Bisignate resonance Raman optical activity: a pseudo breakdown of the single electronic state model of RROA?

Abstract

Raman optical activity (ROA) spectra were calculated for a set of conformers of astaxanthin, which is a non‐rigid molecule exhibiting strong resonance enhancement in the visible range. Single electronic state theory of the Resonance ROA (RROA) predicts the spectrum to be monosigned. For astaxanthin, it appeared that some of the conformers exhibit different sign of the bands than the other conformers. As a result, the conformer population averaged spectrum of astaxanthin can exhibit both signs of the bands, or be monosigned depending on which conformers are dominating, that reflects a departure from the single electronic state approximation. Moreover, use of different basis sets and/or density functional theory (DFT) functionals results in different conformer populations, thus yielding again either monosigned ROA spectrum or with bands of both signs. Consequences of these two findings for the astaxanthin RROA spectrum are discussed. Copyright © 2014 John Wiley & Sons, Ltd.

Structure of supramolecular astaxanthin aggregates revealed by molecular dynamics and electronic circular dichroism spectroscopy

“…Arbitrary models of H and J-aggregates of (3S,3 0 S)-AXT were created from 10 and 8 molecules in the most stable conformation (-GG). 16,19,41 The monomer was optimized at the CAM-B3LYP/ Def2SVP theoretical level, using the Gaussian 09 E.01 package. 53 The initial geometries were constructed according to the structures previously proposed for similar H and J-aggregates of carotenoids; their spectra modelled on the basis of exciton theory and supramolecular exciton chirality reproduced the experimental data reasonably well.…”

“…Chiroptical spectroscopy, such as electronic circular dichroism (ECD) and Raman optical activity (ROA) were found to be very convenient to study aggregation. 19,27,29,32,35,40,41 In addition, we reported a new phenomenon, aggregation-induced resonance Raman optical activity (AIRROA), which seems to be general for chiral xanthophylls, such as astaxanthin, zeaxanthin, lutein and its derivatives. [24][25][26][27] The importance of AIRROA lies in the enormous enhancement of the ''ordinary'' ROA spectra, as these would be immeasurably weak for monomeric (non-aggregated) molecules.…”

The Astaxanthin Aggregation Pattern Greatly Influences Its Antioxidant Activity: A Comparative Study in Caco-2 Cells

“…As the predominant form of natural AST, all-trans-astaxanthin is found to have two different aggregation forms, H-and J-type aggregates, in addition to its monomer form. Although it is well accepted that the structure of AST determines its biological properties, few works could be found to reveal the relationship between AST aggregates and their physical and chemical properties due to the difficulties in obtaining stable AST aggregates [19,20]. Recently, we prepared two colloidal systems with differently aggregated AST to investigate the effect of dehydrated solvent on AST aggregation [21].…”

Vibrational optical activity: From discovery and development to future challenges

“…Since these early discoveries, studies of RROA have focused on a variety of different applications and departures from the SES limit when, for example, one or more additional electronic states become involved in the generation of the RROA spectrum . A conceptual beauty of the SES‐RROA theory is that it gives insight into the contributions of individual excited electronic states to ROA spectra of which there are many simultaneously when the incident laser radiation is no longer in resonance with one or a few excited electronic states.…”

“…For VCD, LLES corresponds to enhanced intensities, and for ROA, it is resonance (RROA) or pre‐resonance ROA, both of which have been discussed above. Particularly important is the development of computational methods that permit LLES VCD and resonance or pre‐resonance ROA calculations where the standard methods for calculations breakdown.…”