We used ion mobility spectrometry to explore conformational adaptability of intrinsically disordered proteins bound to their targets in complex mixtures. We investigated the interactions between a human salivary proline-rich protein IB5 and a model of wine and tea tannin: epigallocatechin gallate (EgCG). Collisional cross sections of naked IBS and IBS complexed with N = 1-15 tannins were recorded. The data demonstrate that IBS undergoes an unfolded to folded structural transition upon binding with EgCG.