α-(Ac)AKRHRKV, a model of the histone H4 amino terminus, uses an unprotonated histidine in phosphate binding

The 1H NMR spectrum of the title peptide at pH 3.3 in 90% H2O was assigned by HOHAHA and NOESY 2D methods. Titration studies in D2O at 300 MHz indicated a histidine side‐chain pKa of 6.3. Peptide backbone NH resonances were studied in 90% H2O at 500 MHz as a function of pH and added phosphate. In acidic solution the peptide was free from conventional secondary structural elements, but near neutrality the valine amide proton resonance remained a sharp doublet, which suggests that it may form a hydrogen bond with some backbone carbonyl group. The other amide resonances broadened and showed significant saturation transfer from the water signal indicating that they exchange with solvent although not all to the same extent. Marked changes in the chemical shift of the histidine aromatic protons in the presence of phosphate and a 70‐fold increase in the 31P line width of inorganic phosphate in the presence of peptide only at pH values above the pKa (6.3) of the histidine imidazole side‐chain implied that the unprotonated imidazole group is specifically involved in phosphate binding. The peptide binds inorganic phosphate with a dissociation constant of 1.6 × 10−5 M−1 at pH 7.4.