Intra-peptide electron transfer

Intra-peptide electron transfer can also occur 166... 

If the electron enters a Rydberg orbital on one of the protonated amine sites, in addition to undergoing a cascade of radiative or non-radiative relaxation steps to lower-energy Rydberg states, it can subsequently undergo intra-peptide electron transfer to either an SS cr or an OCN n orbital after which disulfide or N-Cr, bond cleavage can occur [3r,3u-3w]. 

Because ETD and ECD experiments are carried out at or near room temperature, the SS and N-Ca bonds are expected to sample only distances close to their equilibrium values Rc. Hence, we focus primarily on the Rydberg states having energies close to that of the SS a -attached or OCN Ji -attached state near Rc when considering intra-peptide electron transfer. In Figure 3, this would be the highest Rydberg state shown. 

In summary, ETD and ECD processes involve two kinds of electron-transfer events. The first occurs in the initial capture of an electron by the positively charged polypeptide. The second involves intra-peptide electron transfer from a Rydberg orbital residing on a positively charged site to an SS or OCN bond site. 

The great difference in slopes for short-bridged peptides between the two systems can be rationalized in terms of differences in the nuclear part of the p descriptor. In the charged metallo systems with n = 1-3 prolines, the rates of intra-molecular electron transfer decrease rapidly by about 3 orders of magnitude. For these systems, the LRET mechanism is largely under the control of the nuclear factor (Pn = 9.5 nm" and Pei = 6.5 nm ) (10). The much smaller distance dependence of fcet in the Trp -Tyr system for peptides with n = 0-2, associated largely with the TS pathway and weak nuclear control, is consistent with the neutral character of the Tip and Tyr radicals involved in LRET and, thus, the low value of Xout (3). 

---