The Photoactive Excited State of the B<sub>12</sub>-Based Photoreceptor CarH
Nicholas A. Miller, April K. Kaneshiro, Arkaprabha Konar, Roberto Alonso‐Mori, Alexander Britz, Aniruddha Deb, James M. Glownia, J. D. Koralek, Leena Mallik, Joseph H. Meadows, Lindsay B. Michocki, Tim B. van Driel, Markos Koutmos, S. Padmanabhan, Montserrat Elías Arnanz, Kevin J. Kubarych, E. Neil G. Marsh, James E. Penner‐Hahn, Roseanne J. Sension
Abstract
, adenosylcobalamin (AdoCbl), to sense light. With visible excitation, a nanosecond-lifetime photoactive excited state is formed with unit quantum yield. The time-resolved X-ray absorption near edge structure difference spectrum of this state demonstrates that the excited state of AdoCbl in CarH undergoes only modest structural expansion around the central cobalt, a behavior similar to that observed for methylcobalamin rather than for AdoCbl free in solution. We propose a new mechanism for CarH photoreactivity involving formation of a triplet excited state. This allows the sensor to operate with high quantum efficiency and without formation of potentially dangerous side products. By stabilizing the excited electronic state, CarH controls reactivity of AdoCbl and enables slow reactions that yield nonreactive products and bypass bond homolysis and reactive radical species formation.