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Structures of backbone linkages produced in enzyme-free primer extension reactions: the phosphoramidate of a 3'-amino-2',3'-dideoxynucleoside (1), the phosphoramidate of a 3'-amino-3'-deoxynucleoside (2), the 3',5'-phosphodiester of a natural ribonucleoside (3), and the isomeric 2',5'-phosphodiester of a ribonucleoside (4).
