Intrinsically disordered proteins (IDPs) have roles in myriad biological processes and numerous human diseases. However, kinetic and amplitude information regarding their ground-state conformational fluctuations has remained elusive.' We demonstrate using nuclear magnetic resonance (NMR)-based relaxation dispersion that the D2 domain of p27(Kip1), a prototypical IDP, samples multiple discrete; rapidly exchanging conformational states.)3,By combining NMR with mutagenesis and small angle X-ray scattering (SAXS), we show that these states involve aromatic residue clustering through long-range hydrophobic interactions. Theoretical studies have proposed that small niblecules bind promiscuously to IDPs, causing expansion of their conformational landscapes. However; on the basis of previous NMR-based screening results, we show here that compound binding only shifts the populations of states that existed within the ground state of apo p27-D2 without changing the barriers between states. Our results provide atomic resolution-insight into how a small molecule binds an motions on the low microsecond time scale when probing these types of interactions.