2: Proton Transfer vs. Electron Transfer

2 Proton Transfer vs. Electron Transfer

The obvious and critical distinction between electron and proton transfer is the almost 2000-fold difference in the mass of the particle. The tiny mass of the electron allows transfer by quantum mechanical tunneling to proceed with modest driving forces at a biologically meaningful rate over distances of up to 15 , and non-physiologically beyond 25 . ^[15], ^[25] At these and even much shorter distances, the donor-acceptor interaction is very weak and the electron transfer is clearly in the non-adiabatic regime.

Tunneling probabilities (rates) are proportional to (mass) ^?1/2 and, within the same functional time constraint of 0.1 ms, proton tunneling is limited to no more than 1 . (Figure 1). The necessary close approach of the heavy atom systems will result in sufficient perturbation of the wave functions that adiabatic processes will usually dominate. Indeed, even if the tunneling rate at some average non-bonding distance is adequate to support function, it is evident that it will be greatly modulated by thermal fluctuations of the distance between neighboring atoms-a fluctuation of 0.1 , around 1 , changes the tunneling rate by 1-2 orders of magnitude. ^[26] Thus, the elementary (pairwise) transfer is usually controlled by the dynamics of the system, regardless of the nature of the heavy atom framework.

Figure 1: Proton transfer distances in non-bonded and hydrogen-bonded pairs (schematic charges are omitted for generality) Left Proton (or hydrogen atom) transfer between non-bonded donor (C-H) and acceptor (C) Right Proton transfer...