Stretching the α-helix:: a direct measure of the hydrogen-bond energy of a single-peptide molecule

Atomic force microscopy was used to measure the force required to stretch individual molecules of the peptide cysteine(3)-lysine(30)-cysteine from the alpha-helical state into a linear chain (approximately 200 pN). The measured force versus peptide elongation was used to calculate the work done in breaking the hydrogen bonds which give rise to the helical structure. The average experimental value of the hydrogen-bond energy (20.2 kJ/mol) is in good agreement with reported theoretical calculations. In addition, the stiffness of individual peptides was measured directly using a force modulation technique and found to vary from approximately 0.005-0.012 N/m during elongation. (C) 1999 Elsevier Science B.V. All rights reserved.