Unveiling electrostatic portraits of quinones in reduction and protonation states

Quinones are known to perform diverse functions in a variety of biological and chemical processes as well as molecular electronics owing to their redox and protonation properties. Electrostatics chiefly governs intermolecular interaction behaviour of quinone states in such processes. The electronic distribution of a prototypical quinone, viz.,p-benzoquinone, with its reduction and protonation states (BQS) is explored by molecular electrostatic potential (MESP) mapping using density functional theory. The reorganization of electronic distribution of BQS and their interaction with electrophiles are assessed for understanding the movement of ubiquinone in bacterial photosynthetic reaction centre, by calculating their binding energy with a model electrophile viz., lithium cation (${\text{Li}}^{+}$) at B3LYP/6-311+G(d,p) level of theory. The changes in the values of the MESP minima of BQS states alter their interacting behaviour towards ${\text{Li}}^{+}$. A good correlation is found between the value of MESP minimum of BQS and the ${\text{Li}}^{+}$ binding strength at the respective site. To acquire more realistic picture of the proton transfer process to quinone with respect to its reduction state in the photosynthetic reaction center, interaction of BQS with model protonated motifs of serine, histidine as well as ${\text{NH}}_4^{+}$ is explored. Further, the electronic conjugation of the reduced states of 9,10-anthraquinone is probed through MESP for understanding the switching nature of their electronic conductivity.

Graphical Abstract

Quinones perform important function of proton transfer in photosynthesis and also act as a switch in molecular electronics. This work explores the electronic distribution of reduction and protonation states of p-benzoquinone using molecular electrostatic potential, for understanding the mechanisms of quinone activity in the photosynthesis and its switching nature in electronic conductivity. .