Self-Assembled Supramolecular Chiral Complexes of Alginate and Cyanidin-3-O-Glucoside: Insights into the Molecular Basis of Assembly Formation
Complexes of cyanidin 3-O-glucoside (CND) and alginate (Alg) form long-range supramolecular structures at pH 4, where CND is predominantly uncharged. Under these conditions, the complexes exhibit pronounced chiroptical activity, with Circular Dichroism (CD) bands significantly more intense than those of free CND or of CND/Alg complexes formed at lower pH values. Alginate consists of guluronic and mannuronic acid units, which differ in the axial or equatorial orientation of one carboxyl group, respectively. To gain insight into the molecular basis of CND–alginate complexation, CND was separately complexed with polyguluronic (PolyG) and polymannuronic (PolyM) acids. In both cases, only small aggregates were formed, with sizes around 200 nm as measured by Dynamic Light Scattering (DLS) and 60–120 nm by Atomic Force Microscopy (AFM) in the dry state. These aggregates lacked the characteristic CD bands observed for CND/Alg complexes at pH 4. Molecular dynamics simulations showed that the spacing between successive carboxylate groups interacting with CND molecules in PolyG and PolyM acids is approximately 7.5–8.0 Å. In alginate, however, an additional shorter distance of about 5.0 Å is present, arising from the alternating guluronic and mannuronic residues along the polymer chain. These results suggest that the alternation of guluronic and mannuronic units in alginate plays an important role in promoting CND–CND interactions, thereby driving cooperative assembly and the formation of extended supramolecular structures with enhanced chiroptical response.