Polyelectrolyte complexation of charged IDPs driven by the entropy of counterion release

Abstract

The two IDPs ProTα and H1 bear large opposite net charges (−44 and +53, respectively) and bind to each other with very high affinity; however, the complex remains disordered and represents a limiting case in the spectrum of disorder in IDP complexes [1,2]. Temperature-dependent single-molecule FRET (smFRET) experiments and isothermal titration calorimetry (ITC) reveal ProTα-H1 complexation to be enthalpically unfavourable. Rather, counterion release entropy is an important thermodynamic driving force for complexation, as suggested by affinity measurements with smFRET as a function of salt concentration and using different salts.