Kinetics. Buffer solutions (2.5 mL) were thermostated in a 10-mm cuvet in the constant- temperature cell holder of a Cary 16 or Beckman DB spectrophotometer, at 25.0 i 0.1" C. To initiate reaction, 10 pL of a stock solution of substrate were injected with an Eppendorf pipet. Absorbances at 390 nm were collected to 80% reaction, the final absorbance was read after 10 half-life times, and first-order rate constants were calculated by a nonlinear least- ...