Continuous-wave quantum yields of various cobalamins are influenced by competition between geminate recombination and cage escape

Abstract

Quantum yields of photolysis of the cobalt-carbon bond for three cobalamin compounds were measured with a continuous-wave laser at 442 nm under both aerobic and anaerobic conditions. Aerobically, the initial homolysis product, Co(II) cobalamin, is trapped by oxygen to form aquocobalamin. Use of an excess of the radical trapping reagent 2,2,6,6-tetramethyl-1-piperidinyloxyl, under anaerobic conditions, scavenges the carbon radical and allows detection of the cobalt(II) photoproduct. Quantum yields measured under anaerobic conditions for 5'-deoxyadenosylcobalamin (phi (Co-C alpha),442 = 0.20 +/- 0.03) and methylcobalamin (phi (Co-C alpha),442 = 0.35 +/- 0.03) are in agreement with the values obtained under aerobic conditions (phi (Co-C alpha),442 = 0.19 +/- 0.04 and phi (Co-C alpha),442 = 0.36 +/- 0.04, respectively). Additionally, the quantum yield values for 5'-deoxyadenosylcobalamin and its base-off derivative (phi (Co-C alpha),442 = 0.045 +/- 0.015) match those obtained on a nanosecond time scale [Chen, E., & Chance, M. R. (1990) J. Biol. Chem. 256, 12987-12994]. A comparison of quantum yields obtained anaerobically for 5'-deoxyadenosylcobalamin and methylcobalamin in H2O versus ethylene glycol shows a 4-fold decrease for the former cobalamin and no change for the latter. These quantum yields are evaluated in terms of time-independent radical separation distances.

Photolysis of Adenosylcobalamin and Radical Pair Recombination in Ethanolamine Ammonia-Lyase Probed on the Micro- to Millisecond Time Scale by Using Time-Resolved Optical Absorption Spectroscopy

“…The kinetics in Figure 4 therefore represent the events that occur on the time scale of ≥0.1 μs for each condition. This is consistent with previous studies of AdoCbl photolysis in solution (19, 45). The time dependence of the cob(II)alamin photoproduct concentration in solution was fit by a second-order decay plus a constant function.…”

“…The quantum yield ( φ ) is defined as the concentration of cob(II)alamin photoproduct formed by the laser pulse, divided by the absorbed photon ( h ν) concentration, [ h ν] abs . The concentration of photoproduct is found by using the previously measured difference extinction coefficients for adenosylcob(III)alamin to cob(II)alamin conversion, Δε (M -1 cm -1 ), for AdoCbl (45). The concentration of photoproduct is calculated by using the following expression…”

Time-Resolved Spectroscopic Studies of B12 Coenzymes: The Photolysis of Methylcobalamin Is Wavelength Dependent

“…0.17 from 510 to 550 nm are reported in ref 12. In a more recent study, Chen and Chance reported a 0.35 ( 0.04 quantum yield for the photolysis of methylcobalamin using continuous excitation at 442 nm, 13 in good agreement with our 400 nm measurements.…”

TD-DFT insight into photodissociation of the Co-C bond in coenzyme B12

“…In addition to their pivotal roles in a variety of enzymatic processes, the B 12 derivatives possess complex photophysical and photochemical properties (Endicott and Ferraudi, 1977; Endicott and Netzel, 1979; Rao and Symons, 1982; Chen and Chance, 1990, 1993; Sakaguchi et al, 1990; Chagovetz and Grissom, 1993; Grissom and Chagovetz, 1993; Lott et al, 1995; Natarajan and Grissom, 1996; Kruppa et al, 1997; Walker et al, 1998a,b; Shiang et al, 1999, 2006; Yoder et al, 2001; Cole et al, 2002; Sension et al, 2004, 2005a,b; Harris et al, 2007). The relatively weak organometallic Co-R bond (R = alkyl, hydroxyl, water, cyanide) in these compounds can undergo photodissociation under conditions of simple photon excitation, depending on the nature of the upper axial ligand.…”

“…The photochemistry of cobalt corrinoids has been probed using various experimental techniques such as laser flash photolysis (Endicott and Ferraudi, 1977; Endicott and Netzel, 1979; Chen and Chance, 1990; Chagovetz and Grissom, 1993; Lott et al, 1995), continuous wave (CW) photolysis (Chen and Chance, 1993), kinetic magnetic field effect (MFE) (Grissom and Chagovetz, 1993; Natarajan and Grissom, 1996), chemically induced dynamic electron polarization (CIDEP) (Rao and Symons, 1982; Sakaguchi et al, 1990) as well as nuclear polarization (CIDNP) (Kruppa et al, 1997). Systematic studies of photolysis in B 12 alkyl-derivatives (MeCbl, EtCbl, PropCbl, and AdoCbl) using pump-probe transient absorption spectroscopy found that photoproduct yields for alkylcobalamins are different when excited at 400, or 520/530 nm (Walker et al, 1998a,b; Shiang et al, 1999; Yoder et al, 2001; Cole et al, 2002; Sension et al, 2004, 2005a,b; Harris et al, 2007).…”