Evidence of temperature dependent activation barriers for near-threshold aqueous photoionization of 2′-deoxyguanosine and tryptophan

Alkaline 2′-deoxyguanosine undergoes one- and two-photon laser photoionization at 266 nm. Similar to tryptophan and indole, the one-photon quantum yield increases five-fold between 296 and 362 K. Earlier Arrhenius analyses of tryptophan and indole photoionization rate constants, yielded unreasonably large pre-exponential factors (1013–1018 s−1). An analysis employing pre-exponential factors for diffusion controlled reactions (1010–1011 s−1) provides evidence of activation barriers that decrease with increasing temperature over a range where water structure undergoes significant changes. In the context of Marcus theory, this analysis suggests that curvatures of the electron-transfer free energy surfaces decrease as temperature increases.