The structures of 19 alpha-helical alanine-based peptides, 13 amino acids in length, have been fully optimized using density functional theory and analyzed by means of the quantum theory of atoms in molecules. Two types of N-H...O bonds and one type of C-H...O bond have been identified. The value of the electron density at hydrogen bond critical points corresponding to N-H...O interactions is higher than that of C-H...O interactions. The effect of amino acid substitution at the central position of the peptide on the hydrogen bond network of the alpha-helix has been assessed. The strength of the hydrogen bond network, measured as the summation of the electron density over the hydrogen bond critical points, may be used to explain experimental relative helix propensities of amino acids in cases where solvation and entropic effects cannot.