Eustigmatophyte model of red-shifted chlorophyll a absorption in light-harvesting complexes.

Photosynthetic organisms harvest light for energy. Some eukaryotic algae have specialized in harvesting far-red light by tuning chlorophyll a absorption through a mechanism still to be elucidated. Here, we combined optically detected magnetic resonance and pulsed electron paramagnetic resonance measurements on red-adapted light-harvesting complexes, rVCP, isolated from the freshwater eustigmatophyte alga Trachydiscus minutus to identify the location of the pigments responsible for this remarkable adaptation.

Structural and quantum chemical basis for OCP-mediated quenching of phycobilisomes.

Cyanobacteria use large antenna complexes called phycobilisomes (PBSs) for light harvesting. However, intense light triggers non-photochemical quenching, where the orange carotenoid protein (OCP) binds to PBS, dissipating excess energy as heat. The mechanism of efficiently transferring energy from phycocyanobilins in PBS to canthaxanthin in OCP remains insufficiently understood.

Ultrafast spectroscopy of the hydrophilic carotenoid crocin at various pH.

This study delves into the pH-dependent effects on the excited-state behavior of crocin, a hydrophilic carotenoid with diverse functions in biological systems. Steady-state spectroscopy demonstrates notable changes in absorption and fluorescence spectra, characterized by a pH-dependent blue shift and altered resolution of vibrational bands. Transient absorption spectra further elucidate these effects, highlighting a significant blue shift in the S-S peak with increasing pH.

Direct observation of triplet energy transfer between chlorophylls and carotenoids in the core antenna of photosystem I from Thermosynechococcus elongatus.

Quenching of chlorophyll triplet states by carotenoids is an essential photoprotective process, which prevents formation of reactive singlet oxygen in photosynthetic light-harvesting complexes. The process is usually very efficient in oxygenic organisms under physiological conditions, thus preventing any observable accumulation of chlorophyll triplets. However, it subsequently prevents also the determination of the triplet transfer rate.