Theoretical calculations were performed to study the effects of hydrogen bonding to various amines on the oxidation of phenol to phenoxyl radical. It was found that with ammonia as the hydrogen bond acceptor the phenol oxidation process was a barrierless proton-coupled electron transfer and the shift of the adiabatic phenol oxidation potential by ammonia in the gas phase was as large as about 1 eV. For other amines, it was found that depending on the basicity of the amine, the effects of hydrogen bonding to different amines on the phenol oxidation varied. For those amines that had a proton affinity larger than ca. 204 kcal/mol, the oxidation of the phenol-amine complexes caused a proton transfer and the proton-transferred structure was the only minimum found on the potential surface after oxidation. When the proton affinity of the amine was located in the range of ca. 190-197 kcal/mol, both the proton-transferred and nonproton-transferred structures were found to be minima for the oxidized complex. However, when the proton affinity of the amine was smaller than ca. 189 kcal/mol, no proton transfer occurred in the oxidation. The shift of the adiabatic oxidation potential was found to be roughly in linear correlation with the proton affinity of the amine. For substituted aminoacetylenes, the shift of the adiabatic oxidation potential was also found to be in linear correlation with the Hammett sigma(P) substituent constants. Finally, it was found that the phenol - imidazole-formate complex was not a good model for the tyrosine oxidation in photosystem 11, because this complex had a too low oxidation potential. In fact, because of the strong electron donating effects of formate, in addition to phenol the imidazole moiety in the complex could also be oxidized, which was not observed in the enzymatic systems. Therefore, the Glu189 residue in photosystem II was proposed to be protonated under the physiological condition.