Base-catalyzed deuteration of the methyl groups in protoporphyrin M dimethyl ester (1) proceeds with differential deuteration; rate of deuteration, as measured by NMR spectroscopy of reaction products, follows the order 3-Me> 1-Me> 5-Me> 8-Me. A simple qualitative theory to explain the differential deuteration is discussed, based on primary (vinyl group on subunit bearing the deuteriated methyl) and secondary (vinyl group on adjacent ...