The geometry of the thioester group and its implications for the chemistry of acyl coenzyme a

Abstract

The ground-state geometry of the thioester group has been investigated in an attempt to understand the reactivity of acyl CoAs. The structure of ethyl thiol- d-ribonate tetraacetate has been determined from X-ray diffraction data, and bond lengths, interbond angles, and torsion angles are presented for the two independent molecules in the asymmetric unit of the crystal. In crystals of this compound the ethyl groups are found to be disordered. An analysis of the published data on bond lengths and interbond angles for thioesters, esters, and alkyl aryl/vinyl monosulfides (R 1SR 2, R 1  C( sp 3), R 2  C( sp 2)) and ethers (R 1OR 2) was made using the Cambridge Crystallographic Data File. This survey, together with the results from our studies, shows that in thioesters ▪ the acyl CS bond (marked ∗) is not significantly shorter than a single C( sp 2)S bond in alkyl aryl/vinyl monosulfides. This is in contrast to the well-known conjugation in the corresponding bond in esters ▪ where we find that the acyl  CO bond is shorter than a single C( sp 2)O bond in alkyl aryl/vinyl ethers by 0.076(6) Å. Thus there is much more resonance in O-esters than in thioesters. These findings support recent suggestions that the carbonyl group in acyl CoAs is ketone-like. Therefore thioesters (with little or no double bond character in the CS bond) have a higher free energy of hydrolysis than do O-esters (with appreciable double bond character in the CO bond).