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Ftir Assignment

Functional GroupNotesAlkane C-H bonds are fairly ubiquitous and therefore usually less useful in determining structure.

Absorption peaks above 3000 cm-1 are frequently diagnostic of unsaturation

See "Free vs. Hyrdogen-Bonded Hydroxyl Groups" in the Introduction to IR Spectra for more informationPrimary amines produce two N-H stretch absorptions, secondary amides only one, and tetriary none.

The carbonyl stretching absorption is one of the strongest IR absorptions, and is very useful in structure determination as one can determine both the number of carbonyl groups (assuming peaks do not overlap) but also an estimation of which types.As with amines, an amide produces zero to two N-H absorptions depending on its type.

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 310- and α-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 α-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 α-helical of the three peptides at 1°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 310-helix, along the thermal unfolding pathway.