Effect of the N1 residue on the stability of the α-helix for all 20 amino acids

N1 is the first residue in an alpha -helix. We have measured the contribution of all 20 amino acids to the stability of a small helical peptide CH3CO-XAAAAQAAAAQAAGY-NH2 at the N1 position. By substituting every residue into the N1 position, we were able to investigate the stabilizing role of each amino acid in an isolated context. The helix content of each of the 20 peptides was measured by circular dichroism (CD) spectroscopy. The data were analyzed by our modified Lifson-Roig helix-coil theory, which includes the nl parameter, to find free energies for placing a residue into the N1 position. The rank order for free energies is Asp(-), Ala > Glu(-) > Glu(0) > Trp, Leu, Ser > Asp(0), Thr, Gin, Met, lie > Val, Pro > Lys(+), Arg, His(0) > Cys, Gly > Phe > Asn, Tyr, His(+). N1 preferences are clearly distinct from preferences for the preceding N-cap and alpha -helix interior, pK(a) values were measured for Asp, Glu, and His, and protonation-free energies were calculated for Asp and Glu. The dissociation of the Asp proton is less favorable than that of Glu, and this reflects its involvement in a stronger stabilizing interaction at the N terminus. Proline is not energetically favored at the alpha -helix N terminus despite having a high propensity for this position in crystal structures. The data presented are of value both in rationalizing mutations at N1 alpha -helix sites in proteins and in predicting the helix contents of peptides.