Composition and stability of Cu2+, Zn2+, and Cd2+ galacturonates in aqueous solutions, according to spectrometric data

Abstract

The formation of metal (Cu2+, Zn2+, Cd2+) pectinates characterized by 6–92 nm shifted UV absorption peaks and up to 2.5 times greater intensity of absorption relative to the initial pectin in a solution is confirmed. The maximum absorbance of metal pectinates stable for no less than 48 h is recorded for a strongly acidic medium. The composition of Cu2+, Zn2+, Cd2+ pectinates and Cu2+ polygalacturonate, tetragalacturonate, trigalacturonate, digalacturonate, and monogalacturonate is characterized using different means: ML2 (where M is a metal ion and L is a pectin structural unit or monogalacturonic acid). The stability constants of galacturonates, i.e., Cu2+ pectinate (logβ = 7.40–7.92), Cu2+ (logβ = 7.69) polygalacturonate, Cu2+ tetragalacturonate (logβ = 7.04), Cu2+ trigalacturonate (logβ = 6.77), Cu2+ digalacturonate (logβ = 6.60), Cu2+ monogalacturonate (logβ = 6.41), Zn2+ pectinate (logβ = 6.62), and Cd2+ pectinate (logβ = 6.64) are determined.