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Dynamical factors controlling hopping speed and efficiency in two modified azurin constructs that include a rhenium(I) sensitizer, Re(His)(CO)3(dmp)+, one or two tryptophans (W1, W2) and CuI center were studied by femtosecond time-resolved spectroscopy. Experimental kinetics investigations showed that the two closely spaced (3-4 Å) intervening tryptophans dramatically accelerated long-range electron transfer (ET) from CuI to the photoexcited sensitizer. In order to interpret electron-transfer processes connected with evolution of electronic states (3CT, 3CS1 and 3CS2) the complex systems were investigated by QM/MM molecular dynamics (MD) simulations in solution. The quantum part (QM) was Re(His)(CO)3(dmp) and part of protein chain up to W2, with the rest of the system treated by MM. The QM calculations employed DFT techniques with the PBE0 functiona and D3 dispersion correction.
It was found that TDDFT QM/MM/MD trajectories of low-lying triplet excited states of ReI(His)(CO)3(dmp)+–W1(–W2) exhibited crossings between sensitizer-localized (Re) and charge-transfer [ReI(His)(CO)3(dmp•–)/(W1•+ or W2•+)] (CS1 or CS2) states. Avoided crossing between the lowest Re and CS states on TDDFT trajectories demonstrated that dynamical fluctuations of solvated Re-tryptophan-azurins create situations where oxidation of the tryptophan by an electronically excited Re complex is energetically feasible, leading to a localized CS state. Calculating electronic coupling Hab between their diabatic approximates in the crossing region enabled us to assess whether the studied ET approaches the adiabatic or nonadiabatic limit.
