The Calculated In Vitro and In Vivo Chlorophyll a Absorption Bandshape

Abstract

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 cm −1 occurs. The FC factor distribution for pentacoordinated chlorophyll also describes the native chlorophyll a spectrum but, in this case, either a low-frequency mode ( ν < 200 cm −1) must be added or else the 262-cm −1 mode must increase in coupling by about one order of magnitude to describe the skewness of the main absorption bandshape.