On the molecular conformation of human haemopexin. II. Analysis of circular dichroic spectra.

The circular dichroic (CD) spectra of haemopexin in the far ultraviolet region exhibit an atypical positive maximum at 231 nm, which prevents determination of the secondary structure by the usual methods. We have developed a modified analytical method by which it was possible to calculate the secondary structure of the protein (8% of alpha-helix, 10% of beta-conformation) and the actual wavelength (229 nm) and intensity of the above-mentioned positive band. Measurements and analysis of the CD spectra of haemopexin in an alkaline medium gave information ascribing a major part of intensity of the band at 229 nm to the B1u transition of Tyr; we believe that some other electronic transitions contribute to this band, too. An assignation of the individual extremes of the CD spectrum in the near ultraviolet region to the individual electronic transitions is propounded. The dichroic bands of Trp in this region and the temperature dependence of the CD spectra in the far ultraviolet region corroborate our idea that the haemopexin molecule contains a very compact hydrophobic peptide core. Similarities of the CD spectra of haemopexin to those of the haem. haemopexin complex throughout the ultraviolet region suggest a conformational likeness of these two molecules.