Reduction of OH vibrational frequencies in amino acids by positron attachment.

Positron affinities have been calculated for five amino acid molecules (asparagine, cysteine, glycine, proline, and serine) with the intramolecular COOH···NH2  hydrogen-bonded motif as a function of the OH distance using two computational methods, namely multicomponent molecular orbital theory and pseudopotential model. Since the elongation of the carboxylic OH bond leads to the formation of highly polarized zwitterionic amino acid with COO- ···NH3 +  structure, the calculated positron affinity significantly increases with the elongation of the OH distance. This indicates that the OH bond strength is significantly weakened by positron attachment. We discuss the reduction of OH vibrational frequencies using effective one-dimensional potential energy curves for neutral and positron-attached amino acid molecules.