11 37.4 CatalysisExample Problem 37.1DeVoe and Kistiakowsky [J. American Chemical Society 83 (1961), 274] studied the kinetics of CO2 hydration catalyzed by the enzyme carbonic anhydrase:CO2 + H2O HCO3-1In this traction, CO2 is converted to bicarbonate ion. Bicarbonate is transported in the bloodstream and converted back to CO2, in the lungs, a reaction that is also catalyzed by carbonic anhydrase. The following initial reaction rates for the hydration reaction were obtained for an initial enzyme concentration 0f 2.3 nM and temperature of 0.5 oC:Determine Km and k2 for the enzyme at this temperature.Rate (M s-1)[CO2] (mM)2.7810-51.255.0010-52.58.3310-55.01.6710-420.0
21 37.8 Photochemistry 37.8.1 Photophysical Processes Figure 37.14 A Joblonski diagram depicting various photo-physical processes, where S0 is the ground electronic singlet state, S1 is he first excited singlet state, and T1is the first excited triplet state. Radiative processes are indicated by the straight lines. The nonradiative processes of intersystem crossing (ISC), internal conversion (IC), and vibrational relaxation (VR) are indicated by the wavy lines.
22 37.8 Photochemistry Figure 37.15 Kinetics description of photo-physical processes. Rate constants are indicated for absorption (ka), fluorescence (kf), internal conversion (kic), intersystem crossing from S1 to T1 (ksisc), and phosphorescence (kp)
27 Fluorescence and Fluorescence Quenching 37.8 PhotochemistryFluorescence and Fluorescence QuenchingStern-Volmer plotsFigure 37.16A Stern-Volmer plot. Intensity of fluorescence as a function of quencher concentration is plotted relative to the intensity in the absence of quencher. The slope of the line provides a measure of the quenching rate constant relative to the rate constant for fluorescence.
28 When kf >> kic and kf >> ksisc Measurement of tfWhen kf >> kic and kf >> ksisc
29 Example Problem 37.4Thomaz and Stevens (in Molecular Lumiescence, Lim, 1969)studied the fluorescence quenching of pyrene in solution. Using the following information, determine kf and kq for pyrene in the presence of the quencher Br6C6.Example Problem 37.4[Br6C6] (M)tf (s)0.00052.66×10-70.0011.87×10-70.0021.17×10-70.0038.50×10-80.0055.51×10-8
31 P37.31) For phenanthrene, the measured lifetime of the triplet state (tP) is 3.3 s, the fluorescence quantum yield is 0.12, and the phosphorescence quantum yield is 0.13 in an alcohol-ether class at 77 K. Assume that no quenching and no internal conversion from the singlet state occurs. Determine kp, kTisc, and kSisc/kr.