The reaction of hydroxyl radical ((center dot)OH) with guanine was investigated under restricted pH condition (pH 4.6) using pulse radiolysis technique. The time-resolved optical transient absorption spectra showed two peaks centered at 300 and 330 nm at 4 mu s after the pulse which exhibited different reactivity toward molecular oxygen (O(2)). The peak at 300 nm was found to be relatively more stable than the peak at 330 nm. The peak corresponding to 330 nm decayed within 20 mu s having a first order rate constant 4-7 x 10(4) s(-1) and was pH dependent. On longer time scale, the species decayed by a bimolecular process. The presence of O(2) did not affect its decay rate constant. The center dot OH reacts with guanine at pH 4.6 with a diffusion-controlled second order rate constant of >= 1 x 10(10) dm(3) s(-1). The reaction of Br(2)(center dot-) O(2)(center dot-) and 2-hydroxy-2-propyl radical with guanine was also investigated to differentiate among the one-electron oxidized, one-electron reduced species of guanine and the guanine OH adducts formed in the reaction of (center dot)OH at pH 4.6. On the basis of the spectral characteristics and reactivity toward 02, two guanine-OH adduct species were identified (i) the C4-OH adduct species absorbing at 330 nm which has not been reported so far and (ii) the C8-OH adduct species absorbing at 300 rim in agreement with the known literature absorption features. Quantum chemical calculations using BHandHLYP with 6-31+G(d,p) basis set and excited state calculations using TDDFT for all possible transients complement the assignment of the observed spectral peak at 330 nm to the C4-OH adduct of guanine. Furthermore, steady state radiolysis revealed the formation of 8-hydroxy-guanine whose precursor is known to be the C8-OH adduct species.