Determination of the kinetic constants of the reversible opening of a triazolo-1,4-thienodiazepine in water at different pH values: A striking example of the determination of intimately intricate kinetic and equilibrium constants

Apparent kinetic constants of the reversible opening of a triazolo-1,4-thie nodiazepine were determined by UV-spectrophotometry and by polarography in water at several pH values assuming as a hypothesis a stationary state for the carbinolamine intermediate, Both the apparent kinetic constants, k(1) a nd k(r), exhibited a maximum for the values H-0 = -0.25 and pH = 6.07. A po ssible determination of the elementary kinetic constants of the several aci do-basic species which may be involved in the opening and closing process a ccording to the pH and pKa values was studied and is discussed. The results are consistent with the hypothesis that both the opening of the diazepine cycle and the closing of the opened form proceed through a mechanism sugges ting that the protonated form of the carbinolamine function of the intermed iate is involved. (C) 1999 John Wiley & Sons, Inc.