Quenching of excited states of oxytetracycline and phenol by amino acids: An intermolecular proton donor-acceptor interaction

Abstract

Fluorescence quenching of oxytetracycline and phenol by a number of amino acids has been studied in alkaline and neutral medium respectively. Quenching is assumed to involve an intermolecular H-bond interaction between the colliding species in the excited state, involving OH of phenol or oxytetracycline and carboxylate ion of the amino acids. The observed anomalous increase in the quenching of oxytetracycline fluorescence, which appears as positive deviation from Stern-Volmer plots is discussed. A proton transfer reaction is assumed to be involved due to the occurrence of a stronger H-bond in the case of oxytetracycline fluorescence quenching. However, linear Stern-Volmer plots are obtained in the case of phenol fluorescence quenching by all the amino acids under study and bimolecular rate constants obtained are found to be similar to the rate constant for the diffusional process.