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Modular photosynthetic LH1 complex is applied as a model system to investigate the thermodynamics of a self-assembling membrane protein and the effects of cosolvents and cofactor (carotenoid) on the process. Native chromophores of LH1, bacteriochlorophyll, and carotenoid are excellent intrinsic spectroscopic reporter molecules. Their presence allows us to follow the association of transmembrane helices of LH1, without the use of any external markers, by electronic absorption/emission and circular dichroism. Furthermore, the assembly correctness can be monitored by the intracomplex energy transfer. Both the cosolvent and carotenoid markedly affect ΔH° and ΔS° associated with the complex formation in detergent, but the driving force of the process remains almost constant due to an efficient enthalpy−entropy compensation in the system. In the absence of cosolvent and cofactor, the energy of interactions between transmembrane helices in LH1 equals −580 kJ/mol. ΔH° drastically increases upon the addition of acetone (−1160 kJ/mol) and carotenoid (−1900 kJ/mol), whereas ΔS° lowers from +1.5 kJ/mol·K to −0.4 kJ/mol·K and to −2.6 kJ/mol·K, respectively. The stabilization of the ensemble by cofactor seems to be due to the π−π stacking of aromatic residues of LH1 polypeptides with the carotenoid π-electron system. The cosolvent, lowering the medium permittivity and thus enhancing helix−helix interactions, has an ordering effect on the system (ΔS° < 0). This effect of cosolvent on ΔH° and ΔS° of association of transmembrane helices is relevant for crystallization of membrane proteins, as it explains in thermodynamic terms the action of amphiphiles used for crystallization of membrane proteins in the micellar phase. [ABSTRACT FROM AUTHOR]

Pyrazoloquinolines are highly fluorescent, both in liquid solutions and in the solid state, which makes them good candidates for various optical devices. The aim of the current work is to understand the photochemical behavior of pyrazolo[3,4-b]quinoline (PQ), which is quite complicated since in n-alkane solvents PQ tends to form strong complexes with protic solvent constituents (often present as minor impurities), as well as dimers. Both types of H-bond complexes were studied systematically by temperature-dependent conventional absorption and fluorescence spectroscopy; the effect of protic solvent constituents was mimicked by varying the ethanol concentration in n-octane in the range from 0.0 to 0.8%. At room temperature the PQ:ethanol association constant was estimated at 80 M−1and the dimerization constant at 2 × 103M−1. Dimer formation is enhanced upon lowering the temperature in pure n-alkane down to 220 K, and the fluorescence is strongly reduced since the dimer is nonfluorescent. Surprisingly, when irradiating a frozen sample for several minutes at very low temperatures (<40 K), a narrow-banded Shpol’skii-type fluorescence spectrum gradually appears. To explain this unusual photochemical behavior, PQ and its deuterated analogue were studied using low-temperature absorption and fluorescence spectroscopy over the 300−5 K temperature range. In the case of normal (protonated) PQ, very fast excited-state intermolecular double proton transfer is responsible for the efficient quenching of PQ dimer fluorescence. Deuteration significantly slows down this proton transfer process, and in that case under cryogenic conditions a fluorescent dimer is observed. Photoirradiation under cryogenic conditions leads to molecular rearrangement of the dimers and the appearance of monomer spectra. For both H-PQ and D-PQ, these processes were found to be reversible. A simplified reaction scheme, in which the excited tautomeric dimer plays a crucial role, is presented to explain the observations. [ABSTRACT FROM AUTHOR]