Intramolecular aminolysis reactions, catalyzed by a vicinal OH group, were studied computationally as model reactions of ribosome-catalyzed intracomplex aminolysis. The results predicts 2-6-fold higher enthalpic than entropic contribution to the reduction of the activation free energy, caused by the combined proximity effect of the attacking syn-NH2 group and the catalytic syn-OH group. The acceleration predicted by the theory is supported by the observed instantaneous lactamization of ornithinyl adenosine as compared to the much slower lactamization of ornithinyl 2'-deoxyadenosine. This observation is the first unambiguous experimental demonstration of chemical catalysis of the 1,2-diol monoester aminolysis by its vicinal 2-OH groupthe reaction naturally catalyzed by ribosome.