Chemical compounds and antibody chains used in this study. (a) R = H, p-nitrophenyl 8-methyl-8-azabicyclo[3.2.1]octylphenylphosphonate (phosphonate X); R = biotin, biotinylated phosphonate X (BtX). (b) O,O-Diethyl O-(4-nitrophenyl)phosphate (paraoxon). (c) Amino-acid sequences of A17 κ and λ light chains and the heavy chain. Residues are numbered using the Kabat system (superlinear) and sequential numbering (interlinear). Frameworks (FR1–FR4) are underlined; switch residues of the J segment are designated as FR4 according to Kabat; CDRs are coloured magenta; constant domains κCL, λCL and CH1 are coloured green, yellow and blue, respectively.