Fast and Tight Boronate Formation for Click Bioorthogonal Conjugation

A new click bioorthogonal reaction system was devised to enable the fast ligation (k(ON) ≈340 m(-1) s(-1)) of conjugatable derivatives of a rigid cyclic diol (nopoldiol) and a carefully optimized boronic acid partner, 2-methyl-5-carboxymethylphenylboronic acid. Using NMR and fluorescence spectroscopy studies, the corresponding boronates were found to form reversibly within minutes at low micromolar concentration in water, providing submicromolar equilibrium constant (K(eq) ≈10(5) -10(6) m(-1)). Efficient protein conjugation under physiological conditions was demonstrated with model proteins thioredoxin and albumin, and characterized by mass spectrometry and gel electrophoresis.