Density functional theory (B3LYP and BHandHLYP) and unrestricted second-order Møller–Plesset (MP2) calculations have been performed using 3-21G, 6-31G(d,p), and 6-311 G(2d,2p) basis sets, to study the OH hydrogen abstraction reaction from alanine and glycine. The structures of the different stationary points are discussed. Ring-like structures are found for all the transition states. Reaction profiles are modeled including the formation of prereactive complexes, and very low or negative net energy barriers are obtained depending on the method and on the reacting site. ZPE and thermal corrections to the energy for all the species, and BSSE corrections for B3LYP activation energies are included. A complex mechanism involving the formation of a prereactive complex is proposed, and the rate coefficients for the overall reactions are calculated using classical transition state theory. The predicted values of the rate coefficients are 3.54×108 L⋅mol−1⋅s−1 for glycine and 1.38×109 L⋅mol−1⋅s−1 for alanine. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1138–1153, 2001