Reactivity of acyl and aquo groups in metal—quinone aggregates of photosystem-II

• Published in: Thermochimica acta 1991-12, Vol.191 (2), p.255-264
• Main Authors: Rane, S.Y., Salvekar, J.P., Dass, N.V.R., Kaduskar, P.S., Bakare, P.P.
• Format: Article
• Language: English
• Subjects: Chemistry; Exact sciences and technology; Inorganic chemistry and origins of life; Kinetics and mechanism of reactions
• ISSN: 0040-6031; 1872-762X
• DOI: 10.1016/0040-6031(91)87217-K

Because thermolytic products are analogous to photolytic products, non-isothermal TGA studies of metal-quinones involved in Photosystem-II have been undertaken. The compositions of the compounds C-1-C3 are ML 2 · xH 2O. For C-1 and C-2, M = Co(II), x = 2, and L = Lw in the former and L = O-acylLwOx in the latter; for C-3, M = Ni(II), x = 4 and L = LwOx. The weight loss versus temperature data are treated using the Coats and Redfem relationship. In C-1, the water molecules are ligand participants in the coordination, while in C-2 and C-3 the water molecules are adsorbed. The kinetic data of the thermolysis throw light on the energetics of the ▪ and ▪ ligations in quinone chelates, together with their acyl and aquo functionalities. The E a values of Lw in C-1 and of the acyl and LwOx functional moieties in C-2, are comparable: 55, 58 and 57 kJ mol −1 respectively. The aquation effect dominates in C-1, as the E a of H 2O is ≈ 82 kJ mol −1. The adsorbed water molecules in C-3 have an E a ≈ 7.5 kJ mol −1, while the outer-sphere water molecules participate in the hydrolysis of the O-acyl functional group during the pyrolytic reaction of C-2, which requires an energy ≈ 81.6 kJ mol −1.