Estimation of the bacteriochlorophyll c oligomerisation extent in Chloroflexus aurantiacus chlorosomes by very low-frequency vibrations of the pigment molecules: A new approach.

In green photosynthetic bacteria, the chlorosomal bacteriochlorophyll molecules are organized via self-assembly and do not require proteins to provide a scaffold for efficient light harvesting. Despite numerous investigations, a consensus regarding the spatial structure of chlorosomal antennae has not yet been reached. For the first time, we demonstrated by coherent femtosecond spectroscopy at cryogenic temperature that the very low-frequency (~101  cm-1 ) vibrations of bacteriochlorophyll c pigments in isolated Chloroflexus aurantiacus chlorosomes are sensitive to their oligomerisation extent which depends on the light intensity during the growth of the cell cultures. We explained this sensitivity in terms of the coupling of delocalised vibration modes of BChl c molecules aggregated into chains within their antenna unit building blocks. These findings, together with previously obtained spectroscopy and microscopy data, confirmed that the unit building blocks functioning within Chloroflexus aurantiacus chlorosomal antenna are built up from the rather short (2-5 BChl c pigments) quasi-linear chains. The approach presented here seems to be perspective since it directly reveals structural and dynamical properties of the oligomeric systems.