Helix Stabilization by Glu-⋯ Lys+ Salt Bridges in Short Peptides of De novo Design

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 84; no. 24; pp. 8898 - 8902
• Main Authors: Marqusee, Susan, Baldwin, Robert L.
• Format: Journal Article
• Language: English
• Published: Washington, DC National Academy of Sciences of the United States of America 01.12.1987; National Acad Sciences
• Subjects: Aggregation; Alanine; Amino acids; Biochemistry; Biological and medical sciences; Circular Dichroism; Drug interactions; Fundamental and applied biological sciences. Psychology; Glutamates; Hot Temperature; Hydrogen Bonding; Hydrogen-Ion Concentration; Ions; Low temperature; Lysine; Molecular biophysics; Peptides; Protein Conformation; Salts; Sodium; Sodium Chloride; Structure in molecular biology; Structure-Activity Relationship; Table salt; Thermodynamics; Titration; Tridimensional structure; Models; Helical structure; Peptides; Ionic bond; Stabilization
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.84.24.8898

Four alanine-based peptides were designed, synthesized, and tested by circular dichroism for α -helix formation in H2O. Each peptide has three glutamic/lysine residue pairs, is 16 or 17 amino acids long, and has blocked α -NH2 and α -COOH groups. In one set of peptides (``i+4''), the glutamic and lysine residues are spaced 4 residues or 1 residue apart. In the other set (``i+3''), the spacing is 3 or 2 residues. Within each of these sets, a pair of peptides was made in which the positions of the glutamic and lysine residues are reversed [Glu, Lys (E,K) vs. Lys, Glu (K,E)] in order to assess the interaction of the charged side chains with the helix dipole. Since the amino acid compositions of these peptides differ at most by a single alanine residue, differences in helicity are caused chiefly by the spacing and positions of the charged residues. The basic aim of this study was to test for helix stabilization by (Glu-, Lys+) ion pairs or salt bridges (H-bonded ion pairs). The results are as follows. (i) All four peptides show significant helix formation, and the stability of the α -helix does not depend on peptide concentration in the range studied. The best helix-former is (i+4)E,K, which shows ≈ 80% helicity in 0.01 M NaCl at pH 7 and 0 degrees C. (ii) The two i+4 peptides show more helix formation than the i+3 peptides. pH titration gives no evidence for helix stabilization by i+3 ion pairs. (iii) Surprisingly, the i+4 peptides form more stable helices than the i+3 peptides at extremes of pH (pH 2 and pH 12) as well as at pH 7. These results may be explained by helix stabilization through Glu-⋯ Lys+ salt bridges at pH 7 and singly charged H bonds at pH 2 (Glu0⋯ Lys+) and pH 12 (Glu-⋯ Lys0). The reason why these links stabilize the α -helix more effectively in the i+4 than in the i+3 peptides is not known. (iv) Reversal of the positions of glutamic and lysine residues usually affects helix stability in the manner excepted for interaction of these charged groups with the helix dipole. (v) α -Helix formation in these alanine-based peptides is enthalpydriven, as is helix formation by the C-peptide of ribonuclease A.