PROTON-TRANSFER FROM HETEROCYCLIC-COMPOUNDS .7. METHYLATED GUANOSINE AND INOSINE DERIVATIVES AND THE QUESTION OF ZWITTERIONIC INVOLVEMENT

The rates of detritiation of 1-methyl [8-3H]guanosine, 7-methyl[8-3H]guanosine, and 1-methyl[8-3H]inosine have been measured over a pH range at 85 °C and the results compared with those already available for guanosine and inosine. At low pH where the mechanism involves hydroxide ion attack on the protonated substrate methyl substitution has only a marginal effect on the rate. By contrast the results at high pH show that methyl substitution in the 1 -position brings about a large rate retardation (ca. 103) and leads to the conclusion that in the case of both guanosine and inosine the hydroxide ion reacts with both the neutral and zwitterionic forms.