The central role of thrombin in the blood coagulation cascade makes it an attractive target for the development of inhibitors to treat thromboembolic diseases. Several naturally occurring peptides such as hirudin have been shown to be potent direct thrombin inhibitors (DTIs).¹ Previous work in our group has shown that tyrosine sulfation plays an important role in the activity of these DTIs, with order of magnitude decreases in inhibition constants for thrombin.^2,3 Tyrosine sulfation, however, suffers from well-documented acidic lability,⁴ therefore, there is a need to design acid-stable analogues of sulfotyrosine which retain biological activity. We designed and synthesised two Fmoc-protected sulfonate analogues of sulfotyrosine⁵ via a convergent synthesis with a key Negishi cross-coupling step, which were amenable to standard peptide chemical synthesis procedures and exploited the inherent acid-stability of the C-linked sulfonate moiety over its’ corresponding O-linked sulfate. Incorporation of these synthetic amino acids into two known potent DTIs using Fmoc solid-phase peptide synthesis provided acid-stable DTI analogues. Biological evaluation using thrombin-inhibition assays showed complete retention of anti-thrombotic activity, paving the way for the use such amino acid analogues as suitable mimics of native sulfotyrosine in acid-sensitive applications such as their use in potential anti-thrombotic drug leads.

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