Elucidating Antibody-Hapten Recognition for 25-Hydroxyvitamin D3 toward a Point-of-Need Immunoassay.

Accurate quantification of 25-hydroxyvitamin D3 (25(OH)D3), the gold-standard biomarker for vitamin D nutritional assessment, still presents considerable analytical challenges. Although 25(OH)D3 exhibits intrinsic photosensitivity and thermal lability, these properties can be effectively managed via standardized light-shielding and inert-atmosphere operations. Key bottlenecks for its immunoassay lie in empirical-driven antibody screening lacking systematic theoretical guidance and poor discrimination of structurally similar vitamin D metabolites. To address these bottlenecks, we explored the molecular-recognition mechanism of high-affinity monoclonal antibody mAb-1F3 toward its hapten using molecular-docking and binding-pocket analysis. Four key amino-acid residues (PHE 52, PHE 91, TYR 32, ILE 100) within the antibody complementarity-determining region form a cooperative binding network for specific recognition of the steroid nucleus and C25-hydroxyl moiety of 25(OH)D3. π-alkyl interactions from PHE 52, PHE 91 and TYR 32 stabilize the hydrophobic steroid skeleton, while ILE 100 forms a conserved hydrogen bond (2.8 Å) to realize specific recognition of the C25-hydroxyl group. This structurally synergistic binding mechanism endows mAb-1F3 with high affinity (IC50 = 24.8 ng/mL) and favorable metabolite-discriminating capability. Guided by this mechanistic insight, recombinant mAb-1F3 was prepared and integrated into a rapid colloidal-gold lateral-flow immunoassay (LFIA). Standard matrix-specific sample extraction procedures were adopted to mitigate matrix interference induced by the high lipophilicity of 25(OH)D3 in complex samples. The LFIA completes the detection within 5 min and shows satisfactory linear consistency with LC-MS/MS in raw milk, dietary supplement, and human serum matrices. This work establishes a structure-driven strategy for rational antibody characterization and design, provides critical mechanistic support for conventional empirical antibody-screening workflows, and facilitates performance improvement of vitamin D immunoanalytical methods.