Effect of pH on the conformation and backbone dynamics of a 27-residue peptide in trifluoroethanol: An NMR and CD study

The C-terminal fragment, residues 385-411, from human fibrinogen γ-chain, i.e. KIIPFNRLTIGEGQQHHLG-GAKQAGDV, shows multiple turn conformations in aqueous solution (Mayo, K. H., Burke, C., Lindon, J. N., and Kloczewiak, M. (1990) Biochemistry 29, 3277-3286). The present study investigates the effect of pH and trifiuoroethanol on the conformation and backbone dynamics of this 27-residue peptide. Both circular dichroism (CD) and ¹H-NMR data indicate the normally observed increased helical conformations as a function of increasing trifluoroethanol. ¹H-NMR structural studies done in the presence of 40% trifluoroethanol, pH 5.3, yield a network of nuclear Overhauser effects consistent with significant populations of helix-like conformation. Distance geometry calculations based on nuclear Overhauser effect-derived distance constraints yield a family of structures with relatively well defined N- and C-terminal conformations and an ill defined mid-peptide region from Gly³⁹⁷ to Gly⁴⁰³. Similar conformational populations are observed at pH 2.5. CD studies, however, indicate an increase in average α-helix content on decreasing the pH from 6 to 2. This apparent conflict between CD and NMR results may be explained by a transition from multiple β-turn character at pH 5.3 to increased α-helix structure at pH 2.5. ¹³C_α NMR relaxation data analyzed with the Lipari-Szabo model-free approach provide order parameters that demonstrate little if any influence of pH on backbone motional restrictions within the more flexible mid-peptide domain. At low pH, however, motions become less restricted within N-terminal residues Lys³⁸⁵-Phe³⁸⁹ and more restricted within C-terminal residues Ala⁴⁰⁵-Val⁴¹¹.