You have to go back quite a bit to get to the literature that describes the theoretical basis of the CD spectrum of the alpha helix. This paper is a place to start:
https://www.researchgate.net/profile/Robert_Woody/publication/234869216_Optical_Rotation_of_Oriented_Helices._III._Calculation_of_the_Rotatory_Dispersion_and_Circular_Dichroism_of_the_Alpha_and_310Helix/links/00b49529654d2ca0bc000000.pdf
Article Optical Rotation of Oriented Helices. III. Calculation of th...
The ‘inherently asymmetric’ peptide bonds in proteins absorb in UV region of the spectra (240 nm and below); and the electronic absorption for amides observed are a weak (and broad) n->pi* transition centered around 220 nm (non-bonded nitrogen electron pair), and an intense pi->pi* transition at about 190 nm (double bond). The intensity and energy of these transitions depend on the peptide bond angles (Phi and Psi), and thus the secondary structure of the protein. The optical transitions are affected when the amide chromophores of polypeptide backbone are aligned in arrays. Therefore, the different structural elements produce a characteristic CD spectrum. The most apparent spectral signatures for α-helix are negative bands at 222 nm and 208 nm; and that for β-sheet is a negative band at ~218 nm. The random coil structure displays negative band at around 195 nm.
References:
1. Johnson, W. C., Jr. (1988) Secondary structure of proteins through circular dichroism spectroscopy. Annu Rev Biophys Biophys Chem 17, 145-166
2. Kelly, S. M., Jess, T. J., and Price, N. C. (2005) How to study proteins by circular dichroism. Biochim Biophys Acta 1751, 119-139
3. Greenfield, N. J. (2006) Using circular dichroism spectra to estimate protein secondary structure. Nat Protoc 1, 2876-2890
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