Re-examination of the Kirkwood–Westheimer theory of electrostatic effects. II. Possible conformations of α,ω-amino-acids in aqueous solutions, as deduced from dissociation constants

The first and second dissociation constants of the ten homologous a, α,ω-amino-acids [Formula: see text] have been determined by a potentiometric method at different temperatures, and hence the thermodynamic parameters ΔG0, ΔH0, and ΔS0 were computed. Calculation by Kirkwood–Westheimer theory of ΔG10 for the first (carboxyl) dissociation, assuming the conformation of the polymethylene chain to be intermediate between the random-coil and the fully extended conformation, gives results in good agreement with experiment for the longer chains. A similar calculation of ΔG20 for the second (ammonium) dissociation gives results in fair agreement with theory for the very long chains (n = 7–10), but in disagreement for shorter chains. A tentative explanation based on electrostatic effects on hydration of the ammonium group is advanced.