DETERMINATION OF THE INTRINSIC SITE pA. VALUE AND COOPERATIVITY OF THE SYMMETRICAL HEXADENTATE CHELATOR N,N ',N '-TRIS[2-(3-H YDROXY-2-OXO-1,2- DIH YDROP YRIDIN-1 - YL)ACETAMIDO]ETH YLAMINE

摘要

The three overlapping pK_a values of N,N',N"-tris[2-(3-hydroxy-2-oxo-1,2-dihydropyridin-1-yl)acetamido]ethylamine, a tripodal hexadentate chelator formed from three 3-hydroxy-2(1H)-pyridinone moieties amide linked to tris-(2-aminoethyl)amine, were determined by simultaneous spectrophotometric and potentiometric titration. The data was analysed by non-linear regression with constraints to deal with (a) the highly correlated absorptivities and (b) the highly correlated pK_a values. The three pK_a values were optimized first from the spectrophotometric data (absorbance vs. pH) by non-linear regression to a model in which the molar absorptivity of the ith species (∈_i,) was constrained by the correlation equation ∈_i = ε_0 + (ε_3-ε_0)i/3 with i = 0, 1, 2, 3, where ∈_3 and ∈_0 represent the molar absorptivities of the most protonated and least protonated species, respectively. The molar absorbitivity of the four species defined by three pK_a values is, therefore, linearly related to proton stoichiometry. The pK_a values were then optimized from the potentiometric data (pH 115. titrant volume) by non-linear regression to a model in which the three pK, values were constrained by the correlation equation pK_(ai) = pK_(aint) + b(i - 1) + (i - 2)log(3) where i = 1, 2 or 3. This expresses the three pK, values in terms of only two optimizable parameters, the intrinsic site pAT, (pL_(aint)) and the interaction energy between sites (b). The fixed term (i - 2)log(3) accounts for the statistical effect onthe pK, values of three equivalent ionizable sites. The modified analytical derivatives required for optimization of these parameters by the Gauss-Newton-Marquardt algorithm and the merits of optimizing pK, values with these two correlation equations are discussed. The optimized pK, values were 9.31 ± 0.01, 8.75 ± 0.01 and 8.19 ± 0.01. The separation between pK_a values is 0.58 comprising 0.477 for the statistical effect and 0.081 for the interaction energy while the intrinsic site pK_a, is 8.672 ± 0.005. The tertiary amine at the centre of the tripodal backbone has a pK_a of 5.88 ± 0.03.