Striking changes in molecular recognition by deuteration of aqueous binary solvents

Oxidation of a pair of associating thiols 1 and 2 with oxygen, each with a binding site [—C(O)NHC(O)NH—] and a recognition site (R1 or R2), is examined at various temperatures in aqueous binary solvents of water or deuterium oxide with ethanol, acetonitrile or methanol. The selectivity (r)—a measure of the degree of molecular recognition in the oxidation—represented by the logarithmic ratio of the yield of the unsymmetrical disulfide (4) to twice that of the symmetrical disulfide (3) was examined as a function of the mole fractions of water (xw) and deuterium oxide (xdo). It is found that, on deuteration of aqueous binary solvents containing ethanol (xw= 0.50 and 0.75), the temperature dependence of molecular recognition (r) alters strikingly from a clear bell-shaped type (a maximum at 35°C) to a plateau-like type in the range 20–70°C. It is clarified further that, in aqueous acetonitrile at xw= 0.20 and 0.75, deuteration of water causes the temperature dependence of r to become fairly dull. In contrast, it is found that the deuteration effect in water–methanol is much less than that in water–ethanol or in water–acetonitrile. Furthermore, conversion of the NH groups in 1 and 2 to the ND groups occurs very quickly. A possible cause of the large deuterium effect is discussed. Copyright © 2005 John Wiley & Sons, Ltd.