Why is CMP-ketodeoxyoctonate highly unstable?

CMP-ketodeoxyoctonate (CMP-KDO) and analogs, including CMP-5-deoxy-5-fluoro-KDO, CMP-5-deoxy-KDO, and CMP-5-epi-KDO, were prepared from CTP and the corresponding KDO sugars catalyzed by CMP-KDO synthetase. These analogs were found to be much more stable than CMP-KDO (t1/2 = 0.57 h) yet less stable than CMP-sialic acid (t1/2 = 151 h). Fluorination at the 5-position of CMP-KDO has a 200-fold enhanced stability compared to the 156-fold enhancement for the 3R-fluoro analog, probably due to the loss of H-bonding interactions (for the 5-F derivative) and the cause of remote inductive effect (for the 3- and the 5-F analogs) on the glycosidic cleavage. Hydrolysis of CMP-KDO is perhaps facilitated by an intramolecular hydrogen bond from the 5-OH group with the phosphate oxygen as demonstrated by the 3-5-fold enhanced stability of CMP-5-epi-KDO and CMP-5-deoxy-KDO compared to CMP-KDO and by molecular modeling studies of water-solvated CMP-KDO. Hydrolysis of CMP-KDO also was found to be subject to a substantial solvent isotope effect (kH/kD = 2.7), which is significantly different from the reported solvent isotope effect for the hydrolysis of sialyglycosides (kH/kD = 0.86) and dependent on both buffer and magnesium ion concentrations. Considering these results and molecular modeling studies, it is proposed that the hydrolysis of CMP-KDO under neutral conditions proceeds through a glycosidic cleavage which occurs at the electronically favorable twist-boat conformation, facilitated by intramolecular H-bonding interaction of the 4-, 5- and 7- (or 8-) OH groups and the phosphate oxygen and by the leaving group magnesium ion complexation.