Alpha-helix to random coil transitions: determination of peptide concentration from the CD at the isodichroic point.

A method is presented for determining the concentrations of peptides and proteins having isodichroic points near 203 nm. The existence of an isodichroic point for a given substance indicates a local two-state (alpha-helix, random coil) population. The mean residue ellipticity at the isodichroic point, theta lambda i], is, of course, independent of helix content. For a wide variety of synthetic and natural peptides, including both single helices and coiled coils, it is shown that theta lambda i] is also essentially independent of substance and of whether the transition is induced by temperature, ionic strength, pH, chain length changes, amino acid substitution, or solvent perturbation. Averaging theta lambda i] values culled from various laboratories gives -151 +/- 16 (SD, 7 sources) deg.cm2.mmol-1. In our laboratory, nonpolymerizable rabbit alpha-tropomyosin and two alpha-tropomyosin subsequences yield -135 +/- 10 (SD, 190 values) deg.cm2.mmol-1. Thus, given theta lambda i] for a peptide of known concentration, it is possible to estimate the concentration of any other peptide provided that it has an isodichroic point at which the ellipticity is accurately measurable. It is then possible to calculate theta lambda] at any other wavelength for which theta is known. It is advisable to determine theta lambda i] for the best known peptide in one's own laboratory, since it depends on absolute instrument and cell calibrations and an absolute concentration determination.