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Other methods, particularly those involving base oxidation, have been given some consideration. Osmium tetroxide oxidizes thymine residues much faster than the other bases [9,343]. As applied to DNA, the oxidized residue can be removed by base, whence diphenylamineformic acid treatment affords groups of cytosine nucleotides, C n P n + 1 [344]. Sequences of adenine nucleotides, as yet not sufficiently characterized result from permanganate oxidation, followed by alkali [345]. The problem associated with these reagents is that, as in hydrazinolysis, a nitrogenous residue may be left attached glycosidically and this has to be removed completely before the chain cleavage reaction can be initiated.

Metal ions may act as electrophilic catalysts in the hydrolysis of phosphate esters. In no case is the catalysis understood in detail, and in many cases it is not possible to obtain rate data as the reactions do not always occur in homo­ geneous solution. Nevertheless, the rate enhancements can be very large. Phosphate monoesters have been studied intensively. La 3+ and Ce 3 + are very effective [274,275]. It is often noted, as it is with diester hydrolyses, that the reactions are sharply pH-dependent and that the optimum pH may vary with the concentration both of the metal ion and of the substrate.

It is important to consider the D. M. Brown 42 factors which may apply in determining the rate of the process. The cisstereochemistry of the neighboring 2'-hydroxyl group in the furanose system is probably optimal. A trans-2'-OYi group as in arabino-nuclQOtiae esters is ineffective as a neighboring group [257]. The acidity of the neighboring function in 162 determines the concentration of 163 and hence the rate of hydrolysis under a given set of conditions. In other series, rate measurements are consistent with the view that kohs = Kk [258].