A very intense negative band is observed a t -183 nm in the CD spectrum of fibronectin from bovine plasma. This transition has not previously been reported, probably because it occurs in a spectral region that has not been readily accessible in earlier studies. At longer wavelength, the observed CD is very similar to spectra reported for human and chick material, having positive bands a t -230 and -200 nm, and a negative band a t -215 nm. The low molar ellipticity of the negative band ([el = -2.5 X lo3 deg cm2 dmol-') suggests little or-helix or P-sheet structure. The new transition, and the two positive bands a t higher wavelength, do not correspond to known transitions of the peptide backbone, but all three are present in the CD of N-acetyltyrosineamide. It is therefore suggested that the observed CD behavior of fibronectin arises predominantly from the optical activity of tyrosine side chains. The contribution of this side-chain optical activity to the CD of other proteins is discussed.
On raising pH to ionize tyrosine residues, the positive CD band a t -230 nm is lost in both N-acetyltyrosineamide and in fibronectin. The spectral change is fully reversible in the model compound, but only partially reversible in fibronectin. From this evidence, and the magnitude of the 183-nm band, it is suggested that some or all of the tyrosine residues in fibronectin may be present within ordered domains. The possible role of S-S bonds in maintaining tertiary structure is discussed. The interaction of fibronectin with heparin is accompanied by a large increase in the 183-nm band and by slight enhancement of the negative band at 215 nm, consistent with some limited formation of P-sheet.
Present results indicate that CD may be of considerable value in characterization of the molecular organization and biologically relevant interactions of fibronectins and of related glycoproteins of the extracellular matrix.