Streptavidin binding stoichiometry of biotinylated silicon phthalocyanines: impact of axial ligands

Biotinylated silicon phthalocyanines (SiPcs) represent promising platforms for targeted molecular systems that engage streptavidin through high-affinity biotin–protein interactions. In this study, two SiPc derivatives were synthesized with biotin appended via distinct axial ligands. Although both compounds exhibited similar optical properties in dimethyl sulfoxide, their binding behavior with streptavidin differed markedly. A competitive displacement assay using 4′-hydroxyazobenzene-2-carboxylic acid revealed that compound 1 occupied approximately 3.5 binding pockets per streptavidin tetramer, whereas compound 2 showed negligible displacement even at high concentrations. SDS-PAGE analysis confirmed that the streptavidin tetramer remained intact in the presence of compound 1, ruling out tetramer dissociation as a cause of sub-stoichiometric binding. These findings indicate that steric hindrance or limited spatial accessibility of the biotin moiety, possibly compounded by aggregation in aqueous media, critically impacts functional binding. This study demonstrates that the architecture of axial ligands affects the effective presentation of biotin on SiPc scaffolds and informs the design of supramolecular probes and photofunctional systems responsive to biological targets.