Analysis of the segmental stability of helical peptides by isotope-edited infrared spectroscopy

Isotope-edited infrared spectroscopy has the ability to probe the segmental properties of long biopolymers. In this work, we have compared the infrare d spectra of a model helical peptide (C-12) Ac-W-(E-A-A-A-R)(6)-A-NH2, desc ribed originally by Merutka et al. (Biochemistry 1991;30: 4245-4248) and th ree derivatives that are C-13 labeled at the backbone carbonyl of alanines. The locations of six isotopically labeled alanines are at the N-terminal, C-terminal, and the middle two repeating units of the peptide. Variation in temperature from 1 degrees to 91 degreesC transformed the peptides from pr edominantly helical to predominantly disordered state. Amplitude and positi on of the infrared amide I' absorption bands from C-12- and C-13-labeled se gments provided information about the helical content. Temperature dependen ce of infrared spectra was used to estimate segmental stability. As a contr ol measure of overall peptide stability and helicity (independent of labeli ng), the temperature dependence of circular dichroism spectra in the far-UV range at identical conditions (temperature and solvent) as infrared spectr a was measured. The results indicate that the central quarter of the 32 ami no acids helix has the maximal helicity and stability. The midpoint of the melting curve of the central quarter of the helix is 5.4 +/- 0.8 degreesC h igher than that of the termini. The N-terminal third of the helix is more h elical and is 2.0 +/- 1.4 degreesC more stable than the C-terminus. (C) 200 1 Wiley-Liss, Inc.