Selective molecular recognition of methylated lysines and arginines by cucurbit[6]uril and cucurbit[7]uril in aqueous solution

Abstract

Cucurbit[7]uril selectively binds the epigenetic mark N^ε,N^ε,N^ε-trimethyllysine (LysMe₃, K_CB[7] = (1.8 ± 0.6)× 10⁶ dm³ mol⁻¹) by 3500-fold over lysine ((5.3 ± 0.7) × 10² dm³ mol⁻¹) in aqueous solution, using ion–dipole interactions and the hydrophobic effect, rather than cation–π interactions, as in the “aromatic cages” of p-SO₃-calix[4]arene hosts or chromodomain proteins which recognize LysMe₃. The trend in K_CB[7] of LysMe₃ > LysMe₂ > LysMe > Lys follows the recognition pattern of the chromodomain HP1 and other LysMe_n protein readers. With CB[6], protonation of the guest carboxylate group is required for the formation of inclusion complexes with the LysMe_n series. The CB[7] host also displays modest selectivity between the asymmetric ((2.0 ± 0.3) × 10³ dm³ mol⁻¹) and symmetric ((6.1 ± 0.6) × 10³ dm³ mol⁻¹) dimethylarginines, both of which bind more strongly than the parent arginine or monomethylarginine.