Fourier transform infrared (FTIR) spectroscopy has been used to explore the thermal unfolding of three helical, alanine-based peptides. Each of the peptides follows the general sequence Ac-(AAAX)(n)A-NH2 where X is either Lys(+) or Arg(+) and n = 3 or 4. These particular peptides were chosen because they contain varying amounts of 3(10)- and alpha-helix. The amide I' bands for all three peptides, under helix forming conditions, are between 1632 and 1635 cm(-1). These results are incongruous with the assignment for alpha-helices in proteins where amide I' bands are usually found above 1650 cm(-)1. At elevated temperatures, all the peptides exhibit amide I' bands of 1642 cm(-1), which is the accepted value for random coil. Variable temperature spectra for the 4K peptide (n = 4, X = Lys(+)), which is the most alpha-helical of the three peptides at 1 degrees C, reveal an isosbestic point suggesting a cooperative two-state unfolding transition. The other peptides, however, did not reveal an isosbestic point, thereby indicating the presence of an intermediate, perhaps 3(10)-helix, along the thermal unfolding pathway. (C) 1995 Academic Press, Inc.