Fine-tuning the properties of the thrombin binding aptamer through cyclization: Effect of the 5'-3' connecting linker on the aptamer stability and anticoagulant activity.

A small library of cyclic TBA analogues (named cycTBA I-IV), obtained by covalently connecting its 5'- and 3'-ends with flexible linkers, has been synthesized with the aim of improving its chemical and enzymatic stability, as well as its anticoagulant properties. Two chemical procedures have been exploited to achieve the desired cyclization, based on the oxime ligation method (providing cycTBA I and II) or on Cu(I)-assisted azide-alkyne cycloaddition (CuAAC) protocols (for cycTBA III and IV), leading to analogues containing circularizing linkers with different chemical nature and length, overall spanning from 22 to 48 atoms. The resulting cyclic TBAs have been characterized using a variety of biophysical methods (UV, CD, gel electrophoresis, SE-HPLC analyses) and then tested for their serum resistance and anticoagulant activity under in vitro experiments. A fine-tuning of the length and flexibility of the linker allowed identifying a cyclic analogue, cycTBA II, with improved anticoagulant activity, associated with a dramatically stabilized G-quadruplex structure (ΔT _m  = +17 °C) and a 6.6-fold higher enzymatic resistance in serum compared to unmodified TBA.
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