Properties of zeaxanthin and its radical cation bound to the minor light-harvesting complexes CP24, CP26 and CP29.
Nonphotochemical quenching (NPQ) is a fundamental mechanism in photosynthesis by which plants protect themselves against excess excitation energy and which is thus of crucial importance for plant survival and fitness. Recently, carotenoid radical cation (Car(*+)) formation has been discovered to be a key step in the feedback deexcitation quenching component (qE) of NPQ, whose molecular mechanism and location remains elusive. A recent model for qE suggests that the replacement of violaxanthin (Vio) by zeaxanthin (Zea) in photosynthetic pigment binding pockets can in principle result in qE via the so-called "gear-shift" or electron transfer quenching mechanisms. We performed pump-probe measurements on individual antenna complexes of photosystem II (CP24, CP26 and CP29) upon excitation of the chlorophylls (Chl) into their first excited Q(y) state at 660 nm when either Vio or Zea was bound to those complexes. The Chl lifetime was then probed by measuring the decay kinetics of the Chl excited state absorption (ESA) at 900 nm. The charge-transfer quenching mechanism, which is characterized by a spectral signature of the transiently formed Zea radical cation (Zea(*+)) in the near-IR, has also been addressed, both in solution and in light-harvesting complexes of photosystem II (LHC-II). Applying resonant two-photon two-color ionization (R2P2CI) spectroscopy we show here the formation of beta-Car(*+) in solution, which occurs on a femtosecond time-scale by direct electron transfer to the solvent. The beta-Car(*+) maxima strongly depend on the solvent polarity. Moreover, our two-color two-photon spectroscopy on CP29 reveals the spectral position of Zea(*+) in the near-IR region at 980 nm.
Amarie
Sergiu
S
Institute for Physical and Theoretical Chemistry, Johann Wolfgang Goethe-University Frankfurt, Max-von-Laue-Str. 7, 60438 Frankfurt am Main, Germany.
Wilk
Laura
L
Barros
Tiago
T
Kühlbrandt
Werner
W
Dreuw
Andreas
A
Wachtveitl
Josef
J
D029681:Arabidopsis Proteins / Q000737:chemistry / Q000378:metabolism; D002414:Cations, Monovalent; D002734:Chlorophyll / Q000737:chemistry / Q000378:metabolism; D061146:Chlorophyll Binding Proteins; D004579:Electron Transport; D005609:Free Radicals; D045342:Light-Harvesting Protein Complexes / Q000737:chemistry* / Q000378:metabolism*; D008954:Models, Biological; D055668:Photochemical Processes; D045332:Photosystem II Protein Complex / Q000737:chemistry* / Q000378:metabolism*; D010940:Plant Proteins / Q000737:chemistry* / Q000378:metabolism*; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D013053:Spectrophotometry; D024341:Xanthophylls / Q000737:chemistry* / Q000378:metabolism*; D065146:Zeaxanthins
D029681:Arabidopsis Proteins; C069660:CP29 light harvesting complex; D002414:Cations, Monovalent; D061146:Chlorophyll Binding Proteins; D005609:Free Radicals; D045342:Light-Harvesting Protein Complexes; D045332:Photosystem II Protein Complex; D010940:Plant Proteins; D011994:Recombinant Proteins; D024341:Xanthophylls; D065146:Zeaxanthins; D002734:Chlorophyll; C005613:violaxanthin