The AMI-isonitrile ligation 2.0: Fast, selective, and pH sensitive

The bioorthogonal ligation between isonitriles and azomethine imines (AMIs)—the AMI-isonitrile ligation—combines exquisite chemoselectivity with a stable ligation product, a small molecular reporter, and a pH-dependent rate. In this work, we tailored the modular structure of the dipolar AMI to increase Brønsted basicity and electrophilicity. These additive structural modifications increased the ligation rate by more than two orders of magnitude to 14 M⁻¹s⁻¹ at pH 7, 140 M⁻¹s⁻¹ at pH 6, and >1,000 M⁻¹s⁻¹ at pH 5. The faster reaction rate at lower pH values allowed for the preferential labeling of live cells at acidic versus neutral pH. This environmental sensitivity paves the way for in vivo targeting of acidic milieus, such as tumors.