Short-Range Effects in the Special Pair of Photosystem II Reaction Centers: The Nonconservative Nature of Circular Dichroism

Photosystem II reaction centers extract electrons from
water, providing
the basis of oxygenic life on earth. Among the light-sensitive pigments
of the reaction center, a central chlorophyll a dimer,
known as the special pair, so far has escaped a complete theoretical
characterization of its excited state properties. The close proximity
of the special pair pigments gives rise to short-range effects that
comprise a coupling between local and charge transfer (CT) excited
states as well as other intermolecular quantum effects. Using a multiscale
simulation and a diabatization technique, we show that the coupling
to CT states is responsible for 45% of the excitonic coupling in the
special pair. The other short-range effects cause a nonconservative
nature of the circular dichroism spectrum of the reaction center by
effectively rotating the electric transition dipole moments of the
special pair pigments inverting and strongly enhancing their intrinsic
rotational strength.