Poly(l-proline) interactions with flavan-3-ols units: Influence of the molecular structure and the polyphenol/protein ratio

The interaction of proline-rich proteins with flavan-3-ol monomers was studied using poly(l-proline) as a model. Several parameters were varied: (i) the galloylation and trihydroxylation of the flavan-3-ols and (ii) the polyphenol/protein ratio. The systems were characterized by means of UV–Vis spectroscopy, dynamic light scattering and cryo-electron microscopy. Strong differences were observed between epicatechin and epigallocatechin, that did not form aggregates with poly(l-proline), and epicatechin gallate, epigallocatechin gallate, and catechin, which did. This highlighted the strong influence of structural details on the interactions. When complexes were formed, their stability depended on the monomer/protein ratio and on the initial protein concentration. For low galloylated flavan-3-ol/protein ratios (up to 17) and low poly(l-proline) concentration (0.03 mM), interactions led to the formation of relatively stable particles (average hydrodynamic diameter between 30 and 120 nm). When the ratio was further increased to 27, the particles formed were less stable and equilibrium was reached after a longer time. For the highest ratios (27 and 33), aggregation finally led to precipitation. Increasing the protein concentration (1.06 mM) strongly enhanced polyphenol–protein complexation, resulting in much larger particles and more extensive precipitation.