Solvent Effects on Donor-Acceptor Couplings in Peptides. A Combined QM and MD Study

We present a combined Quantum Chemical/Molecular Dynamics study on electronic coupling between tryptophan-based donor and acceptor in oligopeptides of variable length. Molecular dynamics was performed on Trp-(Pro)n-Trp (n = 1 to 6) molecules in gas phase and aqueous solvent and the electronic coupling matrix element was computed for thermal hole transfer applying semiempirical INDO/S together with the generalized Mulliken-Hush approach. For comparison, we also computed coupling values of 40 000 snapshots applying ab initio Hartree-Fock, showing good agreement with the INDO/S results. We demonstrate that the coupling values strongly fluctuate throughout the molecular dynamic trajectory and the mechanism of electron transfer is affected by the presence of solvent through restriction of the conformational space. Gas-phase calculations show gated electron transfer dominated by direct through-space coupling due to strong conformational changes bringing donor and acceptor in close vicinity. Solvent calculations establish a nongated mechanism dominated by bridge-mediated coupling. In agreement with experimental data, our results point to a donor-acceptor distance of ∼20 Å as a possible point for transition from superexchange to hopping electron transfer mechanism.