Molecular Modelling and Natural Polyphenol Compounds

For most flavonoids, the UV/Vis spectra exhibit at least two absorption bands, one (Band I) ranging from 240 to 280 nm and another (Band II) from 300 to 400 nm. The absorption wavelength is affected by the chemical structure (i.e., conjugation degree, number and position of substituents). Structure-property relationships were partly established from experimental data. They suggested that an increase in the number of OH or OCH₃ substituents leads to a bathochromic (red) shift, whereas methylation and glycosylation lead to a hypsochromic (blue) shift of Band II.

a) Role of the 2,3 double-bond (e.g., flavones/flavonols vs. flavanones/dihydroflavonols)

The absence of the 2,3-double bond in dihydroflavonols (e.g. taxifolin) and flavanones substantially reduces the conjugation as compare to flavonols (e.g., quercetin) and flavones. Apparently, this induces a systematic hypsochromic shift of lambda_MAX by more than 1.0 eV. In fact, the shift is due to the loss of the main absorption band observed in flavones and flavonols. In the presence of the 2,3-double bond (flavones and flavonols) this band is assigned to a HOMO→LUMO transition. In flavanones and dihydroflavonols, this transition becomes completely forbidden due to complete orbital separation (Figure) and concomitantly a pure ICT character of the S₁ state. For these compounds, the first allowed transition which constitutes the major absorption in this region corresponds to a higher energy gap (HOMO-3→LUMO or the shoulder HOMO-1→LUMO) and subsequently to a lower wavelength. Accordingly, flavanones and dihydroflavonols are colourless, while flavones and flavonols are yellow/-ish.

b) Role of OH-substitution in the B-ring (e.g., in flavonols)

It has been largely observed on numerous polyphenols that the substitutent pattern, e.g., number and position of OH groups, sensitively influences the absorption wavelengths. When increasing the number of OH groups in the B-ring of flavonols, the experimental lambda_MAX is systematically red-shifted. It is 359 nm (3.46 eV), 367 nm (3.38 eV), 370 nm (3.36 eV), 370 nm (3.36 eV) and 374 nm (3.32 eV) for galangin (no OH group), kaempferol (one OH group), quercetin (two OH groups), morin (two OH groups) and myricetin (three OH groups), respectively.(Harbone, 1975) This is due to the mesomeric (+M) effect of the hydroxyl group which indeed extends the p-conjugation in the frontier orbitals, as can be seen from our DFT calculations (Figure). Consequently, this induces a bathochromic shift as well as a slight increase of the oscillator strength of the main absorption band, which is formed from the delocalized MOs, thus rationalizing the experimental results.

c) Importance of the 3-OH group

Flavonols are characterized by the presence of an OH group at C3. The absence of this group (i.e. flavones) induces a significant blue shift; for instance lambda_MAX of luteolin (flavone) is 359 nm (3.45 eV), while 385 nm (3.22 eV) is found for quercetin (flavonol). This can be directly attributed to a partial loss of pi conjugation in flavones, which is followed by stabilization of the HOMO, (-6.41 vs. -6.69 eV for quercetin and luteolin, respectively), destabilization of the LUMO, (-2.80 vs. -2.76 eV for quercetin and luteolin, respectively) and subsequently a significant hypsochromic shift. The same effect is observed for other flavonol/flavone pairs, e.g., galangin/chrysin and kampferol/apigenin.

The glycosylation of the 3-OH group of flavonols (from quercetin to rutin) also induces the hypsochromic shift of λ_MAX. This can be attributed to a decrease in the HOMO-LUMO gap i.e., destabilization of the HOMO (-6.52 vs. -6.50 eV for quercetin and rutin, respectively) and the LUMO (-2.82 vs. -2.56 eV for quercetin and rutin, respectively).

Different to flavonols, the absence of the 3-OH group in dihydroflavonols (thus constituting the family of flavanones) does not induce any significant change in lambda_MAX. This is due to the fact that the LCAO coefficient at C3 in the C-ring of dihydroflavonols (e.g., taxifolin) is rather small.

d) Isoflavonoids

Changing the connection point of the B-ring from C2 (flavones) to C3 (isoflavonoids) completely breaks the pi-conjugation over the entire molecule. The frontier orbitals are now spatially largely separated as can be seen for isoluteolin; consequently the HOMO→LUMO electronic transition is very weak (f = 0.02) due to its strong ICT character. The first transition with significant oscillator strength (lambda = 289 nm in vacuo, see Table 3) is an equal mixture of both HOMO-2→LUMO and HOMO→LUMO+1 configurations with 44% and 48%. The corresponding orbitals are located on the A+C-rings (for HOMO-2 and LUMO) and on the B-ring (for HOMO and LUMO+1), thus giving rise to considerable oscillator strength of the transition (f = 0.17).

e) Ring opening (i.e., chalcones)

Chalcones (Fig. 1) display a significant structural change since the C-ring is open. Nonetheless l_MAX (e.g. 376 nm for isoliquiritigenin) is not influenced as compared to flavonols (e.g. 379 nm for kampferol). In fact, chalcones and flavonols display the same conjugated pathway; consequently both HOMO and LUMO, which constitute the first electronic transition are very similar, see Fig. 5d. As flavonols, chalcones are yellow/-ish, see Table 3. Atom O1 (within the C-ring of flavonol) induces a moderate +M effect, which induces a small bathochromic shift of the absorption band of flavonols against the respective chalcones.

Due to the open form, two conformers may exist for chalcones, the s-cis and the s-trans. The former is more stable than the latter by 2.8 kcal/mol. This instability is attributed to a loss in planarity due to steric hindrance in the s-transtrans compound (350 nm) against s-cis (372 nm), see Table 3. Comparison to experiment (348 nm) suggests that in solution mainly the s-trans compound is present. compound. According to the TD-DFT calculations, this induces a loss in p-electron conjugation which induces a blue shift of about 20 nm for the s-

f) Anthocyanidins

The most substantial structural change within the compounds under investigation is found in anthocyanidin through the introduction of the chromenylium ion (see Fig. 1). This significantly enlarges the effective p-conjugation (e.g., in cyanidin) compare to flavan-3-ol (e.g., catechin), the non-charged counterpart, see Fig. 5e. The HOMO-LUMO gap is thus reduced due to a strong stabilization of LUMO vs. HOMO (DE^{stabilization} = 1.2 vs. 0.4 eV, respectively). The properties of the p-electron system of anthocyanidins are responsible for a maximum absorption band around 500 nm (2.48 eV), explaining the blue colours of fruit and flowers that produce these dyes. The bathochromic shift in l_MAX compared to the other flavonoids (e.g., flavonols) is accurately evaluated by TD-DFT (Table 3 and Fig. 2). As for the other flavonoids, upon increasing the number of OH groups, this band is red-shifted (520 nm, 535 nm and 546 nm with pelargonidin, cyanidin and delphinidin, respectively).

g) Size effect

Silybin and dehydrosilybin (Fig. 1) differ from the parent taxifolin and quercetin compounds by condensation of the hydroxyl functionalities in the B-ring. Accordingly, the frontier orbitals are very similar to the parent compounds (see Fig. 5f), so that the first allowed absorption band is assigned to the flavonoid moiety and neither l_MAX nor the oscillator strength change from taxifolin to silybin, see Table 3. A slight red shift is observed from dehydrosilybin to quercetin, which is attributed to the slightly stronger +M effect of the alkoxy functionality compared to the hydroxyl substituent (see Fig. 5f).

This result is of rather general nature, which applies for numerous other large polyphenols since most of these compounds are based on flavonoid, lignan and phenolic acid moieties but do not exhibit extended p-conjugation.