PROTON AFFINITY OF AMINO-ACIDS - THEIR INTERPRETATION WITH DENSITY-FUNCTIONAL THEORY-BASED DESCRIPTORS

The calculation of group electronegativity and hardness for amino acid ''functional groups,'' considered as a biradical taken out of their p rotein environment, is performed for both the alpha-helix and beta-she et geometry of these amino acids. Group electronegativity and hardness are then used to interpret the experimental gas-phase proton affinity sequence of the amino acids. Group hardness was found to play the dom inant role, whereas group electronegativity only had a minor influence on the sequence, thereby stressing the importance of the charged form in the acid-base equilibrium. An explanation for the deviations, seen for some of the amino acids, from the correlation between these group properties and the proton affinity was sought. (C) 1996 John Wiley & Sons, Inc.