Figure 1.

Detection of a fortuitous ligand in RORβ by non denaturing ESI-MS. ESI mass spectrum of RORβ in 25 mM NH₄OAc at pH 6.5 at V_c = 20 V (A) and V_c = 50 V (B). Two series A and B of multiply-charged ions (12 to 14 protonations) are detected. The mass difference (ΔM) between the mass measurements shows the presence of an unexpected bound molecule of ~286 Da. Increasing the accelerating cone voltage from 20 V to 50 V resulted in more energetic collisions with residual gaseous molecules in the interface of the mass spectrometer, which led to the dissociation of the complex. Analysis of this preparation under denaturing conditions (50% CH₃CN, 1% HCOOH) shows that RORβ is pure and homogenous and thus that the extra mass of 286 Da is due to a noncovalent attachment of a small organic molecule to the protein (C). apoRORβ (oval); fortuitous ligand (diamond).