We show that Carr-Purcell-Meiboom-Gill (CPMG) Ca-13 NMR relaxation dispersion measurements are a viable means for profiling mu s-ms ligand dynamics involved in receptor binding. Critically, the dispersion is at natural C-13 abundance; this matches typical pharmaceutical research settings in which ligand isotope-labeling is often impractical. The dispersion reveals ligand Ca-13 nuclei that experience ps-ms modulation of their chemical shifts due to binding. Ca-13 shifts are dominated by local torsion angles (phi, psi, X-1; hence, these experiments identify flexible torsion angles that may assist complex formation. Since the experiments detect the ligand, they are viable even in the absence of a receptor structure. The ps-ms dynamic information gained helps establish flexibility-activity relationships. We apply these experiments to study the binding of a phospho-peptide substrate ligand to the peptidyl-prolyl isomerase Pin1.