Département de chimie moléculaire

Nucleic acid aptamers are involved in a broad field of applications ranging from therapeutics to analytics. Deciphering the binding mechanisms between aptamers and small ligands is therefore crucial to improve and optimize existing applications and to develop new ones. Particularly interesting is the enantiospecific binding mechanism involving small mols. with nonprestructured aptamers. One archetypal example is the chiral binding between L-tyrosinamide and its 49-mer aptamer for which neither structural nor mechanistic information is available. In the present work, the authors have taken advantage of a multiple anal. characterization strategy (i.e., using electroanal. techniques such as kinetic rotating droplet electrochem., fluorescence polarization, isothermal titrn. calorimetry, and quartz crystal microbalance) for interpreting the nature of binding process. Screening of the binding thermodn. and kinetics with a wide range of aptamer sequences revealed the lack of symmetry between the two ends of the 23-mer minimal binding sequence, showing an unprecedented influence of the 5' aptamer modification on the bimol. binding rate const. kon and no significant effect on the dissocn. rate const. koff. The results the authors have obtained lead us to conclude that the enantiospecific binding reaction occurs through an induced-fit mechanism, wherein the ligand promotes a primary nucleation binding step near the 5'-end of the aptamer followed by a directional folding of the aptamer around its target from 5'-end to 3'-end. Functionalization of the 5'-end position by a chem. label, a polydA tail, a protein, or a surface influences the kinetic/thermodn. consts. up to 2 orders of magnitude in the extreme case of a surface immobilized aptamer, while significantly weaker effect is obsd. for a 3'-end modification. The reason is that steric hindrance must be overcome to nucleate the binding complex in the presence of a modification near the nucleation site. [on SciFinder(R)]