GPIIb/IIIa binds to immobilized mAb 314.1 with low affinity in the absence of the drug and more tightly when the drug is present (monovalent interaction). mAb 314.1 and control antibody MOPC were immobilized by amine coupling to a CM3 chip for SPR analysis. GPIIb/IIIa at various concentrations was perfused over the chip surfaces in the absence (A-C) or presence of 0.1 mM quinine (D-F). (A,D) Representative sensorgrams illustrating dose-dependent binding of GPIIb/IIIa to mAb 314.1 without quinine (A) and with 0.1 mM quinine (D). (B,E) Binding isotherms derived from dose-response curves illustrated in (A) and (D), respectively. (C,F) Scatchard plot analysis. Line shown is the linear fit for data points obtained in duplicate studies. In the experiments shown, K_D values for binding of GPIIb/IIIa to immobilized mAb 314.1 were 8.6 × 10⁻⁶ and 1.6 × 10⁻⁶ mol/L in the absence and presence of quinine, respectively.