title: A comparison of the three isoforms of the light-harvesting complex II using transient absorption and time-resolved fluorescence measurements.
abstract: In this article we report the characterization of the energy transfer process in the reconstituted isoforms of the plant light-harvesting complex II. Homotrimers of recombinant Lhcb1 and Lhcb2 and monomers of Lhcb3 were compared to native trimeric complexes. We used low-intensity femtosecond transient absorption (TA) and time-resolved fluorescence measurements at 77 K and at room temperature, respectively, to excite the complexes selectively in the chlorophyll b absorption band at 650 nm with 80 fs pulses and on the high-energy side of the chlorophyll a absorption band at 662 nm with 180 fs pulses. The subsequent kinetics was probed at 30-35 different wavelengths in the region from 635 to 700 nm. The rate constants for energy transfer were very similar, indicating that structurally the three isoforms are highly homologous and that probably none of them play a more significant role in light-harvesting and energy transfer. No signature has been found in the transient absorption measurements at 77 K for Lhcb3 which might suggest that this protein acts as a relative energy sink of the excitations in heterotrimers of Lhcb1/Lhcb2/Lhcb3. Minor differences in the amplitudes of some of the rate constants and in the absorption and fluorescence properties of some pigments were observed, which are ascribed to slight variations in the environment surrounding some of the chromophores depending on the isoform. The decay of the fluorescence was also similar for the three isoforms and multi-exponential, characterized by two major components in the ns regime and a minor one in the ps regime. In agreement with previous transient absorption measurements on native LHC II complexes, Chl b --> Chl a energy transfer exhibited very fast channels but at the same time a slow component (ps). The Chls absorbing at around 660 nm exhibited both fast energy transfer which we ascribe to transfer from 'red' Chl b towards 'red' Chl a and slow transfer from 'blue' Chl a towards 'red' Chl a. The results are discussed in the context of the new available atomic models for LHC II.
authors.affiliation: Department of Biophysics and Physics of Complex Systems, Division of Physics and Astronomy, Faculty of Sciences, Vrije Universiteit, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands. miguel.palacios@philips.com
authors.forename: Miguel A
authors.initials: MA
authors.lastname: Palacios
authors.forename: Joerg
authors.initials: J
authors.lastname: Standfuss
authors.forename: Mikas
authors.initials: M
authors.lastname: Vengris
authors.forename: Bart F
authors.initials: BF
authors.lastname: van Oort
authors.forename: Ivo H M
authors.initials: IH
authors.lastname: van Stokkum
authors.forename: Werner
authors.initials: W
authors.lastname: Kühlbrandt
authors.forename: Herbert
authors.initials: H
authors.lastname: van Amerongen
authors.forename: Rienk
authors.initials: R
authors.lastname: van Grondelle
mesh_terms: D015502:Absorptiometry, Photon; D000042:Absorption; D002734:Chlorophyll / Q000737:chemistry / Q000378:metabolism; D000077194:Chlorophyll A; D004735:Energy Transfer; D005453:Fluorescence; D045332:Photosystem II Protein Complex / Q000737:chemistry* / Q000378:metabolism*; D010940:Plant Proteins / Q000737:chemistry / Q000378:metabolism; D020033:Protein Isoforms / Q000737:chemistry / Q000378:metabolism; D018724:Spinacia oleracea; D013997:Time Factors
chemical_list: D045332:Photosystem II Protein Complex; D010940:Plant Proteins; D020033:Protein Isoforms; D002734:Chlorophyll; D000077194:Chlorophyll A