Samples (E•GMP•NADP+ in red and E•IMP•NADP+ in blue) are in 75 mM Tris-HCl, pH 7.8, 100 mM KCl, 1 mM DTT and 0.5 mM EDTA: (A) substrate monophosphate, (B) cofactor monophosphate, and (C) cofactor diphosphates. The error bars are the standard error in R1 from the exponential fit of signal magnitude versus delay time. The data shown are for one of the three independent experiments.