Excited electronic states and internal conversion in cyanocobalamin

Abstract

Abstract Cyanocobalamin (CNCbl) is a paradigm system for the study of excited electronic states and biological cofactors including the B 12 vitamers. The photophysics of CNCbl has been thoroughly investigated using both ultrafast spectroscopy and time dependent density functional theory (TD-DFT). Here we review the spectroscopic and theoretical investigations of CNCbl with emphasis on the nature of S 1 , the lowest excited electronic state, and extend the spectroscopic measurements to include the ultraviolet region of the spectrum. Ultrafast transient absorption measurements in the visible αβ band region and in the mid-infrared led to assignment of the S 1 state to a ligand-to-metal charge transfer (LMCT) with lengthened axial bonds and a ∼3 kcal/mol barrier for internal conversion to the ground state. The present measurements encompassing the γ band region of the spectrum provide further support for the assignment of the S 1 state. The experiments are in good agreement with the results of TD-DFT calculations which confirm the expected lengthening of the axial bonds in S 1 and account for the observed barrier for internal conversion back to the ground state.