Modeling UV/Vis Absorption Spectra of Food Colorants in Solution: Anthocyanins and Curcumin as Case Studies

Abstract

We present a comprehensive computational study of UV/Vis absorption spectra of significant food colorants, specifically anthocyanins and curcumin tautomers, dissolved in polar protic solvents, namely water and ethanol. The absorption spectra are simulated using two fully polarizable quantum mechanical (QM)/molecular mechanics (MM) models based on the fluctuating charge (FQ) and fluctuating charge and dipoles (FQFμ) force fields. To accurately capture the dynamical aspects of the solvation phenomenon, atomistic approaches are combined with configurational sampling obtained through classical molecular dynamics (MD) simulations. The calculated QM/FQ and QM/FQFμ spectra are then compared with experiments. Our findings demonstrate that a precise reproduction of the UV/Vis spectra of the studied pigments can be achieved by adequately accounting for configurational sampling, polarization effects, and hydrogen bonding interactions.

Keywords: MD; QM/MM; UV/Vis; absorption spectrum; anthocyanins; curcumin; food colorants.

Figure 1

(left) CYD molecular structure of labeling adopted in this work. (right) Dihedral distribution functions (DDF) of the flexible torsions of CYD solvated in water and ethanol, as calculated from OPLS-AA MD.

Figure 2

Radial distribution functions g(r) of intramolecular (top) and intermolecular interactions between selected H_i atoms of CYD and the solvent oxygen atoms (H_i$\cdots$ O_solvent, (a)) and between the O_i atoms of CYD and the solvent hydrogen atoms (O_i $\cdots$ H_solvent, (b)). All RDFs are computed along the OPLS-AA MD trajectories.

Figure 3

Stick and convoluted (normalized) spectra of solvated CYD in (a) water at the QM/FQ level; (b) water at the QM/FQF$\mu$ level; (c) ethanol at the QM/FQ level. All spectra are computed using 200 frames extracted from OPLS-AA MD runs.

Figure 4

QM/FQ MOs involved in the vertical transitions of CYD in water (left) and in ethanol (right) computed for a representative structure extracted from OPLS-AA MD runs.

Figure 5

UV/Vis spectra of solvated CYD computed using all frames (“full”) and the representative structures resulting from clustering (“cluster”) of the OPLS-AA MD runs.

Figure 6

Computed and experimental UV/Vis spectra of solvated CYD. QM/MM frames are extracted from OPLS-AA MDs. Experimental data are taken from ref. [96].

Figure 7

Stick and convoluted (normalized) spectra of solvated EK (left) and KK (right) tautomers of curcumin dissolved in water as computed at the QM/FQ (a,b) and QM/FQF$\mu$ (c,d) levels.

Figure 8

QM/FQ MOs involved in the vertical transitions of the EK (left) and KK (right) tautomers of curcumin computed for a representative snapshot.

Figure 9

UV/Vis spectra of solvated EK and KK tautomers computed using all frames (“full”) and the representative structures resulting from clustering (“cluster”).

Figure 10

Computed and experimental UV/Vis spectra of solvated EK (a) and KK (b) tautomers. Experimental data are taken from ref. [98].