Computational models for structure-hydrophobicity relationships of 4-carboxyl-2,6-dinitrophenyl azo hydroxynaphthalenes.

Some phenyl azo hydroxynaphthalene dyes (e.g., sunset yellow) are certified as approved colorants for food, cosmetics, and drug formulations. The hydrophobicity of 4 newly synthesized azo dyes of the phenyl azo hydroxynaphthalene class was investigated, as a training set, with the goal of developing models for quantitative structure-property relationships (QSPR). Retention behavior of the molecules reversed-phase thin-layer chromatography (RPTLC) was investigated using liquid paraffin-coated silica gel as the stationary phase. Mobile phases consisted of aqueous mixtures of methanol, acetone, and dimethylformamide (DMF). Basic hydrophobicity parameter (Rm(w)), specific hydrophobic surface area (S), and isocratic chromatographic hydrophobicity index (rho(o)) were computed from the chromatographic data. The hydrophobicity index (R-m) decreased linearly with increasing concentration of organic modifiers. Extrapolated Rmw values obtained by using DMF and acetone differ significantly from the value obtained by using methanol as organic modifier [P < 0.05, 1-way analysis of variance (ANOVA), Tukey's multiple comparison test]. Structure-property relationships showed that hydrophobicity was dependent on type and position of naphthalene ring substituents. R-m decreased with the presence of a highly polar substituent (e.g., COOH). Owing to intramolecular interaction, Rm increased when the common hydroxyl group (OH) is positioned ortho to the azo group, relative to para positioning, in 2 positional isomers. Pattern recognition data analysis underscores the utility of rho(o) as a more accurate hydrophobicity descriptor than Rm(w). rho(o) is negatively correlated with theoretically calculated density, surface tension, and refractive index for the molecules. These models could be used to predict toxicity (absorption, distribution, metabolism, excretion, toxicity; ADMET) properties of the azo dyes and may also play useful roles in computer-assisted molecular discovery of nontoxic azo dyes.