Thrombin is a serine protease that plays a key downstream role in the blood coagulation cascade. The central role of thrombin in the formation of fibrin clots makes it an attractive target for the development of inhibitors to treat thromboembolic diseases. Several naturally occurring anti-thrombotic peptides such as hirudin have been isolated and shown to be potent direct thrombin inhibitors (DTIs). Previous work in our group has shown that tyrosine sulfation plays an important role in the activity of DTIs, with 2-3 orders of magnitude increases in inhibitory potency against human thrombin.1 Tyrosine sulfation, however, suffers from well-documented acidic lability.2 Therefore, there is a need to design acid-stable analogues of sulfotyrosine which retain biological activity against thrombin. Such analogues would improve the ease of synthesis and purification of thrombin inhibitors and should improve biological stability in vivo. Based on prior work by Taylor and co-workers,3 we designed a synthetic route to Fmoc-protected analogues of sulfotyrosine (sTyr) (Fig. 1.) via a key Negishi coupling step. Incorporation of these unnatural amino acids into two known potent thrombin inhibitors, using Fmoc solid-phase peptide synthesis provided the acid-stable analogues. We have evaluated the biological activity by performing thrombin inhibition experiments and could show that the activity of the peptides containing sTyr mimics is maintained.