Biophysical characterization of polyphenol aggregates in Moringa oleifera leaves water extract: stability and surface exposure effect on antioxidant activity under dilution.

Abstract

The aqueous extract of Moringa oleifera leaves has been previously characterized for its polyphenolic composition, yet the behavior of its colloidal aggregates under dilution remains largely unexplored. In this study, we investigate the structural and chemical properties of these aggregates at room temperature, focusing on their stability and surface exposure upon dilution. Although the aggregates break up as dilution increases, they never fully dissolve within the conditions explored. Both multi-angle static light scattering and dynamic light scattering highlight aggregates fragmentation and size heterogeneity under dilution. UV-vis absorption spectroscopic data strongly suggest that the aggregates of different sizes present in the extract are homogeneously constituted, as their spectra are similar to those of the main polyphenol components. The Folin-Ciocâlteu assay reveals an increase in gallic acid equivalent values normalized for extract concentration, suggesting that fragmentation prompted by dilution enhances the exposure of reactive sites. A very basic model, considering only one kind of aggregate with uniform density, is employed to support this interpretation. Assuming this model, the Folin-Ciocâlteu assay data allow to grasp the law regulating the change of the aggregate average size under dilution, i.e., a power law. Additionally, in-liquid atomic force microscopy imaging confirms the presence of smaller but still aggregated particles at high dilution, enabling the calculation of a height distribution, that is consistent with the model prediction. These findings provide insights into the dynamic behavior of polyphenol-rich aggregates in aqueous systems and their potential implications for bioavailability and reactivity.