The unfolded- and folded-state infrared (IR) spectra of peptides studied to date show a common pattern, i.e., the amide I peak of the unfolded state is typically shifted toward higher frequencies with respect to the folded peak. Here, we study by means of a theoretical-computational method, the Perturbed Matrix Method (PMM), the IR spectra in the amide I region of two beta-hairpin peptides. The computed spectra are in good agreement with the experimental ones, thus providing an explanation of the physical origin underlying the differences of the unfolded- and folded-state spectra. (C) 2010 Elsevier B.V. All rights reserved.