Investigating excited state dynamics of salinixanthin and xanthorhodopsin in the near-infrared

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 S₂ → S₁ and from S₁ → S₀ of the carotene. (2) The rate of the internal conversion from S₂ → S₁ 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) S₁ of salinixanthin is verified to lie ∼12700 cm^-1 above the ground electronic surface, excluding its involvement in the retinal sensitization in XR. (4) The oscillator strength of the S₁ → S₂ 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 S₂ lifetimes and kinetic schemes used to model these data are discussed.