AI Article Synopsis
- The study used ultrafast transient absorption spectroscopy to explore how different cobalamins (AdoCbl, MeCbl, PrCbl) behave under acidic conditions (pH 2) where a water molecule replaces a key nitrogen ligand.
- The research found that in base-off cobalamins, the photolysis process is influenced by the competition between internal conversion and bond breaking, unlike in base-on forms where photolysis occurs with nearly perfect efficiency.
- The absence of the axial nitrogen ligand alters the electronic structure, leading to a significant drop in the probability of forming free radicals, reducing the quantum yield for primary radical formation.
Article 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.
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