To probe the sequence requirements for stabilization of a reverse turn conformation in a short peptide in water solution, the behavior of two series of peptides was investigated by nuclear magnetic resonance (NMR) spectroscopy. The peptides have the general sequences XPGDV and AXGDV, where X is a representative subset of all 20 naturally occurring amino acids. The residues chosen at positions 3 and 4, Gly and Asp, respectively, were shown to give the greatest population of reverse turns in a previous study [Dyson, H. J., Rance, M., Houghten, R. A., Lerner, R. A. & Wright, P. E. (1988) J. Mol. Biol. 201, 161-200]. Within this framework, the identity of the first residue of the turn (X in XPGDV) does not greatly influence the turn population, although a small but significant increase is observed for residues such as Ala which have a preference for backbone conformations in the alpha region of (phi,psi) space. The series AXGDV was initially studied for completeness only, since it was expected that the turn would not be stabilized in such a small linear peptide in the absence of proline. In contrast, it appears that a significant population of type II turn conformations is to be found in peptides in the series AXGDV, although proline remains one of the most favorable residues at position 2. These results indicate that while residues at all positions within the turn can influence the turn population, the presence of Gly-Asp as the third and fourth members of the sequence gives a strong bias towards type II turn formation regardless of the residues at positions 1 and 2. Our results give a final prediction that the sequence with the highest intrinsic propensity for turn formation is APGD.