Barrier free proton transfer

The remaining part of this article demonstrates the possible involvement of stable nucleobases anions in the formation of DNA strand breaks - the concept which has been overlooked by the radiation research community so far. In Section 21.2 we describe proton transfer (PT) induced by electron attachment to the complexes between nucleobases and various proton donors - a process leading to the strong stabilization of nucleobases anions. We start with the description of methodology used to register the photoelectron spectra of anions. Next the basic characteristics of barrier free proton transfer (BFPT) induced by excess electrons in the complexes of nucleobases are described. Further, we review our results concerning the anionic binary complexes of nucleobases. Then excess electron induced BFPT/PT is characterized for the anions of AT and GC base pairs. Finally, the possible consequences of interactions between DNA and proteins in the context of electron attachment are... 

Basic Characteristics of Barrier Free Proton Transfer Induced by Electron Attachment... 

Excess electron attachment induces barrier-free proton transfer in binary complexes of uracil with H2Se and H2S but not with H20. J Phys Chem B 107 7889-7895. 

Haranczyk M, Rak J, Gutowski M, Radisic D, Stokes ST, Nilles JM, Bowen KH (2004). Effect of hydrogen bonding on barrier-free proton transfer in anionic complexes of uracil with weak acids (U...HCN)- versus (U...H2S). Isr J Chem 44 157-170. 

Here AG is the free energy of activation, w, is the free energy required to bring the reactants into an encounter complex, with reactants and solvation spheres correctly oriented for reaction, Wp is the corresponding term for the separation of the products, 2/4 is the free-energy barrier for proton transfer within the encounter complex if AG is zero, and A = AG - w, - Wp. 

Marcus showed that if within a log k — log X relationship variations in the rate constant solely reflect changes in the equilibrium constant, then log k, or in Marcus s formulation the free energy of activation AG may be expressed in terms of a free energy of the proton-transfer step of the reaction AGr, and an intrinsic energy barrier to proton-transfer for reaction of a substrate and. acid for which AGg = 0 ... 

Figure 13. Exaggerated free energy surface showing two volcanoes with contours for a concerted proton transfer. At all values of the C-O distance there is a barrier to proton transfer. The Marcus theory of proton transfer predicts that a = since the saddle occurs at the value of x where the thermodynamics driving the proton transfer is in balance (QRS). The surface has been drawn for a symmetrical case, but this conclusion still holds for an unsymmetrical case. The strip cartoon shows the mechanism for the hydration dehydration reaction of a ketone.

Fig. 1 Free energy reaction coordinate profiles for hydration and isomerization of the alkene [2] through the simple tertiary carbocation [1+], The rate constants for partitioning of [1 ] to form [l]-OSolv and [3] are limited by solvent reorganization (ks = kteorg) and proton transfer (kp), respectively. For simplicity, the solvent reorganization step is not shown for the conversion of [1+] to [3], but the barrier for this step is smaller than the chemical barrier to deprotonation of [1 ] (kTtOTg > kp).

Since the rate for the tunneling of a proton is strongly dependent on barrier width, it is necessary that the molecular systems to be studied constrain the distance of proton transfer. Also, since the various theoretical models make predictions as to how the rate of proton transfer should vary with a change in free energy for reaction as well as how the rate constant should vary with solvent, it is desirable to study molecular systems where both the driving force for the reaction and the solvent can be varied widely. 

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