Distortion-induced tuning of chlorophyll a electronic properties in LHC proteins

In eukaryotic light-harvesting complexes, the protein host tunes chlorophyll-a (Chl a) site energies through a number of mechanisms (e.g. hydrogen bonding; excitonic/charge-transfer (CT) mixing). However, whether protein-imposed macrocycle distortions also contribute to fine-tuning this energy remains unclear. We have investigated three Chl a-only algal antennas - VCP (from Nannochloropsis oceanica), rVCP (Trachydiscus minutus), and CLH (Chromera velia). These complexes show distinct red-shifted absorption features, which at 77 K range from 670 nm up to an extremely red-shifted band at 700 nm (rVCP). By analysis of marker bands in excitation-selective resonance Raman spectra, we have determined the contribution of (i) hydrogen bonding to the keto group (1630-1700 cm^-1 region), and/or (ii) macrocycle distortion (bands near 1555 and 1612 cm^-1) to these red-shifts. Finally, the absence of vibrational features in fluorescence line-narrowing measurements for the reddest rVCP band is consistent with strong excitonic coupling, which we suggest is at the origin of this large red shift. Our results indicate that tuning of the functional properties of Chl a antennas may occur via distortion of the macrocycle.