Use of Circular Dichroism to Determine Secondary Structure of Neuropeptides
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Circular dichroism (CD) spectroscopy is currently the method of choice to study the conformation of proteins in low resolution
(1
). Chiral substances absorb right and left circularly polarized lights to different extents, demonstrating differences in
absorbance (△A) and molar extinction coefficients (△ε). A typical CD curve is the wavelength dependence of △ε, or of the molar
([Θ]M
) or mean residue ([Θ]MR
) ellipticities (2
). Protein CD concentrates on the amide chromophore between 180 and 260 nm, where the nπ_and_ππ_transitions_occur._Although~1____________a_single_amide_group_is_achiral~E_amide~Famide_interactions_~Achiral_perturbation~B_give_rise_to_a_positive_CD_in_proteins_~A~Kcite~M3~1~K~Hcite~M~1~B._Proteins_and_polypeptides_with_a_single_type_of_secondary_structure_such_as_a_helix~E_parallel_or_antiparallel_β pleated
sheets, regularly repeating β turns, or unordered structure exhibit characteristic CD curves (4
,5
) (see
Note 1
). Many algorithms for curve analysis have been developed (6
–9
), but they offer pitfalls (10
), and some exceed the capabilities of commercial instrumentation (11
).
