Analysis of host-assisted guest protonation exemplified for p-sulfonatocalix[4]arene -: Towards enzyme-mimetic pKa shifts

The pD dependence of the complexation of p-sulfonatocalix[4]arene (CX4) with the azoalkanes 2,3-diazabicyclo[2.2.1]hept-2-ene (1), 2,3-diazabicyclo[2.2.2]oct-2-ene (2), 2,3-diazabicyclo[2.2.3]non-2-ene (3), and 1-methyl-4-isopropyl-2,3-diazabicyclo- [2.2.2]oct-2-ene (4) in D2O has been studied. The pD-dependent binding constants, determined by H-1 NMR spectroscopy, were analyzed according to a seven-state model, which included the CX4 tetra- and penta-anions, the protonated and unprotonated forms of the azoalkanes, the corresponding complexes, as well as the complex formed between CX4 and the deuteriated hydronium ion. The variation of the UV absorption spectra, namely the hypsochromic shift in the near-UV band of the azo chromophore upon protonation, was analyzed according to a four-state model. Measurements by independent methods demonstrated that complexation by CX4 shifts the pK(a) values of the guest molecules by around 2 units, thereby establishing a case of host-assisted guest protonation. The pK(a) shift can be translated into improved binding (factor of 100) of the protonated guest relative to its unprotonated form as a result of the cation-receptor properties of CX4. The results are discussed in the context of supra-molecular catalytic activity and the pK(a) shifts induced by different types of macrocyclic hosts are compared.