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

Grzegorz Zajac,Malgorzata Baranska,Jiří Kessler,Petr Bouř,Ewa Machalska,Agnieszka Kaczor

doi:10.1039/c8cp01742e

Abstract

Biomolecular aggregation is omnipresent in nature and important for metabolic processes or in medical treatment; however, the phenomenon is rather difficult to predict or understand on the basis of computational models. Recently, we found that electronic circular dichroism (ECD) spectroscopy and closely related resonance Raman optical activity (RROA) are extremely sensitive to the aggregation mechanism and structure of the astaxanthin dye. In the present study, molecular dynamics (MD) and quantum chemical (QC) computations (ZIndo/S, TDDFT) are used to link the aggregate structure with ECD spectral shapes. Realistic absorption and ECD intensities were obtained and the simulations reproduced many trends observed experimentally, such as the prevalent sign pattern and dependence of the aggregate structure on the solvent type. The computationally cheaper ZIndo/S method provided results very similar to those obtained by TDDFT. In the future, the accuracy of the combined MD/QC methodology of spectra interpretation should be improved to provide more detailed information on astaxanthin aggregates and similar macromolecular systems.

Highlights

We concentrate on the astaxanthin dye (3,30-dihydroxy-b,bcarotene-4,40-dione, AXT, Fig. 1), because it is a convenient system to study, important in biology and potentially useful in human medicine
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 molecules

Summary

Introduction

We concentrate on the astaxanthin dye (3,30-dihydroxy-b,bcarotene-4,40-dione, AXT, Fig. 1), because it is a convenient system to study, important in biology and potentially useful in human medicine. The H-aggregates are characterised by tight ‘‘card-pack’’ stacking where the polyene chains are more or less parallel to each other, while J-aggregates with ‘‘head-to-tail’’ or ‘‘herring bone’’ orientation of the conjugated chains are looser.[26,33,34,38,39] Formation of a particular assembly depends on the balance of intermolecular forces between molecular constituents, such as hydrogen bonds, electrostatic forces, and van der Waals interactions.[30,34,36] In spite of gross phenomenological models of the H and J aggregates, detailed mutual arrangements of the monomers in them is not known 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 EA and ECD curves were obtained by convolution with Gaussian functions of 0.2 eV half width at full maximum, using the CD Spec Tech program[62,63] and averaged over the MD snapshots

Results and discussion

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Conclusions