Kinetic evidence for hemiacetal formation during the oxidation of dextran in aqueous periodate

A kinetic analysis is described of the periodate oxidation of a dextran in which all the 93% of oxidisable D-glucose residues contained a 2,3,4-triol system. Measurements were made of the periodate consumed and the formic acid liberated by the dextran, the periodate consumed and the formaldehyde liberated by samples that had been partially oxidised and then reduced with sodium borohydride, and the glycerol and erythritol released from these samples by acid hydrolysis. Initially, the oxidisable D-glucose residues decayed according to second-order kinetics. After the first oxidative attack, ~ 40% of the singly oxidised residues very rapidly consumed a second mole of periodate, while the remainder consumed further periodate at about one-seventh of the rate of an intact D-glucose residue. Residues cleaved between positions 3 and 4 were generated 7.5 times faster than residues cleaved between positions 2 and 3, but the two kinds of singly oxidised residue subsequently decayed at similar rates. Towards the end of their reaction, the rate of decay of intact, oxidisable D-glucose residues declined in a way that was simply correlated with the proportion of doubly oxidised residues in the chains. A simple scheme is presented that explains these facts in terms of intra-residual hemiacetal formation by singly oxidised residues, and inter-residual hemiacetal formation between doubly oxidised residues and intact D-glucose residues adjacent to them in the chains.