Xanthorhodopsin (xR) is a recently discovered retinal protein that contains, in addition to the retinal chromophore, a carotenoid (salinixanthin) absorbing at 456, 486, and 520 nm, which functions as a light-harvesting antenna. We have studied the interactions between the two chromophores by monitoring the absorbance and circular dichroism (CD) spectroscopies of artificial pigments derived from synthetic retinal analogues characterized by shifted absorption maxima. In addition, we have followed the binding process of the synthetic chromophores to the apomembrane of xR. We have revealed that the CD spectrum of xR originated mainly from the carotenoid chromophore without a significant contribution of the retinal chromophore. Because the binding process rate of these analogues is slower compared to all-trans retinal, it was possible to detect and analyze the major alterations in the CD spectrum. It was revealed that the main changes occur as a result of binding site occupation by the retinal chromophore and not because of the formation of the retinal-protein covalent bond.

Xanthorhodopsin is a light-driven proton pump in the extremely halophilic bacterium Salinibacter ruber. Its unique feature is that besides retinal it has a carotenoid, salinixanthin, with a light harvesting function. Tight and specific binding of the carotenoid antenna is controlled by binding of the retinal. Addition of all-trans retinal to xanthorhodopsin bleached with hydroxylamine restores not only the retinal chromophore absorption band, but causes sharpening of the salinixanthin bands reflecting its rigid binding by the protein. In this report we examine the correlation of the changes in the two chromophores during bleaching and reconstitution with native all-trans retinal, artificial retinal analogues, and retinol. Bleaching and reconstitution both appear to be multi-stage processes. The carotenoid absorption changes during bleaching occurred not only uponhydrolysis of the Schiff base but continued while the retinal was leaving its binding site. In the case of reconstitution, the 13-desmethyl analogue formed the protonated Schiff base slower than retinal, and provided the opportunity to observe changes in carotenoid binding at various stages. The characteristic sharpening of the carotenoid bands, indicative of its reduced conformational heterogeneity in the binding site, occurs already when the retinal occupies the binding site but the covalent bond to Lys-227 via a Schiff base is not yet formed. This is confirmed by the results for retinol reconstitution, where the Schiff base does not form but the carotenoid exhibits its characteristic spectral change from the binding.

Excited-state dynamics of xanthorhodopsin (XR) and of salinixanthin (SX) in ethanol were investigated by ultrafast pump-hyperspectral probe spectroscopy. Following excitation to the strongly allowed S(2) state of the SX chromophore, transient spectra were recorded photoselectively in the range 430-850 nm. Global kinetic analysis of these data shows the following. (1) Efficient energy transfer from S(2) of the SX in XR to its retinal moiety is verified here. The lifetime of S(2) in SX is, however, determined to be approximately 20 fs, much shorter than previously reported. (2) Branching ratios of excitation transfer from S(2) to S(1), to S*, and to retinal in XR are measured leading to species associated difference spectra (SADS) for all the states involved. Strong protein effects are detected on these branching probabilities. (3) S(1) and S* absorption bands in both systems exhibit anisotropy well below the expected r = 0.4, indicating an angle of approximately 25 degrees between the S(0) --> S(2) and S(1) --> S(n)/S* --> S(n) transition dipoles. The latter allows confident assignment of the debated S* absorption band to an excited state of SX, and not to "hot" S(0). In light of the extremely fast IC from S(2) to lower excited singlets, possible involvement of ballistic IC in SX, and of coherent energy transfer in XR, are discussed.

The octahedral titanium catalyst Ti(Ph2PNpy)2(NEt2)2 (1) was found to be an efficient catalyst for the hydroamination of methylenecyclopropane with either aromatic or aliphatic amines. For a symmetrical methylenecyclopropane, only one product is obtained in high yield. The reaction of complex 1 with ethylamine and aniline produces the dimeric titanium complexes Ti2(Ph2PNpy)2(μ-NEt)2 and Ti2(Ph2PNpy)2(μ-NPh)2, respectively.

Excited state dynamics of native Xanthorhodopsin (XR), of an XR sample with a reduced prosthetic group, and of the associated Carotenoid (CAR) salinixanthin (SX) in ethanol were investigated by hyperspectral Near Infrared (NIR) probing. Global kinetic analysis shows that: (1) unlike the transient spectra recorded in the visible, fitting of the NIR data requires only two phases of exponential spectral evolution, assigned to internal conversion from S(2) → S(1) and from S(1) → S(0) of the carotene. (2) The rate of the internal conversion from S(2) → S(1) in the reduced sample is well fit with a decay time of 130 fs, significantly longer than in XR and in SX, both of which are well fit with τ ≈ 100 fs. This increased lifetime is consistent with a ∼30% efficiency of ET from SX to retinal in XR. (3) S(1) of salinixanthin is verified to lie ∼12,700 cm(-1) above the ground electronic surface, excluding its involvement in the retinal sensitization in XR. (4) The oscillator strength of the S(1) → S(2) transition is determined to be no more than 0.16, despite its symmetry allowedness. (5) No long lived NIR absorbance decay assignable to the carotenoid S* state was detected in any of the samples. Inconsistencies concerning previously determined S(2) lifetimes and kinetic schemes used to model these data are discussed.