Femtosecond anisotropy excitation spectroscopy to disentangle the Q(x) and Q(y) absorption in chlorophyll a

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 C[double bond, length as m-dash]O 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.