The enthalpy of the alanine peptide helix measured by isothermal titration calorimetry using metal-binding to induce helix formation.

The goal of this study is to use the model system described earlier to make direct measurements of the enthalpy of helix formation at different temperatures. For this we studied model alanine peptides in which helix formation can be triggered by metal (La(3+)) binding. The heat of La(3+) interaction with the peptides at different temperatures is measured by isothermal titration calorimetry. Circular dichroism spectroscopy is used to follow helix formation. Peptides of increasing length (12-, 16-, and 19-aa residues) that contain a La(3+)-binding loop followed by helices of increasing length, are used to separate the heat of metal binding from the enthalpy of helix formation. We demonstrate that (i) the enthalpy of helix formation is -0.9 +/- 0.1 kcal/mol; (ii) the enthalpy of helix formation is independent of the peptide length; (iii) the enthalpy of helix formation does not depend significantly on temperature in the range from 5 to 45 degrees C, suggesting that the heat capacity change on helix formation is very small. Thus, the use of metal binding to induce helix formation has an enormous potential for measuring various thermodynamic properties of alpha-helices.