Fluorometric study of molecular association of pectin and algin with ferulic and p-coumaric acids: Evaluation of association capacity of dietary fibers towards antioxidants

Abstract

Antioxidants undergo molecular association with dietary fibers in plant foods, and the actual nutritional functions depend on the conditions of association. As an approach to quantitative interpretation of the association effects, the present study proposes a method to evaluate association capacity, based on fluorometric titration of ferulic and p-coumaric acids with pectin and algin. The titration curve is formulated on the model that active sites on a fiber chain consist of segments each capable of binding to a single antioxidant molecule. The average segment sizes NS determined from observed curves range from 5 to 12 monomer units, and the mean association constant KA from 5 × 104 to 18 × 104 M−1 per segment, depending on fiber–antioxidant combinations. These parameters suggest that, although individual monomer units have weak binding forces, each segment involves multiple binding sites so that accumulating forces lead to the high thermodynamic stability of the molecular associates; the association capacity is presented by the half association concentration of antioxidant CG1/2 = 2Cm/NS – 2/KA at a monomer-based fiber concentration Cm. The extended application of the method to selected fiber–nutrient combinations would help to describe the association at molecular levels and to design better nutritional agents.

Keywords

• Dietary fiber
• Antioxidant
• Molecular association
• Polymer complex
• Fluorescence titration