The calculated in vitro and in vivo chlorophyll a absorption bandshape

The room temperature absorption bandshape for the Q transition region of chlorophyll a is calculated using the vibrational frequency modes and Franck-Condon (FC) factors obtained by line-narrowing spectroscopies of chlorophyll a in a glassy (Rebane and Avarmaa, Chem. Phys. 1982; 68:191- 200) and in a native environment (Gillie et al., J. Phys. Chem. 1989; 93:1620-1627) at low temperatures. The calculated bandshapes are compared with the absorption spectra of chlorophyll a measured in two different solvents and with that obtained in vivo by a mutational analysis of a chlorophyll-protein complex. It is demonstrated that the measured distributions of FC factors can account for the absorption bandshape of chlorophyll a in a hexacoordinated state, whereas, when pentacoordinated, reduced FC coupling for vibrational frequencies in the range 540-850 cm1 occurs. The FC factor distribution for pentacoordinated chlorophyll also describes the native chlorophyll a spectrum but, in this case, either a low-frequency mode ( < 200 cm1) must be added or else the 262-cm1 mode must increase in coupling by about one order of magnitude to describe the skewness of the main absorption bandshape.