A Theoretical Investigation of Xanthophyll–Protein Hydrogen Bonding in the Photosystem II Antenna

Photoprotective nonphotochemical quenching (NPQ) in higher
plants
is the result of the formation of energy-quenching traps in the light-harvesting
antenna of photosystem II (PSII). The primary driving forces behind
NPQ are the protonation of the thylakoid lumen and the de-epoxidation
of the xanthophyll violaxanthin to zeaxanthin in the antenna. There
is currently some disagreement over whether de-epoxidation occurs
only at the peripheral, V1, binding site of the major LHCII or also
at the internal, L2, site of the minor antenna CP29 complex of PSII.
We have used density functional theory (DFT) to study of hydrogen
bonding between xanthophylls and the protein scaffold of LHCII and
CP29. We argue that a lack of hydrogen bonding for violaxanthin in
LHCII is consistent with it being weakly bound and accessible for
de-epoxidation. Conversely, the strong violaxanthin–protein
hydrogen bonding at the L2 site of CP29 is consistent with evidence
that it is not readily accessible for de-epoxidation and therefore
quenching by zeaxanthin at the L2 of CP29 is an unlikely candidate
for in vivo NPQ.