Role of Functional Groups in the Complexation of Structural Analogs of Aromatic Amino Acids with Pectin

Spectrophotometry is used to determine constants of stability for coordination compounds of apple pectin with indole and L-tyrosine—structural analogs of aromatic amino acids (tryptophan and phenylalanine). It is found that the presence of phenolate oxygen in tyrosine favors bonding to pectin and is attributed to this species’ participation in the forming of H bonds with the carbonyl functional group of the polysaccharide, which ensures better stability of the pectin–tyrosine complex than that of a pectin–phenylalanine system. It is shown that the key contribution to the stability of pectin complex comes from the indole moiety of tryptophan. Based on analysis of thermodynamic characteristics, it is shown that the formation of 1 : 1 molecular complexes for pectin–indole and pectin–tyrosine systems is governed only by the enthalpy component (∆H⁰ < 0, ∆S⁰ < 0). The thermal properties of modified pectins are determined.