Direct Measurement of Salt-Bridge Solvation Energies Using a Peptide Model System: Implications for Protein Stability

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 93; no. 7; pp. 2985 - 2990
• Main Authors: Wimley, William C., Gawrisch, Klaus, Creamer, Trevor P., White, Stephen H.
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences of the United States of America 02-04-1996; National Acad Sciences; National Academy of Sciences
• Subjects: Amino Acid Sequence; Amino Acids; Biochemistry; Buffers; Calorimetry; Carboxyl compounds; Chemical equilibrium; Chromatography, High Pressure Liquid; Electrostatics; Free energy; Hydrogen-Ion Concentration; Kinetics; Magnetic Resonance Spectroscopy; Models, Molecular; Molecular Sequence Data; Octanols; Oligopeptides - chemical synthesis; Oligopeptides - chemistry; Physics; Protein Conformation; Protein stability; Salt; Salts; Solubility; Solvation; Spectrometry, Mass, Fast Atom Bombardment; Structure-Activity Relationship; Thermodynamics
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.93.7.2985

The solvation energies of salt bridges formed between the terminal carboxyl of the host pentapeptide AcWL-X-LL and the side chains of Arg or Lys in the guest (X) position have been measured. The energies were derived from octanol-to-buffer transfer free energies determined between pH 1 and pH 9. 13C NMR measurements show that the salt bridges form in the octanol phase, but not in the buffer phase, when the side chains and the terminal carboxyl group are charged. The free energy of salt-bridge formation in octanol is approximately -4 kcal/mol (1 cal = 4.184J), which is equal to or slightly larger than the sum of the solvation energies of noninteracting pairs of charged side chains. This is about one-half the free energy that would result from replacing a charge pair in octanol with a pair of hydrophobic residues of moderate size. Therefore, salt bridging in octanol can change the favorable aqueous solvation energy of a pair of oppositely charged residues to neutral or slightly unfavorable but cannot provide the same free energy decrease as hydrophobic residues. This is consistent with recent computational and experimental studies of protein stability.