c. Intermolecular DEER analysis of free hnRNP A1 DEER with singly spin - labelled proteins was applied to characterize protein - protein interactions . Contributions to the DEER signal that enable this analysis are schematically displayed in Figure S6 . The DEER primary data for hnRNP A1 singly spin - labelled at sites 231 , 271 , and 316 are shown in Figure S6 , analogous to the data in main text Figure 1H. The samples in this series were at 50 μM protein concentration , labelling efficiency as determined by spin counting is annotated ( no spin dilution ) . Deuterated glycerol 50 % ( v : v ) was used as cryoprotectant in this series . A distance analysis was performed in the same way as for the distance restraints ( homogenous background correction followed by single - Gaussian fit ) and is shown as well . This analysis implicitly assumes that all of the short - range interactions that are not removed by the background fit arise from dimerized protein and that the monomers and dimers are otherwise homogenously distributed in the sample ( background dimensionality of the DEER fit is three ) . The distance distributions are very broad and very similar for all three sites , which demonstrates that the proteins are not interacting in a well - defined manner , but rather form a disordered complex or a condensed phase . Note that distance data derived in this way can only serve as a guideline to monitor proteinprotein interactions . They are probably a poor approximation of the actual distance distributions , given that a fraction of hnRNP A1 is likely in the LD state , where local spin label concentration is expected to be very high .