The a-helix-stabilizing effect of different amino acid residues at the helical termini of short peptides in aqueous solution has been determined. Several dodecapeptides containing alanine, asparagine, aspartate, glutamine, glutamate, and serine at the amino terminus and arginine, lysine, and alanine at the carboxyl terminus were synthesized, and the a-helical content of each peptide was measured by using circular dichroism spectroscopy. The trend in a-helix-inducing ability of these amino acids was found to be as follows: In an a-helix, NH donors of the first four residues and CO acceptors of the last four residues lack intrahelical hydrogenbond partners. Presta and Rose (19) hypothesized that a necessary condition for helix formation is the presence of residues flanking the helix termini that have side chains to supply hydrogen-bond partners for unpaired main-chain NH and CO groups. Richardson and Richardson (20), by surveying several proteins of known structures, showed that at the amino termini of helices, there is a preponderance of amino acids with side chains that could hydrogen-bond with the free NH groups of the helix. Gierasch and coworkers (21) have shown that side-chain-backbone hydrogen bonding, as proposed by Presta and Rose, may also stabilize helix formation in peptides. Site-directed mutagenesis studies involving the amino-terminal residues of a-helices in native proteins have shown the significance of such hydrogen bonding in protein stability (22,23).We decided to test the Presta and Rose hypothesis in a peptide designed to fold into an isolated a-helix (24). The rationale for the design is as follows. (i) We chose a short a-helical segment (12 residues) resembling most a-helices found in native proteins. Further, we expected the capping effects to be more prominent in a short peptide. (ii) We introduced a proline and a glycine, amino acids traditionally known to be helix breakers, at positions 2 and 11, respectively, to demarcate the helical ends (25). We also hoped that the presence of the helix breakers might enable the side chains of the end residues to turn around and hydrogen-bond with the free NH and CO groups at the helical termini. (iii) The presence of a glutamate at position 4 and a lysine at position 7 increases solubility of the peptide and may also stabilize the helix by salt bridge formation (14). (iv) The rest of the amino acids were chosen to be alanine, which has a high tendency to form a-helices (26). (v) The respective positively and negatively charged side chains of arginine at the carboxyl end and aspartate at the amino end are expected to interact favorably with the helical dipole and stabilize the a-helix (13). (vi) The negatively charged carboxyl group of Arg-12 is amidated to eliminate repulsive interactions -between itself and the negative pole of the helical dipole. (vii) The a amino group of residue 1 was not acylated to prevent such an amide carbonyl from hydrogen bonding with the free NH groups at the amino terminus. The sequences of our model peptide an...