“…Since phosphorylation is an essential step for the antiherpes activity of the pyrimidine nucleoside analogs (DeClercq, 1982;Gupta, Tourigny, Stuart, DeClercq, Quail, Ekiel, E1-Kabbani & Delbaere, 1987), it was felt that one possible reason for the loss of bioactivity of N4-methyl-MMdCyd may be that the conformation of the molecule has been altered. This hypothesis is based on studies which have shown that the conformation of the deoxyribofuranose moiety is important in determining substrate specificity towards the viral enzyme (E1-Kabbani, Ekiel, Delbaere, Quail, Ekiel, E1-Kabbani, Tourigny, Delbaere, Stuart & Gupta, 1986;Quail, Tourigny, Delbaere, E1-Kabbani, Stuart & Gupta, 1988;Gupta et al, 1987;Tourigny, Stuart, Ekiel, Aduma & Gupta, 1989;Jia, Tourigny, Stuart, Delbaere & Gupta, 1990). The crystal structure of N4-methylMMdCyd was determined by X-ray diffraction analysis.…”