequency differences between the first and last residues.