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A procedure is described in detail by which the algal polysaccharide, i-carrageenan, can be modified and fragmented to products of more regular structure and shorter chain length. Since most chains of this product can form uninterrupted helices they undergo the disorder-order transition without the gelation that is a feature of the behaviour of the natural polymer.The disorder-order transition becomes much sharper and occurs at higher temperaturcs with increasing chain length. The sign and magnitude of the accompanying shift in optical rotation can be estimated satisfactorily from the solid-state geometry of the helix using our semi-empirical method for correlating optical rotation with the conformations at glycosidic linkages. In solutions of the helical form, but not the coil form, cationic dyes such as methylene blue or acridine orange show induced optical activity and a changed visible spectrum which we attribute to binding and stacking on the helix. This also causcs a shift in the temperature of the helix-coil transition. We discuss the possibility that the tendency of dyes to stack could drive polysaccharides into conformations that they would not otherwise adopt, with implications for their use as 'conformational probes'.It is now several years since we reported in a preliminary communication [I] that the algal polysaccharide, i-carrageenan, can be converted to short chain segments of modified structure, which show a co-operative, temperature-induced conformation change in aqueous solution. We suggested that this conformation change was between random coil and double-helix forms, chiefly because of the then proposed [2] (and since confirmed [3]) double-helical structure in the orientated fibres. Various properties of the transition have since been investigated, and many results now support the hypothesis of a change from coil to double helix. These include: molecular weight changes [4,5], heat changes by calorimetry and from variation of equilibrium constant with temperature [6], thc axial ratio as measured by small angle X-ray scattering (unpublished data) and nuclear magnetic relaxation [7]. Preliminary reports also demonstrate the correlation of the sign and magnitude of the optical rotation shift with the geometry of the double helix [8] and that the disorderorder transition shows second-order kinetics [9]. Meanwhile more polysaccharides have been shown to exhibit orderdisorder transitions [I0 -161. However the short chain segments of modified r-carrageenan remain the simplest and bestcharacterised example, and they are still frequently used to develop general methods for investigation and characterisation of other systems. In contrast to all these results, a recent report from another laboratory [17] has claimed that, under certain conditions differcnt from our own, carrageenans can undergo a conformational changc which is intraniolecular. r-Carrageenan may be isolated from a number of marine algae including Eucheuma spinosum [I 81, Eucheuma uncinutum [ 191, Eucheuma isoforme [I 91, Aghardhiell...

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SynopsisA 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.

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