A Neutral, Water-Soluble, α-Helical Peptide: The Effect of Ionic Strength on Helix-Coil Equilibrium J. Martin Scholtz, Eunice J. York, John M. Stewart,

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A Neutral, Water-Soluble, α-Helical Peptide: The Effect of Ionic Strength on Helix-Coil Equilibrium J. Martin Scholtz, Eunice J. York, John M. Stewart, and Robert L. Baldwin

Background

Alanine-based Peptides  In a previous study it was suggested that the α-helical polypeptide backbone itself is responsible for stability of the helix in water.  Scholtz, J.M.; Marqusee, S.; Baldwin, R.L.; York, E.J.; Stewart, J. M.; Santoro, M; Bolen, D. W. Proc. Natl. Acad. Sci. U.S.A. 1991, 88,  These peptides are being used to study helical propensities of amino acids.

Objective  To characterize a synthesized uncharged alanine-based peptide.  Test a prediction based on 1943 theory of Kirkwood for the thermodynamic interaction between a dipolar ion and an electrolyte. Prediction: Increasing ion strength will stabilize the helix by shifting the equilibrium between the helix and random coil toward the helix.

Methods  A 16 residue peptide used: Ac-(AAQAA) 3 Y(NH 2 )  Circular Dichroism Spectroscopy Uses circularly polarized light Is used to determine secondary structure  Alpha helices give a characteristic CD spectrum

Thermal-unfolding Curve

Helical Content of Peptide as a Function of Ion Strength ■-Na 2 SO 4 ●-NaCl ▲-CaCl 2

Changes in ΔG for Helix Stability as a Function of Ionic Strength ■-Na 2 SO 4 ●-NaCl ▲-CaCl 2 Equation used to calculate free energy: ΔG= -RT ln (σs π-2 ) Hofmeister series

Changes in ΔG for Helix Stability after Subtraction of the Specific Hofmeister Effect ■-Na2SO4 ●-NaCl ▲-CaCl2 The magnitude of the dipole moment of the α-helix is estimated from the slope of the line and is 51D, yielding a 3.2D dipole moment per residue

Conclusion  Agreement with previously determined values may be coincidental.  A detailed investigation of Kirkwood’s theory may reveal that electrolyte ions interact with partial charges NH and C=O near the ends of the helix that are not H bonded rather than with the macrodipole.