A bifunctional spin label for ligand recognition on surfaces

In situ monitoring of biomolecular recognition, especially at surfaces, still presents a significant technical challenge. Electron paramagnetic resonance (EPR) of biomolecules spin-labelled with nitroxides can offer uniquely sensitive and selective insights into these processes, but new spin-labelling strategies are needed. We report the synthesis and study of a bromoacrylaldehyde spin-label (BASL), which features two attachment points with orthogonal reactivity. We have created the first examples of mannose and biotin ligands coupled to aqueous carboxy-functionalized gold nanoparticles through a spin-label. EPR spectra were obtained for the spin-labelled ligands both free in solution and attached to nanoparticles. The labels were recognised by the mannose-binding lectin, Con A, and the biotin-binding protein avidin-peroxidase. Binding gave quantifiable changes in the EPR spectra from which binding profiles could be obtained that reflect the strength of binding in each case.