TY  - JOUR
A1  - Betke, Alexander
A1  - Lokstein, Heiko
T1  - Two-photon excitation spectroscopy of photosynthetic light-harvesting complexes and pigments
JF  - Faraday discussions
N2  - In addition to (bacterio)chlorophylls, (B)Chls, light-harvesting complexes (LHCs) bind carotenoids, and/or their oxygen derivatives, xanthophylls. Xanthophylls/carotenoids have pivotal functions in LHCs: in stabilization of the structure, as accessory light-harvesting pigments and, probably most importantly, in photoprotection. Xanthophylls are assumed to be involved in the not yet fully understood mechanism of energy-dependent (qE) non-photochemical quenching of Chl fluorescence (NPQ) in higher plants and algae. The so called "xanthophyll cycle" appears to be crucial in this regard. The molecular mechanism(s) of xanthophyll involvement in qE/NPQ have not been established, yet. Moreover, excitation energy transfer (EET) processes involving carotenoids are also difficult to study, due to the fact that transitions between the ground state (S-0, 1(1)A(g)(-)) and the lowest excited singlet state (S-1, 2(1)A(g)(-)) of carotenoids are optically one-photon forbidden ("dark"). Two-photon excitation spectroscopic techniques have been used for more than two decades to study one-photon forbidden states of carotenoids. In the current study, two-photon excitation profiles of LHCII samples containing different xanthophyll complements were measured in the presumed 1(1)A(g)(-) -> 2(1)A(g)(-) (S-0 -> S-1) transition spectral region of the xanthophylls, as well as for isolated chlorophylls a and b in solution. The results indicate that direct two-photon excitation of Chls in this spectral region is dominant over that by xanthophylls. Implications of the results for proposed mechanism(s) of qE/NPQ will be discussed.
Y1  - 2019
U6  - https://doi.org/10.1039/c8fd00198g
SN  - 1359-6640
SN  - 1364-5498
VL  - 216
SP  - 494
EP  - 506
PB  - Royal Society of Chemistry
CY  - Cambridge
ER  - 
TY  - JOUR
A1  - Lokstein, Heiko
A1  - Betke, Alexander
A1  - Krikunova, Maria
A1  - Teuchner, Klaus
A1  - Voigt, Bernd
T1  - Elucidation of structure-function relationships in plant major light-harvesting complex (LHC II) by nonlinear spectroscopy
JF  - Photosynthesis research
N2  - Conventional linear and time-resolved spectroscopic techniques are often not appropriate to elucidate specific pigment-pigment interactions in light-harvesting pigment-protein complexes (LHCs). Nonlinear (laser-) spectroscopic techniques, including nonlinear polarization spectroscopy in the frequency domain (NLPF) as well as step-wise (resonant) and simultaneous (non-resonant) two-photon excitation spectroscopies may be advantageous in this regard. Nonlinear spectroscopies have been used to elucidate substructure(s) of very complex spectra, including analyses of strong excitonic couplings between chlorophylls and of interactions between (bacterio) chlorophylls and "optically dark'' states of carotenoids in LHCs, including the major antenna complex of higher plants, LHC II. This article shortly reviews our previous study and outlines perspectives regarding the application of selected nonlinear laser-spectroscopic techniques to disentangle structure-function relationships in LHCs and other pigment-protein complexes.
KW  - Excitonic interactions
KW  - Laser spectroscopy
KW  - Light-harvesting complex (LHC II)
Y1  - 2012
U6  - https://doi.org/10.1007/s11120-011-9700-y
SN  - 0166-8595
VL  - 111
IS  - 1-2
SP  - 227
EP  - 235
PB  - Springer
CY  - Dordrecht
ER  -