Protein structure determination from CD

The (right handed) a-helrx is the dominant secondary structure in many proteins and on average accounts for about one third of the residues in globular proteins. It is a well-defined structural motif where the nth peptide unit forms hydrogen bonds between its C-0 and the N-H of the (n + 4)th peptide and between its N-H and the (n - 4)th C-0 there is a 0.15 nm translation and 100° rotation between two consecutive peptide units, giving 3.6 amino add residues per turn. Its helix pitch (number of residues times distance between a-Cs on neighbouring residues) is 0.54 nm. 

The CD spectra of a-helices are characterized by a negative band with separate maxima of similar magnitude at 222 nm (the n- ir transition) and 208 nm which is part of the transition (see Section 5.3 and Rgure 15). The a-helix is the only motif where the transition has such a long wavelength component. The a-helix CD is also larger in magnitude than that due to other motifs so it is apparent upon the most casual inspection of a spectrum. It is no 

The 3io helix is a right handed helix with 3.0 residues per turn and a helix pitch of 0.6 nm. 

An alternative efficient formation of hydrogen bonds occurs between a sheet of parallel or antiparallel runs of amino acids these are known as a p-sheets. The runs of amino acids face in alternate directions so that alternate amino acids hydrogen bond to neighbouring runs on each side. The spectroscopic characterization of p-sheets has proved more difficult than that of a-helices due to the practical reason that they are less soluble in solvents with a good UV transmission, and due to the intrinsic reason that they are generally structurally less well-defined they may be parallel or antiparallel and of varying lengths and widths. Furthermore, an extended p-sheet is usually found to show a marked twist, rather than to be planar. Such tertiary structure influences the overall CD spectrum. 

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