| Abstract |
A new triethylbenzene-derived tetraazamacrocycle containing pyridyl spacers, L, was prepared and its dinuclear copper(II) complex was used as a receptor for the recognition of phosphorylated peptides in aqueous solution. A detailed study of the acid-base behaviour of L and its copper(II) complexation properties as well as of the cascade species with phosphorylated anions including two peptidic substrates was carried out in a H2O/MeOH (50 : 50 v/v) solution using different techniques, such as potentiometry, X-band EPR and DFT calculations. The association constants of the dinuclear receptor with the phosphorylated peptides and other anionic species revealed a clear preference towards phenylic phosphorylated substrates, with values ranging 3.96-5.35 log units. Single-crystal X-ray diffraction determination of the dicopper(II) complex of L showed the copper centres at a distance of 5.812(1) angstrom from one another, with the phosphate group of the PhPO42- substrate well accommodated between them. X-band EPR studies indicated a similar structure for this cascade complex and for the other cascade complexes with the phosphorylated anions studied. DFT studies of the [Cu2L(mu-OH)](3+) complex revealed a different conformation of the ligand that brings the two copper centres at a very short distance of 3.94 angstrom aided by the presence of a bridging hydroxide anion that provides a Cu center dot center dot center dot O center dot center dot center dot Cu angle of 167.3 degrees. This complex is EPR silent, in line with the singlet ground state obtained using CASSCF(2,2) calculations and DFT calculations with the broken-symmetry approach. This species coexists in solution with a complex in a different conformation, and having a Cu center dot center dot center dot Cu distance of 6.63 angstrom, in lower percentage. |
