Unusual 1H NMR chemical shifts support (His) Cε1–H•••O==C H-bond: proposal for reaction-driven ring flip mechanism in serine protease catalysis - Ash, E L; Sudmeier, J L; Day, R M; Vincent, M; Torchilin, E V; Haddad, K C; Bradshaw, E M; Sanford, D G; Bachovchin, W W (Proc Natl Acad Sci U S A, 2000)