DETERMINATION OF THE ACID DISSOCIATION-CONSTANTS FOR WR-1065 BY PROTON NMR-SPECTROSCOPY

The acid dissociation constants for N-(2-mercaptoethyl)-1,3-diaminopropane (WR-1065) were determined in D2O by 360- and 500-MHz NMR spectroscopy. Results obtained at 0.21 M initial ionic strength and 26°C were corrected to 25°C yielding pK(D1) = 8.28 ± 0.04, pK(D2) = 9.88 ± 0.07, and pK(D3) = 11.58 ± 0.03. Correction of these values for the effect of the deuterium isotope upon the ionization reaction yielded dissociation constants in water of pK(H1) = 7.69 ± 0.09, pK(H2) = 9.35 ± 0.09, and pK(H3) = 11.10 ± 0.08. Analysis of the changes in chemical shift with pD indicated that the first ionization occurs largely through ionization of the thiol group (~67%) and to a lesser extent the secondary ammonium group (~30%), whereas the third ionization involves mainly the secondary ammonium group (~65%) and to a lesser extent the primary ammonium group (~30%). Estimates of the microscopic pK values for WR-1065 were also obtained from the results.