Binding of Acetylated Lysine by Using a Water Soluble Aryl Extended Calix[4]pyrrole

: Post-translational modifications of lysine in histones, as methylation and acetylation, have well established functions in epigenetics and are emerging as important actors in broader biological regulation. Currently, the detection of acetylated lysine (Kac) in water solution as free amino acid or protein residue remains challenging. Acetylated lysine is a neutral amino acid, and the lack of ion-dipole interactions causes the decrease in binding affinity displayed by synthetic molecular receptors with respect to the other lysine modifications. Here, we report molecular modeling calculations and 1 H NMR experiments to investigate the binding properties of two different calix[4]pyrrole receptors towards Kac. Computational analyses reveal that tetra-aryl-extended calix[4]pyrrole (1) preferentially binds the cis-Kac conformer over the trans one due to steric considerations and more favorable interactions. Experimental 1 H NMR titration experiments confirm the formation of a 1 : 1 complex between receptor 1 and cis-Kac, with a Ka exceeding 103  M-1 . Conversely, the super-aryl-extended calix[4]pyrrole 2 is less efficient in binding Kac, due to unfavorable solvation/desolvation effects, as proven by 1 H NMR experiments. Moreover, receptor 1 showed a higher affinity for Kac over other lysine modifications, such as methylated lysines.