Trimethyl-p-benzoquinone Provides Excellent Structural, Spectroscopic, and Thermochemical Models for Plastoquinone-1 and Its Radical Anion

Trimethyl-p-benzoquinone (TMQ) has been proposed to furnish an accurate thermochemical model for plastoquinones, key electron acceptors in oxygenic photosynthetic electron transfer. Free energy perturbation/molecular dynamics simulations combined with hybrid Hartree−Fock/density functional (HF/DF) calculations confirm that TMQ and plastoquinone-1 have approximately equal aqueous one-electron reduction potentials, within the accuracy of the calculations. HF/DF calculations using the B3LYP/6-31G(d) method also show that TMQ and its radical anion have (1) structures almost identical to those of PQa model for plastoquinone-1 without the isoprenoid chain's methyl groupsand its radical anion, respectively, have (2) spin densities for TMQ•- and PQ•- which differ by 0.01 electrons at most, and have (3) key CO and CC stretching frequencies for the TMQ/PQ and TMQ•-/PQ•- pairs differing by only 1−8 cm-1. Thus, TMQ and TMQ•- are excellent models for the structures, spin densities, and vibrational frequencies of plastoquinones and their radical anions, respectively.