Femtosecond Stimulated Emission Probing the Forbidden 1B_u^- and 3A_g^- Levels of Carotenoids and ²⁵Mg-NMR Signal Probing the Axial Coordination / Interaction of Bacteriochlorophylls

Abstract

We have found that coherent excitation, with approximate 30 fs pulses, of the optically-allowed 1B_u⁺(0) and the overlapped, optically-forbidden 1B_u^-(2) or 3A_g^-(1) levels of carotenoids (spheroidene or lycopene) generates stimulated emission from the 1B_u^-(0) and 3A_g^-(0) level [T. Miki, Y. Kakitani, Y. Koyama and H. Nagae, Chem. Phys. Lett. 457 (2008) 222 ]. Also, we have shown that the axial coordination to the Mg atom of bacteriochlorophyll, i.e., the penta-coordinated state, in the presence or absence of additional axial interaction, can be probed by the width of the ²⁵Mg-NMR signal [Y. Kakitani, Y. Koyama, Y. Shimoikada, T. Nakai, H. Utsumi, T. Shimizu and H. Nagae, accepted by Biochemistry].
We are applying these techniques to determine the 1B_u^-(0) and 3A_g^-(0) levels of the above carotenoids, and the axial coordination / interaction of B800 and B850 bacteriochlorophyll a, when bound to the LH2 complexes from Rba. sphaeroides 2.4.1 and Rsp. molischianum.