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 alpha-helical turn in CH2Cl2. A ROESY NMR experiment corroborates the presence of the alpha-helical turn through observation of a d(alpha N) (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, k(et) = 5.6 +/- 0.3 x 10(8) s(-1), in this compound. Evaluation of the electronic coupling matrix element, H-ab, gives a value of 3.2 +/- 0.8 cm(-1), approximately 100-fold larger than the expected value of similar to 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. (C) 2000 Elsevier Science S.A. All rights reserved.