Table 1. Y· Lifetimes for α₂-Variant/Substrate Combinations.

α2-variant	substrate	τ/μsa
wt	CDP	18 (1)
Y731F	CDP	24 (1)
C439S	CDP	25 (2)
wt	[3′-2H]-CDP	26 (1)

^aPhotogenerated Y· lifetimes monitored by TA spectroscopy at 412.5 nm and fit to monoexponential decay from 3 to 76.5 μs (Figure S3). Weighted averages represent duplicate sets of three measurements each, on two separate protein preparations of [Re]-F₃Y-β₂ and wt-α₂. Samples contain 50 μM met-[Re]-F₃Y-β₂, 75 μM α₂ variant, 1 mM or 0.5 mM CDP or [3′-²H]-CDP, 3 mM ATP, 10 mM Ru(NH₃)₆Cl₃, and assay buffer at pH 8.2.