Hydrophilization of guaiazulene-based blue pigment: improving its stability in acidic conditions by substitution with polyallylamine

Abstract

Guaiazulene, an azulene compound with a blue color more saturated than that of naturally derived blue pigments, shows potential as a food coloring agent. However, poor water solubility and loss of blue color due to degradation under acidic conditions of guaiazulene hinder its development as a food coloring agent. The synthesis of blue guaiazulene derivatives with high water solubility and stability under acidic conditions has not been achieved. In this study, 7-isopropyl-1,4-dimethylazulene-2-carboxylic acid, a blue guaiazulene derivative easily condensed with amines, has been prepared. By condensing the guaiazulene derivative with poly(allylamine) whose amino groups provide hydrophilic properties, guaiazulene-substituted poly(allylamine) has been synthesized. The introduction degree of guaiazulene can be facilely adjusted by changing the equivalent amount of 7-isopropyl-1,4-dimethylazulene-2-carboxylic acid. The water solubility of the guaiazulene unit in the polymer with a 3.2% introduction degree is 8.7 mg mL⁻¹, which is 10⁴ times higher than that of guaiazulene (0.00084 mg mL⁻¹). Due to preferential protonation of the basic amino groups, the polymer is stable for one month in solution at pH 4 for one month. Therefore, introducing guaiazulene into poly(allylamine) results in high water solubility and improved stability even under acidic conditions.