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.
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.
Peng
Jian
J
Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109-1055, USA.
Tang
Kuo-Chun
KC
McLoughlin
Kaitlin
K
Yang
Yang
Y
Forgach
Danika
D
Sension
Roseanne J
RJ
D003038:Cobamides / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D008024:Ligands; D008968:Molecular Conformation; D056004:Molecular Dynamics Simulation; D010777:Photochemistry; D010782:Photolysis; D011789:Quantum Theory; D013056:Spectrophotometry, Ultraviolet; D013997:Time Factors; D014805:Vitamin B 12 / Q000031:analogs & derivatives* / Q000737:chemistry; D014867:Water / Q000737:chemistry
D003038:Cobamides; D008024:Ligands; C450702:n-propylcobalamin; D014867:Water; C019476:mecobalamin; D014805:Vitamin B 12