On the tautomerization process of glycine in aqueous solution

The experimental activation energy for the tautomerization of glycine (zwit terion --> neutral form) has been reported to be 14.6 kcal/mol. It has been generally assumed that this energy barrier is needed for proton transfer t o occur. However, previous theoretical results do not support this interpre tation. In the present work, we examine this question using density functio nal calculations, extended basis sets and a polarizable continuum solvent m odel. Our results suggest that the limiting step for the tautomerization pr ocess corresponds basically to H-atom reorientation in the -COOH group. Thi s could be a general feature in the tautomerization of amino acids. (C) 200 0 Elsevier Science B.V. All rights reserved.