PEGylation of Fluorescein by Enzyme-Catalyzed "Click" Michael Addition

This paper reports the first "Click" Michael addition catalyzed by Candida antarctica lipase B (CALB) between fluorescein o-acrylate and thiol-functionalized poly(ethylene glycol)s (HS-PEG-SH, M_n = 1200 g mol^-1 , Đ = 1.14, and M_n = 2200 g mol^-1 , Đ = 1.09). The progress of the reactions is monitored with ¹ H-NMR spectroscopy. In the absence of CALB, the reaction does not go to completion even after 18 h but completes in less than 2 min when CALB is added. Similarly, the reaction with HS-PEG-SH having M_n = 2200 g mol^-1 and Đ = 1.09 completes in less than 2 min by CALB catalysis. The structures of the products are also confirmed by ¹³ C-NMR. This enzyme-catalyzed "Click" Michael addition is found to be a powerful tool to synthesize fluorescein-based polymeric conjugates for a wide variety of applications.