Thermochemical Investigation of the Oxygenation of Vitamin K

Abstract

Discovery of a new oxygenation reaction of naphthohydroquinone anions makes possible a determination of the heat of reaction (DELTAH(ox)) of oxygen with the potassium salt derived from deprotonation of the hydroquinone form of vitamin K. From that value (-33.52 +/- 0.60 kcal/mol), the heat of deprotonation of vitamin KH2 (-30.03 +/- 1.20 kcal/mol), and the heat of deprotonation of water (-6.05 +/- 0.3 kcal/mol), the enthalpy change for converting vitamin KH2 to vitamin K oxide is established to be -57.5 kcal/mol, in reasonable agreement with our previous estimate of -62.4 kcal/mol for the oxygenation of the parent naphthohydroquinone. Indeed, in similar fashion the heat of oxygenation of the parent naphthohydroquinone was determined to be -58.47 kcal/mol, and this permits the assignment of a heat of formation to naphthoquinone epoxide of DELTAH(f)degrees = -47.6 kcal/mol. Heats of oxygenation and deprotonation of a variety of related phenols and naphthols provide perspective on cation and substitution effects. These data provide strong support for the base strength amplification mechanism for the biological action of vitamin K proposed by two of us (P.D. and S.W.H.).