Reactivity of p-nitrostyrene oxide as an alkylating agent. A kinetic approach to biomimetic conditions

The alkylating potential of p-nitrostyrene oxide (pNSO) - a compound used as a substrate to study the activity of epoxide hydrolases as well as in polymer production and in the pharmaceutical industry - was investigated kinetically. The molecule 4-(p-nitrobenzyl)pyridine (NBP), as a model nucleophile for DNA bases, was used as an alkylation substrate. In order to gain insight into the effect of the hydrolysis of pNSO, as well as the hydrolysis of the NBP-pNSO adduct on the pNSO alkylating efficiency, these two competing reactions were studied in parallel with the main NBP-alkylation reaction. The following conclusions were drawn: (i) pNSO reacts through an S _N2 mechanism, with NBP to form an adduct, pNSO-NBP (AD). The rate equation for the adduct formation is: r = d[AD]/dt = k _alk[NBP][pNSO]- kADhyd [AD] (k _alk, and kADhyd being the alkylation rate constant and the NBP-pNSO adduct hydrolysis rate constant, respectively); (ii) the alkylating capacity of pNSO, defined as the fraction of initial alkylating agent that forms the adduct, is similar to that of mutagenic agents as effective as β-propiolactone. The instability of the pNSO-NBP adduct formed could be invoked to explain the lower mutagenicity shown by pNSO; (iii) the different stabilities of the α and β-adducts formed between NBP and styrene oxides show that the alkylating capacity f = k _alk[NBP]/(k _alk[NBP] + k _hyd) (k _hyd being the pNSO hydrolysis rate constant) as well as the alkylating effectiveness, AL = f/kADhyd, are useful tools for correlating the chemical reactivity and mutagenicity of styrene oxides; (iv) a pNSO-guanosine adduct was detected.