Leucine modifications by He/O2 plasma treatment in phosphate-buffered saline: bactericidal effects and chemical characterization

Abstract

We studied the chemical and bactericidal properties of plasma-modified amino acid leucine (Leu) treated in phosphate-buffered saline (PBS) using an atmospheric-pressure plasma jet operating with He/O2 gas. We determined the initial formation of hypochlorite by the reaction of plasma-supplied O atoms with chloride ions in PBS, followed by the subsequent chlorination of Leu. The principal chemical process involved the formation of Leu monochloramine and Leu dichloramine. The products of plasma-modified Leu further possessed post-discharge reactivity characterized by the decay of Leu chloramines to the secondary products, i.e. isovaleraldehyde, isovaleronitrile, and N-chloromethyl-3-butaldimine. Our study of the effects of Leu on the viability of Escherichia coli bacteria in the plasma-treated PBS solution demonstrated time-and pH-dependent cytotoxic behavior, which were associated with the formation of different reaction products. At shorter treatment times, the addition of Leu hindered the bactericidal properties of plasma-treated PBS caused by the consumption of bactericidal hypochlorite in reaction with Leu and formation of non-bactericidal Leu monochloramine. Acidification to pH = 3 and longer plasma treatment substantially enhanced the bactericidal effects of Leu solutions, which were tentatively assigned to the formation of Leu dichloramine.