Preferred conformation of peptides from C alpha,alpha- symmetrically disubstituted glycines: aromatic residues.

The conformational preference of C(alpha,alpha-diphenylglycine (D-phi-g) and C(alpha,alpha)-dibenzylglycine (Dbz) residues was assessed in selected derivatives and small peptides by conformational energy computations, ir absorption, 1H-nmr, and x-ray diffraction. Conformational energy computations on the two monopeptides strongly support the view that these C(alpha,alpha)-symmetrically disubstituted glycines are conformationally restricted and that their minimum energy conformation falls in the fully extended (C5) region. The results of the theoretical analyses appear to be in agreement with the solution and crystal-state structural propensities of three derivatives and seven di- and tripeptides.