Charge Transfer in Peptides

Summary

One-electron oxidation of TyrOH-TrpH or TrpH-TyrOH in aqueous solutions by N^·₃ radicals occurs predominantly at the tryptophyl residue. The corresponding indolyl radicals (absorbing at 510 nm) are subsequently transformed into phenoxyl radicals (absorbing at 390/405 nm):

The first-order radical transformation rates are independent of the (initial) concentration of N^·₃ or peptide and unaffected by urea (as a modifier of hydrogen bond structures). Intermolecular conversion of indolyl into phenoxyl radicals, e.g. by reaction of GlyH-Trp^· with TyrOH-GlyH, is very slow and inefficient. It is concluded that reactions (5) and (7) occur by intramolecular charge transfer across the peptide bond.