The trimeric main light-harvesting complex (LHC-II) is the only antenna complex of higher plants of which a high-resolution 3D structure has been obtained (Kühlbrandt, W., Wang, D., and Fujiyoshi, Y. (1994) Nature 367, 614-621) and which can be refolded in vitro from its components. Four different recombinant forms of LHC-II, each with a specific chlorophyll (Chl) binding site removed by site-directed mutagenesis, were refolded from heterologously overexpressed apoprotein, purified pigments, and lipid. Absorption spectra of mutant LHC-II were measured in the temperature range from 4 to 300 K and compared to likewise refolded wild-type complex and to native LHC-II isolated from pea chloroplasts. Chls at different binding sites have characteristic, well-defined absorption sub-bands. Mixed occupation of binding sites with Chls a and b is not observed. Temperature-dependent changes of the mutant absorption spectra reveal a consistent shift of the major difference bands but an irregular behavior of minor bands. A model of the spectral substructure of LHC-II is proposed which accounts for the different absorption properties of the 12 individual Chls in the complex, thus establishing a first consistent correlation between the 3D structure of LHC-II and its spectral properties. The spectral substructure is valid for recombinant and native LHC-II, indicating that both have the same spatial arrangement of Chls and that the refolded complex is fully functional.

In the present study the rate of triplet transfer from chlorophyll to carotenoids in solubilized LHCII was investigated by flash spectroscopy using laser pulses of approximately 2 ns for both pump and probe. Special attention has been paid to calibration of the experimental setup and to avoid saturation effects. Carotenoid triplets were identified by the pronounced positive peak at approximately 507 nm in the triplet-singlet difference spectra. DeltaOD (507 nm) exhibits a monoexponential relaxation kinetics with characteristic lifetimes of 2-9 micros (depending on the oxygen content) that was found to be independent of the pump pulse intensity. The rise of DeltaOD (507 nm) was resolved via a pump probe technique where an optical delay of up to 20 ns was used. A thorough analysis of these experimental data leads to the conclusion that the kinetics of carotenoid triplet formation in solubilized LHCII is almost entirely limited by the lifetime of the excited singlet state of chlorophyll but neither by the pulse width nor by the rate constant of triplet-triplet transfer. Within the experimental error the rate constant of triplet-triplet transfer from chlorophyll to carotenoids was estimated to be kTT > (0.5 ns)-1. This value exceeds all data reported so far by at least one order of magnitude. The implications of this finding are briefly discussed.

Line-narrowed and temperature-dependent fluorescence spectra are reported for the solubilized trimeric lightharvesting complex of Photosystem II (LHC II). Special attention has been paid to eliminate effects owing to reabsorption and to ensure that the line-narrowed fluorescence spectra are virtually unaffected by hole burning or scattering artifacts. Analysis of line-narrowed fluorescence spectra at 4.2 K indicates that the lowest Q y -state of LHC II is characterized by weak electron-phonon coupling with a Huang-Rhys factor of ∼ 0.9 and a broad and strongly asymmetric one-phonon profile with a peak frequency ω m of 15 cm -1 and a width of Γ ) 105 cm -1 . The 4.2 K fluorescence data are further consistent with the assignment of the lowest Q y -state at ∼ 680.0 nm and an inhomogeneous width of ∼80 cm -1 gathered from a recent hole-burning study (Pieper et al. J. Phys. Chem. A 1999, 103, 2412. The temperature dependence of the fluorescence spectra of LHC II is simulated using the low-energy Q y -level structure reported in the latter study as well as the parameters of electron-phonon coupling determined in the present study. Up to a temperature of 120 K, the calculations reveal that this model satisfactorily describes the basic features of the fluorescence spectra such as thermal broadening and, especially, the blue-shift of the fluorescence peak with increasing temperature. An unexpected red-shift of the fluorescence peak above 150 K is attributed to conformational changes of the protein environment. The shape of the temperature-dependent fluorescence spectra indicates that the low-energy Q ystates are populated according to a Boltzmann distribution representing the thermal equilibrium of excitation energy.

We measured the absolute lengths of three single crystal silicon samples by means of an imaging Twyman-Green interferometer in the temperature range from 7 K to 293 K with uncertainties of about 1 nm. From these measurements we extract the coefficient of thermal expansion with uncertainties in the order of 1 × 10 −9 /K. To access the functional dependence of the length on the temperature usually polynomials are fitted to the data. Instead we use a physically motivated model equation with 7 fit parameters for the whole temperature range. The coefficient of thermal expansion is obtained from the derivative of the best fit. The measurements conducted in 2012 and 2014 demonstrate a high reproducibility and the agreement of two independently produced samples supports single crystal silicon as reference material for thermal expansion. Although the results for all three samples agree with each other and with measurements performed at other institutes, they significantly differ from the recommended values for thermal expansion of crystalline silicon.