Theoretical determination of absolute free energy of reduction of plastoquinone-9 in photosystem II and of plastoquinone- n in DMF

Abstract

We employed static continuum electrostatics and multi-conformation continuum electrostatics (MCCE) methods to determine the reduction potential ( E red 0 ) of PQ-9 in a section of Photosystem II (PSII). Both methods relied on the finite difference Poisson–Boltzmann (FDPB) solution. The static method brings out a E red 0 value (0.01 V) that is close to the experimental one (0.05 V), thereby demonstrating that the surrounding environment critically decides the net free energy change. The E red 0 value obtained from MCCE (0.04 V) is even closer to the observed value, thereby indicating the importance of protein side-chain and proton motions in the electron transfer process. Furthermore, density functional theory-dielectric polarisable continuum model (DFT-DPCM) was employed to calculate the absolute free energy of reduction of plastoquinone- n (PQ- n, where n is the number of isoprenoid units) in N, N dimethyl formamide (DMF) solvent. The DFT-DPCM method produced reduction potential values of −0.59 and −0.65 V for PQ-1 and PQ-9, respectively. These are more or less in agreement with the experimentally reported values of −0.64 and −0.62 V, respectively.