C-peptide, which contains the 13 NH2-terminal residues of RNase A, shows partial helix formation in water at low temperature (1 degree C, pH 5, 0.1 M NaCl), as judged by CD spectra; the helix is formed intramolecularly [Brown, J. E. & Klee, W. A. (1971) Biochemistry 10, 470-476]. We find that helix stability depends strongly on pH: both a protonated histidine (residue 12) and a deprotonated glutamate (residue 9 or 2 or both) are required for optimal stability. This information, together with model building, suggests that the salt bridge Glu-9- ... His-12+ stabilizes the helix. Formation of the helix is enthalpy driven [van't Hoff delta H, - 16Kcal/mol (1 cal = 4.18 J)] and the helix is not observed above 30 degrees C. Proton NMR data indicate that several side chains adopt specific conformations as the helix is formed. These results have two implications for the mechanism of protein folding. First, they indicate that short alpha-helices, stabilized by specific side-chain interactions within the helix, can be stable enough in water to function as folding intermediates. Second, they suggest that similar experiments with peptides of controlled amino acid sequence could be used to catalogue the intrahelix interactions that stabilize or destabilize alpha-helices in aqueous solution. These data might provide the code relating amino acid sequence to the locations of alpha-helices in proteins.