Porphyrin to quinone electron transfer across a depsipeptide which forms an α-helical turn

A porphyrin linked to a quinone via a three residue depsipeptide, l-proline-l-lactate-l-lactate has been used to investigate the effect of a protein folding interaction on electron transfer rates through a protein medium. Infrared analysis in the amide A (NH stretch region) demonstrates that the major conformation of the compound is an α-helical turn in CH2Cl2. A ROESY NMR experiment corroborates the presence of the α-helical turn through observation of a dαN (i,i+3) through-space contact. Time correlated single photon counting fluorescence lifetime measurements on the porphyrin in the presence and absence of the acceptor quinone indicate that electron transfer is very efficient, ket=5.6±0.3×108 s−1, in this compound. Evaluation of the electronic coupling matrix element, Hab, gives a value of 3.2±0.8 cm−1, approximately 100-fold larger than the expected value of ∼0.03 cm−1 for electron transfer along the depsipeptide backbone in the absence of any folding interactions. This result demonstrates that long-range non-covalent protein folding interactions can have profound effects on rates of electron transfer.