It is well documented that helices in proteins can decrease the pK(a) of residues located at the N-terminus, but the real nature of this perturbation remains unclear. In the present work, the origin of the effect of 3(10)- and a-polyalanine helices on the pK(a) of an N-terminal cysteine residue is examined in gas phase as well as in aqueous solution by means of density functional theory. In a systematic study of the helix dipole, the proton affinity (PA), and the pK, of the N-terminal cysteine, in relation to both the helix length and the strength of the hydrogen bonds between the helix backbone amides and the S gamma of the N-terminal cysteine, a direct relation between the terminal hydrogen bonds and the pK(a) perturbation is revealed.