Effect of Phenyl Derivatization on the Stabilities of Antimicrobial N-Chlorohydantoin Derivatives

Phenyl substitution at position 5 on the hydantoin ring of a series of N-halamine derivatives, including an antimicrobial siloxane used for coating surfaces, has been employed to better understand the biocidal activities and stabilities of these compounds in the presence of water and exposure to ultraviolet irradiation. This phenyl derivatization of the hydantoin ring at its 5 position does, in fact, affect the antimicrobial efficacy of the siloxane against Staphylococcus aureus and Escherichia coli O157:H7, and its stability toward hydrolyses and ultraviolet light exposure. The phenyl substitution caused weakening of the N-Cl bond at the 1 position on the hydantoin ring, leading to an increase in biocidal activity but a decrease in stability. Model compounds were studied experimentally and theoretically at the (U)B3LYP/6-311++G(2d,p) level of theory in order to better explain the observations. It was found that the phenyl moiety at position 5 on the hydantoin ring can interact with the oxidative chlorine atom bonded to the amide nitrogen atom at position 1 which then results in a less stable N-Cl bond.