Direct measurement of vibrational modes in LHCII trimers using femtosecond stimulated Raman spectroscopy

Juan Manuel Artés, Yusaku Hontani, Janneke Ravensbergen, John Kennis, Rienk van Grondelle
Biophysics Photosynthesis and Energy. Vrije Universiteit Amsterdam, De Boelelaan 1081-1087. 1081HV Amsterdam

Photosynthesis performed by plants, algae and photosynthetic bacteria is the biological process that transforms solar energy into chemical energy with high efficiency. A complete understanding of the mechanisms and factors that modulate the efficiency of this process is a challenge centered at the nexus of fundamental physics and biology. In that sense, the direct investigation of vibrational modes in the photosynthetic process could provide new clues for a complete understanding of the mechanisms involved in the early stages of the light harvesting processes and regulatory mechanisms such us Non Photochemical Quenching.

To explore that possibility we used femtosecond stimulated Raman spectroscopy to study the excited states of trimeric Light-Harvesting Complexes from plants. Herein we show the first time-resolved results of the vibrational modes of xanthophylls and chlorophylls in excited LHCII trimers. By selectively exciting the different pigments, we obtain the time evolution of the vibrational spectra and relate it to different processes in the light harvesting mechanism. Results after exciting lutein 2 show ground state bleach of the main carotenoid modes and the transient population of energy transfer states S2 and S1, simultaneous to the bleach of chlorophyll modes. In contrast, results after exciting chlorophyll a show simultaneous ground state bleach of chlorophyll and carotenoid modes nearly from time 0, pointing to the possibility of coupled modes between the different pigments.

These preliminary results constitute a proof of concept of the application of fsRaman to complex photosynthetic samples and pave the way for future research aiming for a complete model of the light harvesting mechanisms and its regulation at the molecular level.

Figure: fsRaman data obtained on LHCII trimers excited at 520 nm.