Understanding the basic chemistry of the interaction of radicals with DNA bases is imperative when trying to elucidate the potential effects of radiation on DNA. Experimental evidence points to guanine as having the highest affinity of all the DNA bases for undergoing damage. In this study we use Car-Parrinello molecular dynamics (CPMD) in conjunction with traditional quantum chemical methods to research the possible mechanisms for the gas-phase damage of guanine in the presence of an OH radical. Our findings indicate that, for most of the sampled starting configurations, dehydrogenation of guanine (to yield guanine radical and water) is spontaneous and free energetically most favored at all hydrogen sites except at C8. Furthermore, through state-of-the-art analysis, we are able to separately track the nuclear and electronic rearrangement during the dehydrogenation reactions, and find examples of both concerted and stepwise abstraction of the proton and electron. We also find spontaneous hydroxylation at C8 and C4 in accordance with experimental findings.