Simulations of the polarisation-dependent Raman intensity of -carotene in photosystem II crystals

In order to clarify possibilities to identify the β -carotene ( β -Car) radicals in secondary electron transfer (ET) reactions in the photosystem II core complex (PSIIcc), Raman intensities of all 96 β -Car cofactors in the unit cell of PSIIcc-dimer crystals as a function of polarisation and crystal orientation were simulated based on the 2.9 A resolution structure. The Raman-active symmetry A g in the C 2 h group is assigned to the β -Car modes ν 66 and ν 67 . Simulations are in agreement with experiment for off-resonant excitation at 1064 nm. Resonant measurements at 476 and 532 nm excitation can not be explained, which is attributed to mode mixing in the excited state and the existence of different spectral pools. The identity of the β -Car oxidised in secondary ET can not be resolved by Raman measurements on PSIIcc-dimer crystals. Additional simulations show that similar measurements on PSIIcc-monomer crystals could provide a possible route to solve this issue.