A Theoretical Study on Monitoring Explosives Degradation by Pentaerythritol Tetranitrate Reductase Using THz Spectroscopy

Abstract

Pentaerythritol tetranitrate (PETN) reductase is an enzyme produced by bacteria (Enterobacter cloacae PB2) that have the ability to use nitroaromatic (trinitrotoluene – TNT, picric acid) or nitroester (glycerol trinitrate – GTN, PETN) explosives as a sole nitrogen source for growth. The reaction of PETN reductase comprises a reductive and an oxidative half-reaction, explosives being degraded during the oxidative half-reaction. Since the side chains of the residues that form the active site of PETN reductase assume different conformations during the two half-reactions, we investigated whether THz spectroscopy could discriminate between the different states of PETN reductase and whether their discrimination could be useful in the detection of explosives and consequent monitoring of explosives degradation process. In order to answer these questions, we simulated and compared the THz spectra of the hydrated enzyme in the reduced state, in the oxidized state bound to an inert substrate, namely a thiocyanate ion and in the oxidized state bound to picric acid. Our results show that PETN reductase structures in reduced and oxidized states present a significantly different THz absorption, the enzyme in reduced state being the one that absorbs more THz radiation. In oxidized state, the enzyme bound to thiocyanate absorbs less THz radiation then the enzyme bound to picric acid. By discriminating between the conformations of PETN reductase during the two half-reactions involved in explosives degradation, THz spectroscopy should allow the evaluation of explosives degradation state.