Structures and thermodynamics of biphenyl dihydrodiol stereoisomers and their metabolites in the enzymatic degradation of arene xenobiotics.

A key step in the metabolic degradation of biphenyl xenobiotics is catechol formation upon dehydrogenation of cis- and trans-dihydrodiols in prokaryotic and eukaryotic pathways, respectively. Structure and thermodynamics of stereoisomers of cis-, trans-2,3-biphenyl-dihydrodiols (I) and their dehydrogenation products (hydroxyketones, II), as well as final catechol (2,3-biphenyldiol, III) are studied by means of ab initio MP2/6-311++G(2df,2p)//MP2/6-311G(d,p) calculations. Formation of stereoisomers I and II is exothermic and endergonic, whereas III is enthalpically and entropically driven. Dehydrogenations are endothermic (DeltaHR0 approximately 1.5-4 kcal mol(-1)) and exergonic (DeltaGR0 approximately -5 to -7.5 kcal mol(-1)) without noticeable differences between cis and trans pathways, although the same keto stereoisomer II-(2S) is found to be the more favored product from both cis- and trans-I. The final II --> III tautomerization is thermodynamically enhanced (DeltaHR0 approximately -27, DeltaGR0 approximately -28 kcal mol(-1)) but the process is shown to have a large activation energy if it had to occur via unimolecular path. Although this tautomerization is generally assumed to be a nonenzymatic process as it involves rearomatization of an oxygenated ring, proton transfer with an anionic intermediate might be a more probable process.