Circular Dichroism of Proteins and Nucleic Acids

Except for peptides with many aromatic amino acids, the CD spectra of a-helical polypeptides comprised of more than about 10 amino acid residues are relatively independent of the exact amino acid composition [31]. A minimum of 2-3 turns of helix (7-11 residues) appears to be sufficient to generate a typical spectrum [40]. The CD of p-sheet structure is more variable, probably mainly as a result of different amounts of twisting [40]. P-tums also have highly variable spectra. 

The contributions that various secondary structural elements make to a protein of unknown structure can be estimated by fitting the observed CD spectrum with a sum of basis spectra like those shown in Fig. 9.9B, C [27-29, 41-44]. A web-server providing a variety of algorithms for this analysis is available [33,45]. 

Exciton interactions of tryptophan sidechains can give rise to pairs of CD bands in the region of 220 nm, along with weaker bands in the near UV [53, 54], The bands around 220 nm stem from the indole Bb transition. Coupling of peptide transitions with the transiticms of tryptophan and tyrosine sidechains also can contribute significantly to the CD spectra of proteins [44, 55, 56]. 

Circular dichroism spectra of DNA and RNA also depend strongly on the molecular structure. As shown in Fig. 9.11, B-form DNA typically has a CD band with positive rotational strength near 185 nm and negative bands in the regions of 200 and 250 nm, while Z-form DNA has positive bands at 180 and 260 nm and negative bands at 195 and 290 nm. 

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