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Kinetic isotope effects. Values are reported as means ± standard errors with the number of independent trials in parentheses. The second value for the nitrogen nucleophile (asterisked) was previously determined using mass spectrometry instead of scintillation counting as the readout. Calculated values for the transition state in figure 4 are shown in italics. Full substrates are shown; cyt=cytidine, cap=caproic acid, bio=biotin, pmn=puromycin.
