Affinity capillary electrophoresis study of the linkage existing between proton and zinc ion binding to bacitracin A1

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Abstract
Measurements by capillary electrophoresis (CE) of bacitracin A1 effective mobility at different pH values permitted to estimate the five acidic dissociation constants and the Stokes radii at different protonation stages of the macrocyclic dodecapeptide. The pKa values were 3.6 and 4.4 for the two carboxylic groups of the lateral chains of D-Asp-11 and D-Glu-4, respectively, 6.4 for the aza-atom of the imidazole ring of His-10, 7.6 for the amino group of N-terminal Ile-1 and 9.7 for the δ-amino group of D-Orn-7, very close to the values obtained by other researchers by titration experiments. In agreement with a rigid macrocyclic structure the Stokes radii of different protonated forms ranged only between 14.3 and 14.8 Å. Best fitting procedures performed on experimental mobility measured at two different pH values (5.50 and 6.72) in the presence of increasing Zn+2 concentration allowed confirming the model that assumes the binding of Zn+2 to P0 peptide form with a 1.5×103 M-1 intrinsic association constant. Following to Zn+2 binding, the pKa of the amino group of N-terminal Ile-1 is shifted from 7.6 to 5.9 and the Stokes radius is reduced of about 3 Å. The mean charge of the bacitracin A1-Zn+2 complex resulted +1.67 and +1.12 at pH 5.50 and 6.72, respectively. These results suggest that the amino group of N-terminal Ile-1 is not essential for Zn+2 binding.