Interflavan bonds

Results obtained with colorimetric methods are highly empirical. Estimations of total PAs are often expressed as catechin or epicatechin equivalents, which make the data difficult to interpret and compare across different samples. Qualitative data such as subunit structures, interflavan bond types, and proportions of oligomers with different degrees of polymerization - is not available with any of these non-chromatographic methods. 

Proanthocyanidins with one A-type linkage have two less hydrogen than those of the B-type proanthocyanidins. A procyanidin trimer gave rise to [M-H] m/z 865, whereas a procyanidin trimer with one-type linkage yielded [M H] m/z 863. A-type interflavan bond differs from B-type bound in that they do not undergo QM cleavage. Thus,... 

Evidence of such adducts in wine fractions has been provided, as detailed in Chapter 9A. These include F-A+ (Alcalde-Eon et al. 2006 Boido et al. 2006) and F-A-A+ (Alcalde-Eon et al. 2006) adducts based on different flavanol and anthocyanin units and (F) -A+ adducts deriving from different flavanols monomers and oligomers (Hayasaka and Kennedy 2003). Proanthocyanidins arising from these reactions cannot be distinguished from those extracted from grapes. However, detection of F-A+ adducts without prior fractionation (Morel-Salmi et al. 2006) confirmed the occurrence of the acid-catalyzed interflavanic bond breaking process in wines. 

The central C-pyran ring tends to have a cationic charge, natural for the flavylium forms of anthocyanins, or formed by acidic cleavage of the interflavan bonds connecting the flavanol units of tannins. Therefore, the C-ring reacts as an electrophilic moiety and undergoes nucleophilic addition. 

Based on the flavonoid reactivity pattern (figure 2), the dichotomy between electrophilic and nucleophilic characters is strongly related to pH. The more acidic the pH is, the more cationic charges prevail. Thus, the balance between flavylium cations (AH+) and hydrated forms (AOH) is pH-dependent as well as the interflavanic bond cleavage yielding the flavanol cations (F+). In addition, acetaldehyde protonation is also controlled by pH. Therefore, it can be expected that reactions involving flavylium, flavanol or protonated acetaldehyde cations are favored at lower pH values. Based on carbocation reactivity, these cationic species can be classified as follows ... 

Anthocyanin-polyflavan-3-ols with B-Type and A-Type Interflavan Bonds... 

Figure 2. MALDI-TOF positive reflectron mode mass spectra of the anthocyanm-polyflayan-3-ol oligomers of Hyred cranberry fruit and spray dried juice, (A) Anthocyanins [Mfr. (B) Anthocyanin linked to a single flavan-3-ol through a CH3-CH bridge [M]. (C) Anthocyanin linked to a polyflavan-3-ol of 2 degrees of polymerization through a CH3-CH bridge, containing either an A-type or a B-type interflavan bond [M]. ...

A series of anthocyanins linked to polyflavan-3-ols of 2 degrees of polymerization (DP2) with either an A-type or B-type interflavan bonds (A 2 amu) were observed cyanidin-pentoside-DP2 (A-type = m/z 1021.2, B-type = m/z 1023.1), peonidin-pentoside-DP2 (A-type = m/z 1035.1, B-type = m/z... 

Mass calculations were based on the equation Anthocyanin + 28 +288a 2b, where anthocyanin represents the molecular wei t (MW) of the terminal anthocyanin, 28 is the MW of the CH-CH3 bridge, a is the DP of the extending flavan-3-ol units, and b is the number of A-type interflavan bonds. Formation of each A-type interflavan ether linkage leads to the loss of two hydrogen atoms (A 2 amu). Masses were not observed. 

Polymers of + ) catechin and (-)-epicatechin have an intrinsic fluorescence because chromophores are an integral part of each monomer unit. The time-resolved emission from well-characterized dimers can be used to determine the relative populations of two rotational isomers at the interflavan bond between monomer units. When combined with the solid-state conformations, molecular mechanics calculations, and rotational isomeric state analysis, the interpretation of the time-resolved fluorescence leads to the unperturbed dimensions of the polymers. Significant population of both rotational isomers causes the chains to have unperturbed dimensions comparable with those in atactic polystyrene molecules of the same molecular weight. 

A short homopolymer of (+ )-catechin with 4p 8 interflavan bonds. Heavy lines denote the main chain considered in the evaluation of o see definition on p 291). Hydrogen atoms bonded to carbon atoms are omitted. 

The unperturbed dimensions of the high polymers depend very strongly on the relative populations of the two rotational isomers at the interflavan bond. If one rotational isomer in a homopolymer were populated to the exclusion of the other, the local conformation would describe a helix and the overall shape would be that of a rod. Significant population of the second rotational isomer converts the rod to a random coil. The molecular mechanics calculations do not provide the relative energies of the two rotational isomers with the accuracy required for description of the unperturbed dimensions. 

Dimers exhibit steady-state emission in the same spectral range as the monomers, but with reduced quantum yield (JO). An important difference exists between the time-resolved emission of the monomers and various dimers. In the absence of artificial constraints on the rotation about the interflavan bond, the time dependence of the emission from a dimer cannot... 

The values of the parameters for ( )-epicatechin-(4p-- 6)-(-f )-catechin and (-)-epicatechin-(4(3—>8)-(H-)-catechin, are presented in the third and fourth lines in Table I. The lifetimes are shorter for the dimers than for the monomers. The two decay processes do not contribute equally to the time dependence of the emissions from the dimers. The preexponential factor is larger for the shorter lifetime. Two independent types of experiments that will be described, show that rotational isomerism at the interflavan bond is responsible for the heterogeneity of the time-dependence emission from the dimers. 

The fifth line in Table I presents the decay parameters for the constrained dimer, structure 4. The bridging ring forces the population of a single rotational isomer at the interflavan bond. The time dependence of the emission for the constrained dimer is satisfactorily described by the single exponential decay (equation 1). In contrast, a satisfactory description of the emission from unconstrained dimers requires the sum of two exponential decays (equation 2). 

The description of the emission in the constrained dimer by equation 1, and the successful correlation of the populations deduced for the peracetylated dimer from 400-MHz proton NMR, with the preexponential factors in equation 2 show that the heterogeneity of the emission in the unconstrained dimers arises from the population of two rotational isomers at the interflavan bond. The populations of these rotational isomers in the free phenol forms can be deduced from the preexponential factors on the third and fourth lines of Table 1. This assignment of the populations provides the necessary ingredient for a rotational isomeric state analysis of the unperturbed dimensions of the polymers. 

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