The effect of adding explicit water molecules to the neutral (N) and zwitterionic (Z) forms of the glycyl radical has been examined. The results show that a minimum of three water molecules is required to stabilize the Z radical as a local minimum, with an energy gap of 123 kJ mol(-1) between the N and Z forms at this point, in favor of the N form. Increasing the number of water molecules to similar to 20 leads to a converged Z-N energy difference of similar to 50 kJ mol(-1 )still in favor of the N form, even though the radical is not considered fully solvated from a structural point of view. Thus, energetic convergence is determined mainly by solvation of the polar functional groups, and a complete coverage of the entire molecule is not necessary. Because aqueous closed-shell glycine exists as a zwitterion while aqueous glycyl radical prefers the neutral form, the conversion between the two necessitates a change along the hydrogen-abstraction reaction pathway. In this regard, the transition structure for alpha-hydrogen abstraction by the center dot OH radical has greater resemblance to glycine than to the glycyl radical. Overall, the barrier for hydrogen abstraction from Z glycine is larger than that from the N isomer, and this might act to provide some protection against radical damage to the free amino acid in the (aqueous) biological environment.