Ultrafast Excited-State Dynamics and Photolysis in Base-Off B₁₂ Coenzymes and Analogues: Absence of the trans-Nitrogenous Ligand Opens a Channel for Rapid Nonradiative Decay

Abstract

Ultrafast transient absorption spectroscopy was used to investigate the photochemistry of adenosylcobalamin (AdoCbl), methylcobalamin (MeCbl), and n-propylcobalamin (PrCbl) at pH 2 where the axial nitrogenous ligand is replaced by a water molecule. The evolution of the difference spectrum reveals the internal conversion process and spectral characteristics of the S(1) excited state. The photolysis yield in the base-off cobalamins is controlled by competition between internal conversion and bond homolysis. This is in direct contrast to the process in most base-on alkylcobalamins where primary photolysis occurs with near unit quantum yield and the photolysis yield is controlled by competition between diffusive separation of the radical pair and geminate recombination. The absence of the axial nitrogenous ligand in the base-off cobalamins modifies the electronic structure and opens a channel for fast nonradiative decay. This channel competes effectively with the channel for bond dissociation, dropping the quantum yield for primary radical pair formation from unity in base-on PrCbl and AdoCbl to 0.2 ± 0.1 and 0.12 ± 0.06 in base-off PrCbl and AdoCbl, respectively. The photolysis of base-off MeCbl is similar to that of base-off AdoCbl and PrCbl with competition between rapid nonradiative decay leading to ground state recovery and formation of a radical pair following bond homolysis.

B12-Mediated, Long Wavelength Photopolymerization of Hydrogels

“…By transient absorption, alkyl-cobalamins photolyze at 510 nm ( α / β band) with near quantitative yields, but radical recombination within a solvent cage reduces the observed quantum yield to 20–30%. 32,37 This Φ hom is comparable to the reported quantum yields of other common initiators, such as ~29% for Irgacure 2959 (I2959, λ max < 300 nm) and ~25% for eosin Y/ triethanolamine ( λ max ~ 510 nm, 20 mM amine coinitiator) system. 38,39 We investigated the Φ hom of both initiators exposed to 520 or 660 nm light with a constant photon flux (1 mW/ cm 2 , Table 1, 2Figure 2).…”

Excited electronic states and internal conversion in cyanocobalamin

“…The lower axial ligand can also be replaced by a water molecule resulting in a base-off cobalamin compound. This base-off form dominates in solution at low pH [9], in certain transport proteins [10] and a small number of methyl transferase proteins [2]. The upper axial ligand is an alkyl group in the enzymatically active B 12 coenzymes.…”

“…Ultrafast broadband transient absorption spectroscopy has been used to study the photochemistry and photophysics of naturally occurring and synthetically prepared cobalamin compounds [9,[15][16][17][18][19][20][21][22][23][24][25][26][27]. Alkylcobalamins are photolabile.…”

Electronic structure of the S1 state in methylcobalamin: Insight from CASSCF/MC‐XQDPT2, EOM‐CCSD, and TD‐DFT calculations

“…[42][43][44][45][46][47][48][49][50][51] As demonstrated by Sension et al, useful information about the low-lying excited electronic states of B 12 derivatives can be obtained with the aid of transient absorption spectroscopy (TAS). [42][43][44][45][46][47][48][49][50][51] As demonstrated by Sension et al, useful information about the low-lying excited electronic states of B 12 derivatives can be obtained with the aid of transient absorption spectroscopy (TAS).…”