Photoprotective non-photochemical quenching (NPQ) in higher plants is the result of the formation of energy quenching traps in the light-harvesting antenna of photosystem II (PSII). It has been proposed that this quenching trap is a lutein molecule closely associated with the chlorophyll terminal emitter of the major light-harvesting complex LHCII. We have used a combination of time-dependent density functional theory (TD-DFT) and the semiempirical MNDO-CAS-CI method to model the chlorophyll-lutein energy transfer dynamics of the highly quenched crystal structure of LHCII. Our calculations reveal that the incoherent "hopping" of energy from Chla612 to the short-lived, dipole forbidden 2(1)A(g)(-) state of lutein620 accounts for the strong fluorescence quenching observed in these crystals. This adds weight to the argument that the same dissipative pathway is responsible for in vivo NPQ.

Raman and electronic absorption spectra corresponding to the S0-S2 electronic transition of various carotenoid and polyene molecules are theoretically analyzed using the density functional theory (DFT) approach. The results demonstrate the linear dependence between the frequency of the so-called ν1 band corresponding to the C═C stretching modes in the Raman spectra and the S0-S2 electronic transition for molecules of different conjugation lengths. From these calculations the following relationship have been identified: (i) the effective conjugation length shortens in conformers of carotenoids containing β-rings whereas it increases in polyene upon s-cis isomerization at their ends, (ii) methyl groups connected to the conjugated chain of carotenoids induce a splitting of the ν1 band in the Raman spectra, (iii) the effective conjugation lengths of all-trans-polyenes and corresponding all-trans-carotenoids are the same as follows from the Raman ν1 frequency, but they are different as defined from S0-S2 electronic transition energies. The results well correlate with the experimental observations.

Quantum chemical calculations have been employed for the investigation of the lowest excited electronic states of lutein, with particular reference to its function within light harvesting antenna complexes of higher plants. Through comparative analysis obtained by using different methods based on gas-phase calculations of the spectra, it was determined that variations in the lengths of the long C-C valence bonds and the dihedral angles of the polyene chain are the dominant factors in determining the spectral properties of Lut 1 and Lut 2 corresponding to the deformed lutein molecules taken from crystallographic data of the major pigment-protein complex of photosystem II. By MNDO-CAS-CI method, it was determined that the two singlet B(u) states of lutein (nominally 1B(u)(-)* and 1B(u)(+)) arise as a result of mixing of the canonical 1B(u)(-) and 1B(u)(+) states of the all-trans polyene due to the presence of the ending rings in lutein. The 1B(u)(-)* state of lutein is optically allowed, while the 1B(u)(-) of a pure all-trans polyene chain is optically forbidden. As demonstrated, the B(u) states are much more sensitive to minor distortions of the conjugated chain due to mixing of the canonical states, resulting in states of poorly defined particle-hole symmetry. Conversely, the A(g) states are relatively robust with respect to geometric distortion, and their respective inversion and particle-hole symmetries remain relatively well-defined.