The influence of hydroxyl and methoxyl groups on the retention of some phenolic compounds
Abstract
The paper examines the effect of the introduction of two (hydroxy and methoxy) functional groups that are most important for natural phenolic compounds on the retention of compounds in reverse-phase HPLC. In the proposed approach, it is not recommended to use the retention of substances in a randomly selected chromatographic system for calculations or conclusions, but to use only the asymptotic retention of substances in eluents with zero organic modifier content.
Analysis of the results on the retention of similar glycosides of six main natural anthocyanidins and a set of common substituted cinnamic acids often found in natural objects showed that the addition of OH groups to the benzene ring in one or both meta positions (the para position in these compounds is occupied by the OH group) always leads to an increase in the hydrophilicity of molecules. In this case, the addition of methoxy groups to the same positions leads to an increase in hydrophobicity in terms of retention in mobile phases with zero organic modifier content, although the order of elution in randomly selected compositions of mobile phases may change. For two substituted cinnamic acids, 4-methoxycinnamic and 3,4-dimethoxycinnamic, which are rarely found in natural objects, unique results were obtained. Thus, when a methoxy group is added to cinnamic acid at position 4, the lipophilicity increases, but only slightly. And the addition of a second methoxy group to position 3, instead of increasing lipophilicity, leads to a significant decrease in this parameter. To explain the obtained result, calculations using AM1 in the HyperChem program and using the MM molecular mechanics method in the Chen3D program were used. The latter method predicts the location of the atoms of the methoxy group (except hydrogen atoms) in the plane of the molecule, while the second methoxy group, added in the ortho position to the already existing methoxy group, is moved out of the plane of the molecule due to steric effects, which made it possible to explain the nonequivalence two methoxy groups in ortho position to each other.
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References
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