Chemical shift perturbations vs. peptide length. (A) Amide chemical shift changes for 0.1 mM U-[15N]APLF FHA domain in the presence of 0.8 mM XRCC1pSpT-9 (blue bars) or XRCC1pSpT-18 (orange bars). (B) 1H,15N-HSQC spectra of U-[15N]APLF FHA domain as a function of XRCC1pSpT-9 (left panel) or XRCC1pSpT-18 (right panel). Peptide concentrations (in μM): 0 (red), 10 (orange), 20 (yellow), 50 (green), 100 (blue), 200 (purple), 400 (cyan) and 800 (pink). Titration studies were performed in a NMR buffer containing 25 mM HEPES, 25 mM MES, 150 mM NaCl, 1 mM EDTA, pH 7.4.