LIGAND-BINDING TO 4-ATOM-LINKED CAPPED PORPHYRINS

Equilibrium measurements and kinetic behavior from seconds to picoseco nds are described for the binding of O-2 or CO to the four-atom-linked capped porphyrin systems Fe(Por)(base), where Por = OC(2)OPor (1) or OC-(CO)NPor (2) and base = 1-methylimidazole (1-MeIm) or 1,2-dimethyli midazole (1,2-Me(2)Im). Binding of O-2 or CO to the amide-linked syste m Fe(2)(base) would impose nonplanarity on the amide linkages; this do es not occur as this system does not bind O-2 or CO even with gas pres sures of 100 atm. In contrast, the ether-linked system Fe(1)(base), wi th its sterically crowded pocket, is able to bind CO, but with excepti onally high values of P-1/2(CO) of 100, 40, and 17 Torr at +25, 0, and -20 degrees C, respectively, for Fe(1)(1-MeIm) and similar to 5 x 10( 3) Torr at 25 degrees C for Fe(1)(1,2-Me(2)Im). The thermodynamics of binding of CO to Fe(1)(1-MeIm) involves a normal entropy term of Delta S degrees = -29(3) eu but a less negative than normal enthalpy term o f Delta H degrees = -5.8(8) kcal/mol. Binding of CO to Fe(1)(1-MeIm) i s not isosbestic below -30 degrees C. The value of P-1/2(O2) of Fe(1)( 1-MeIm) is 61 Torr at -43 degrees C; above that temperature the bindin g is not reversible. Kinetic analysis of Fe(1)(1-MeIm)(CO) yields the exceptionally low value of the association constant k(B)(CO) of 18.5(5 ) M(-1) s(-1) and a normal value of the dissociation constant k(B)(-CO ) of 0.044(2) s(-1). In that system, after subpicosecond photolysis, t here is evidence that there may be similar to 15% geminate recombinati on occurring with a half-life of 14 ps. This is the first hint of trap ping in a model system, but it is still far from the type of trapping seen in proteins, which persists for hundreds of nanoseconds. For the 1-MeIm complex at high pressures of CO, and especially for the 1,2-Me( 2)Im complex with CO, which can only be detected at high pressures, ki netics observed following photodissociation are complex.