Mass Spectrometric Determination of Association Constants of Bovine Serum Albumin (BSA) with para-Sulphonato-Calix[n]arene Derivatives

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Electrospray Ionization Mass Spectrometry (ESI/MS) has been used to determine the association constants (K As) and binding stoichiometries for parent para-Sulphonato-calix[n]arenes and their derivatives with bovine serum albumin (BSA). K A values were determined by titration experiments using a constant concentration of protein. K A measurements were carried out in a methanol–formic acid solution. 5,11,17,23–tetra-Sulphonato-calix[4]arene (1a) and 25-mono-(2-aminoethoxy)-5,11,17,23-tetra-Sulphonato-calix[4]arene (1d) interact strongly with BSA showing 3 non-equivalent binding sites with K A1 = 7.69 × 10M−1, K A2 = 3.85 × 10M−1, K A3 = 0.33 × 10M−1 and K A1 = 1.69 × 10M−1, K A2 = 2.94 × 10M−1, K A3 = 0.60 × 10M−1, respectively. The strength of the interactions between the calixarene and BSA is inversely proportional to the size of macrocyclic ring: n = 4 > n=6>>n=8.