Surface plasmon resonance study on the interaction between lactose-installed poly(ethylene glycol)-poly(D,L-lactide) block copolymer micelles and lectins immobilized on a gold surface

Poly(ethylene glycol)-poly(D,L-lactide) (PEG-PLA) block copolymer micelles bearing lactose moieties on their surface were prepared, and their interaction with the lectin Ricinus Communis Agglutinin (RCA-I) was evaluated by surface plasmon resonance. The one-pot, ring-opening anionic polymerization used in the preparation of the alpha-acetoxy-PEG-PLA yielded one for which the molecular weight of the blocks (4900 and 4500 g/mol for the PEG and PLA blocks, respectively) was controlled by the initial monomer/initiator ratio. This process also conveyed a quite narrow molecular weight distribution (M-w/M-n = 1.05). Micelles prepared from this copolymer had a size in the 30-nm range, also with low size distributions (polydispersity index < 0.1), as established by dynamic light scattering measurements. The coupling of p-aminophenyl-beta-D-lactopyranoside (Lac) moieties on the acetal-deprotected ends of the PEG block proceeded in a quantitative way, while the particle size remained unchanged, despite the chemical nature of the reaction. The lactose functionality, defined as the number of lactose molecules per 100 copolymer chains, was found to be as high as 80%. The interaction of Lac-installed micelles with surface-immobilized RCA-I lectins was found to be specific and to proceed in a cooperative manner. Further, the specificity in the complex formation provided a tool for more fundamental studies, including a review on the critical association concentration of these self-assembled structures, resulting in values in good accordance with the ones established using pyrene as a fluorescent probe (6-8 mg/L). Finally, the importance of multivalency was also stressed, as enhanced binding was observed for lactose functionalities higher than 40%.