FTIR spectroscopy of alanine-based peptides: assignment of the amide I' modes for random coil and helix.
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)nA-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 degree 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.
Martinez
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G
Department of Chemistry and Biochemistry, University of California, Santa Cruz 95064, USA.
Millhauser
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G
D000409:Alanine / Q000737:chemistry*; D000577:Amides; D000595:Amino Acid Sequence; D006358:Hot Temperature; D006860:Hydrogen Bonding; D008969:Molecular Sequence Data; D010455:Peptides / Q000737:chemistry*; D011489:Protein Denaturation; D017433:Protein Structure, Secondary; D017550:Spectroscopy, Fourier Transform Infrared
D000577:Amides; D010455:Peptides; D000409:Alanine