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 gamma-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 trifluoroethanol 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 alpha-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 beta-turn character at pH 5.3 to increased alpha-helix structure at pH 2.5. 13C alpha 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.
Fan
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Department of Biochemistry, University of Minnesota, Minneapolis 55455, USA.
Mayo
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D000595:Amino Acid Sequence; D002942:Circular Dichroism; D005340:Fibrinogen / Q000737:chemistry*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D009682:Magnetic Resonance Spectroscopy; D008969:Molecular Sequence Data; D010446:Peptide Fragments / Q000737:chemistry*; D011487:Protein Conformation; D014270:Trifluoroethanol / Q000494:pharmacology*
D010446:Peptide Fragments; D014270:Trifluoroethanol; D005340:Fibrinogen