Synthesis, structural and biological evaluation of GlyAla based lanthanide macrocyclic conjugates as supramolecular ribonuclease mimics

Abstract

The glycine–alanine conjugated ligands of cyclen (1,4,7,10,-tetraazacyclododecane) 1 and 2, possessing methyl and benzyl alanine esters respectively, and the corresponding lanthanide complexes 1La, 1Eu, 1Tb, 1Yb, 2La, 2Eu, and 2Tb were designed with the aim of mimicking the nature of the hydrophobic cavity of ribonucleases. X-ray crystallographic investigations showed that 2Tb has a typical monocapped square antiprism geometry, where the Tb(III) ion is central, coordinating to the four amino moieties of the cyclen ring and four of the oxygens of amide carbonyl groups of the glycine residues of the four pendant arms, with the ninth coordinated site being occupied by a water molecule. All the complexes were shown to promote the hydrolysis of the phosphodiester bond of 2-hydroxypropyl p-nitrophenyl phosphate (HPNP, τ 1/2=5.78×10 3 h) with 1Tb being the most efficient in promoting such hydrolysis at pH 7.4 and at 37 °C for the 1Ln family with τ 1/2=4.9 h. For the 2Ln family, 2La was most effective in promoting hydrolysis of HPNP, with τ 1/2=3.7 h. The rate of hydrolysis was also investigated for 1La and 2La as function of pH, with both complexes displaying bell-shaped pH dependence within the physiological pH range. For 1Ln the highest activity was observed at pH 7.0, with τ 1/2=4.6 h, whereas for 2La it occurred at pH 7.4. Beyond pH 8, the rate of both complexes was shown to be almost linearly increased. The ability of 1Eu and 2Eu to cleave a 23-mer sequence from the mRNA of the GAG–HIV gene was also investigated. It was found that both gave rise to cleavage of the sequence at every nucleotide residue after 4 h of incubations at pH 7.4 and 37 °C.