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Phosphate modifications of 7 and 8. The sites of modification (•) and the contacts made with MS2 coat protein are indicated on the secondary structure. The chemical structures and stereochemistry of the phosphate modifications are shown on the right. Note that rules of nomenclature (46) require that when an oxygen is replaced by a sulfur to give the RP phosphorothioate, the same oxygen substituted with a methyl group yields the SP methylphosphonate.
