Conformation of cyclo(L-alanylglycyl-ε-aminocaproyl), a cyclized dipeptide model for a β bend. 3. Infrared and Raman spectroscopic studies
Infrared and Raman spectra of cyclo(L-alanylglycyl-ε-aminocaproyl) have been obtained in the solid state and in solution. The spectra have been analyzed with the aid of a normal mode analysis, carried out on computed low-energy conformations of the molecule. The results of this analysis and spectra of model compounds can be used to interpret the effects of conformation on vibrational frequencies. In the solid state, the spectra are found to be consistent only with a type II β bend. A type II bend is the predominant conformation in solution as well. The presence of large amounts of other low-energy bend conformations (of bend types I, III, I′, and III′) in solution is clearly ruled out as being inconsistent with one or several of the observed Vibrational frequencies. Small amounts of type I and III bends may be present in aqueous solution. This result agrees with the conclusions reached from theoretical conformational energy calculations and from NMR and CD spectroscopic measurements, reported in the two preceding papers of this series. The spectra of the open-chain dipeptide analogue N-acetyl-N′-methyl-L-alanylglycinamide differ quantitatively from those of the cyclic molecule, and they indicate considerably more conformational flexibility in the open-chain peptide. © 1981 American Chemical Society.
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