Environmental effects on vibrational properties of carotenoids: experiments and calculations on peridinin

Abstract

Carotenoids are employed in light-harvesting complexes of dinoflagellates with the two-fold aim to extend the spectral range of the antenna and to protect it from radiation damage. We have studied the effect of the environment on the vibrational properties of the carotenoid peridinin in different solvents by means of vibrational spectroscopies and QM/MM molecular dynamics simulations. Three prototypical solvents were considered: cyclohexane (an apolar/aprotic solvent), deuterated acetonitrile (a polar/aprotic solvent) and methanol (a polar/protic solvent). Thanks to effective normal mode analysis, we were able to assign the experimental Raman and IR bands and to clarify the effect of the solvent on band shifts. In the 1500-1650 cm(-1) region, seven vibrational modes of the polyene chain were identified and assigned to specific molecular vibrations. In the 1700-1800 cm(-1) region a strong progressive down-shift of the lactonic carbonyl frequency is observed passing from cyclohexane to methanol solutions. This has been rationalized here in terms of solvent polarity and solute-solvent hydrogen bond interactions. On the basis of our data we propose a classification of non-equivalent peridinins in the Peridinin-Chlorophyll-Proteins, light-harvesting complexes of dinoflagellates.

Can far-IR action spectroscopy combined with BOMD simulations be conformation selective?

“…The quality of the potential energy surface is entirely contained in the ''ab initio'' force field used in the dynamics, calculated at the DFT/BLYP + dispersion level in the work presented here. The good to excellent agreement of the absolute (and relative) positions of the different active bands obtained in our theoretical works (see for instance dynamical spectra in the gas phase, [51][52][53][54][55] in the liquid phase, 43,[56][57][58][59] and at solid-liquid and liquid-air interfaces [60][61][62] ) is a demonstration that this level of theory is correct.…”

Theoretical Methods for Vibrational Spectroscopy and Collision Induced Dissociation in the Gas Phase

“…3.2). The good to excellent agreements of the absolute (and relative) positions of the different active bands obtained in our theoretical works (see for instance dynamical spectra in the gas phase [37][38][39][40][41], in the liquid phase [31,[42][43][44][45], and at solid-liquid and liquid-air interfaces [46][47][48]) is a demonstration (though a posteriori) that this level of theory is correct. -Even more crucial in this discussion, the calculation of IR spectra with MD is related only to the time-dependent dipole moment of the molecular system, requiring neither any harmonic expansion of the transition dipole moment nor the knowledge of normal modes, in contrast to harmonic calculations.…”

Femtosecond transient infrared and stimulated Raman spectroscopy shed light on the relaxation mechanisms of photo-excited peridinin

“…Assignments have been proposed in the literature, also based on quantum chemical computations. 34,35 Vanishing intensity is expected for the ester C= =O in the Raman spectrum, while it should be visible in the FTIR spectrum; detectable Raman activity is predicted for the lactonic C= =O because of the conjugation with the polyene chain.…”