A computational study of substituted flavylium salts and their quinonoidal conjugate-bases: S0 -> S1 electronic transition, absolute pKa and reduction potential calculations by DFT and semiempirical methods

Abstract

As transições eletrônicas para os cátions flavílio e bases quinonoidais de dezessete sais deste cátion foram estudadas nos níveis semiempírico e DFT (teoria do funcional da densidade). O efeito do solvente nos espectros eletrônicos foi incluído pelo Modelo Contínuo Polarizado, PCM. As transições eletrônicas de menor energia foram assinaladas como transições HOMO→LUMO. Ambos os níveis de teoria forneceram bons resultados para as transições eletrônicas dos cátions flavílio, enquanto apenas os cálculos por TDDFT-PCM puderam ser empregados para as transições das bases quinonoidais. Foram feitos cálculos de pK a absoluto para nove sais de flavílio em nível DFT. Os valores de pK a calculados pela nossa parametrização do PCM forneceram resultados excelentes, com um desvio médio absoluto de menos de meia unidade de pK a . Foram calculados por DFT potenciais de redução para cinco cátions flavílio. Os resultados teóricos encontrados ficaram em boa concordância com os resultados experimentais após a correção de um desvio sistemático.The electronic transitions for flavylium cations and quinonoidal bases of 17 substituted flavylium salts have been studied at semiempirical and DFT (density functional theory) levels. Solvent effect on electronic spectra was included by Polarizable Continuum Model, PCM. We assigned longest-wavelength absorption maxima to HOMO→LUMO transition. Both levels of theory gave good results for electronic transitions of flavylium cations whereas only TDDFT-PCM calculations could be used for electronic transitions of their quinonoidal bases. We also performed absolute pK a calculations of nine flavylium salts at DFT level. The pK a calculated values by our PCM parameterization gave excellent results with mean absolute deviation less than a half of one pK a unit. One-electron reduction potentials were carried out for 5 flavylium cations at DFT level. The theoretical results found were in good agreement with experimental values after adjustment for a systematic deviation.Keywords: flavylium salts, anthocyanins, quinonoidal base, pK a calculation, time dependent-DFT IntroductionAnthocyanins constitute the major red and purple pigments in plants and can be found in fruits, flowers and leaves. 1,2 Interest in the anthocyanins stems from the fact that they are omnipresent in our diet, exhibit unusual chemical and photochemical properties, [3][4][5][6][7][8][9] and have potential for application as food dyes 1 and antioxidant additives. 10,11 The basic chromophore of anthocyanins is the 7-hydroxyflavylium ion (Figure 1). In nature, the flavylium ion typically has hydroxyl substituents at positions 3 (always glycosylated) and 5 (occasionally glycosylated) and the phenyl or B-ring has one or more hydroxyl or methoxy substituents. 1 The colors of natural and synthetic anthocyanins range from yellow to purple, depending on the degree of substitution of the 7-hydroxyflavylium ion chromophore. Rationalization of the chemical and photochemical properties of anthocyanins is complicated by the fact that, in aqueous solu...

Optical properties of anthocyanins in the gas phase

“…The spectra obtained from hybrid functionals are reported in Figure 3, in agreement with previous theoretical calculations obtained with different techniques [32,33,4,34,35]. While the classification of the spectra in three categories and their overall qualitative shape are not influenced by the choice of functionals, there are significant differences that affect the color expressed by them.…”

“…Even though many theoretical results [32,33,4,43,34,35] (including the ones reported in the previous section) were able to qualitatively account for this feature, its microscopic origin has not been explained. Here, we investigate the intimate correlation between the number and the position of the OR groups and the energy and the symmetry of the molecular orbitals, involved in the low-frequency absorption process.…”

Photochemistry of anthocyanins and their biological role in plant tissues

“…2) [1,2,19]. At pH < 2, anthocyanins exist predominantly in the reddish-colored flavylium cation form, AH + , which is a weak acid with a first pK a (deprotonation of the 7-hydroxy group) in the range of 4-5 [20]. Deprotonation of AH + produces a bluish quinonoidal base A.…”

Ultrafast Internal Conversion in a Model Anthocyanin–Polyphenol Complex: Implications for the Biological Role of Anthocyanins in Vegetative Tissues of Plants

“…As in previous works, [29,33] the geometries of MMF and PCA were fully optimized at the mPW1PW91/6-31 + G(d) level by using the integral equation formalism for the polarizable continuum model, IEFPCM, [34] to describe the implicit solvent water. The electronic transition energies and oscillator strengths were calculated by using time-dependent DFT (TD-DFT) with IEFPCM [35] at the mPW1PW91/6-31 + G(d) level.…”