Investigation of the electrochemical behavior of catechol and 4-methylcatechol in the presence of methyl mercapto thiadiazol as a nucleophile: application to electrochemical synthesis

The Peculiar H-Bonding Network of 4-Methylcatechol: A Coupled Diffraction and In Silico Study

The crystal structure of 4-methylcatechol (4MEC) has, to date, never been solved, despite its very simple chemical formula C7O2H8 and the many possible applications envisaged for this molecule. In this work, this gap is filled and the structure of 4MEC is obtained by combining X-ray powder diffraction and first principle calculations to carefully locate hydrogen atoms. Two molecules are present in the asymmetric unit. Hirshfeld analysis confirmed the reliability of the solved structure, since the two molecules show rather different environments and H-bond interactions of different directionality and strength. The packing is characterised by a peculiar hydrogen bond network with hydroxyl nests formed by two adjacent octagonal frameworks. It is noteworthy that the observed short contacts suggest strong inter-molecular interactions, further confirmed by strong inter-crystalline aggregation observed by microscopic images, indicating the growth, in many crystallization attempts, of single aggregates taller than half a centimetre and, often, with spherical shapes. These peculiarities are induced by the presence of methyl group in 4MEC, since the parent compound catechol, despite its chemical similarity, shows a standard layered packing alternating hydrophobic and polar layers. Finally, the complexity and peculiarity of the packing and crystal growth features explain why a single crystal could not be obtained for a standard structural analysis.

Quinone-induced protein modifications: Kinetic preference for reaction of 1,2-benzoquinones with thiol groups in proteins

Oxidation of polyphenols to quinones serves as an antioxidative mechanism, but the resulting quinones may induce damage to proteins as they react through a Michael addition with nucleophilic groups, such as thiols and amines to give protein adducts. In this study, rate constants for the reaction of 4-methylbenzoquinone (4MBQ) with proteins, thiol and amine compounds were determined under pseudo first-order conditions by UV-vis stopped-flow spectrophotometry. The chemical structures of the adducts were identified by LC-ESI-MS/MS. Proteins with free thiols were rapidly modified by 4MBQ with apparent second order rate constants, k2 of (3.1±0.2)×10(4)M(-1)s(-1) for bovine serum albumin (BSA) and (4.8±0.2)×10(3)M(-1)s(-1) for human serum albumin at pH 7.0. These values are at least 12-fold greater than that for α-lactalbumin (4.0±0.2)×10(2)M(-1)s(-1), which does not contain any free thiols. Reaction of Cys-34 of BSA with N-ethylmaleimide reduced the thiol concentration by ~59%, which resulted in a decrease in k2 by a similar percentage, consistent with rapid adduction at Cys-34. Reaction of 4MBQ with amines (Gly, Nα-acetyl-l-Lys, Nε-acetyl-l-Lys and l-Lys) and the guanidine group of Nα-acetyl-l-Arg was at least 5×10(5) slower than with low-molecular-mass thiols (l-Cys, Nα-acetyl-l-Cys, glutathione). The thiol-quinone interactions formed colorless thiol-phenol products via an intermediate adduct, while the amine-quinone interactions generated colored amine-quinone products that require oxygen involvement. These data provide strong evidence for rapid modification of protein thiols by quinone species which may be of considerable significance for biological and food systems.