Distortions of the Xanthophylls Caused by Interactions with Neighboring Pigments and the LHCII Protein Are Crucial for Studying Energy Transfer Pathways within the Complex

It has been proposed that photoprotective
non-photochemical quenching
(NPQ) in higher plants arises from a conformational change in the
antenna which alters pigment–pigment interactions. This brings
about the formation of energy quenching “traps” that
capture and dissipate excitation energy as heat. We have used the
semiempirical AM1-CAS-CI method combined with the transition density
cube (TDC) approach to model chlorophyll (Chl) to xanthophyll (Xanth)
resonant Coulomb couplings in the crystal structure of LHCII. Due
to its proposed role as the NPQ quenching site we have focused on
lutein interactions and have explored how distortions to lutein conformation,
as well as interpigment distances and relative orientations, affect
this coupling. Our calculations indicate that distortions as well
as Chl-lutein angle have a significant effect on coupling, whereas
interpigment distances have a relatively minor effect. We therefore
conclude that particular attention to the distortions of the Xanths
should be given for calculation of energy transfer pathways and study
of the NPQ mechanism.