Porphyrin to quinone electron transfer across a depsipeptide which forms an alpha-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 f olding interaction on electron transfer rates through a protein medium. Inf rared analysis in the amide A (NH stretch region) demonstrates that the maj or 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 correlat ed single photon counting fluorescence lifetime measurements on the porphyr in in the presence and absence of the acceptor quinone indicate that electr on transfer is very efficient, k(et) = 5.6 +/- 0.3 x 10(8) s(-1), in this c ompound. 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 expec ted value of similar to 0.03 cm(-1) for electron transfer along the depsipe ptide backbone in the absence of any folding interactions. This result demo nstrates 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.