LOPTC binds to a unique site in SBL1. (A) DEER data were recorded at 60 K for F270R1 (50 μM) and varied amounts of LOPTC (15.4–80 μM, top to bottom). (B) The modulation depth of decays in panel A was compared to a calibrated standard for determination of the fraction of the spins giving a defined dipolar interaction between F270R1 and bound LOPTC. Experimental results are presented as a Hill plot (solid circles, linear fit of slope 1.1, solid line), with LOPTC concentrations in μM. These data are compared to a plot calculated with Kd of 60 μM, on the assumption that 80% of the protein (40 μM) contributed to the dipolar interaction.