Femtosecond anisotropy excitation spectroscopy to disentangle the Qx and Qy absorption in chlorophyll a

Abstract

Chlorophyll a (Chl a) belongs to the most important and most investigated molecules in the field of photosynthesis. The Q-band absorption is central for energy transfer in photosystems and the relative orientation of the Q_y transitions of interacting chlorophylls governs the energy transfer. Chl a was well investigated, but a quantitative separation of Q_x and Q_y contributions to the Q-band of the Chl a absorption spectrum is still missing. We use femtosecond Vis-pump – IR-probe anisotropy excitation spectroscopy to disentangle the overlapping electronic Q_x and Q_y contributions quantitatively. In an anisotropy excitation spectrum we trace the dichroic ratio of a single vibration, i.e. the keto CO stretching vibration at 1690 cm⁻¹, as a function of excitation wavelength. The change in dichroic ratio reflects altering Q_y and Q_x contributions. We identified Q_x00 (0–0 transition of Q_x) and Q_x01 transition at (636 ± 1) nm and (607 ± 2) nm, respectively, and the Q_y01 and Q_y02 at (650 ± 6) nm, and (619 ± 3) nm, respectively. We find that Q_x absorption, contributes to 50% to 72% at 636 nm and 49% to 71% at 606 nm to the Chl a absorption at room temperature. The Q band was well modelled by a single vibronic progression for the Q_x and Q_y transition of (700 ± 100) cm⁻¹, and the energy gap between Q_x00 and Q_y00 was found to be (820 ± 60) cm⁻¹. This precise description of the hexa-coordinated Chl a absorption spectrum will foster more accurate calculations on energy transfer processes in photosystems, and advance the detailed understanding of the intricate interaction of chlorophyll molecules with the solvent.