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A series of iota-carrageenans containing different amounts of nu-carrageenan (0-23 monomer %) have been prepared from neutrally extracted carrageenan of Eucheuma denticulatum. nu-Carrageenan is the biochemical precursor of iota-carrageenan. The conformational order-disorder transition and rheological properties of these carrageenans were studied using optical rotation, rheometry, size exclusion chromatography coupled to multiangle laser light scattering, and high-sensitivity differential scanning calorimetry. The helix forming capacity of iota-carrageenan turns out to decrease monotonously with increasing amount of nu-units. In contrast, the rheological properties of iota-carrageenan are remarkably enhanced by the presence of a small amount of nu-units, yielding a maximum twofold increase in G' at 3% nu-units. It is concluded that the structure-forming capacity of iota-carrageenan, containing a small amount of nu-carrageenan, is significantly higher than that of pure iota-carrageenan. This phenomenon is explained in terms of the balance between the helical content and the number of cross-links between chains, taking into consideration the fact that nu-units introduce "kinks" in the chain conformation enabling neighboring chains to connect. Increasing amounts of nu-units increase the number of cross-links in the network, resulting in increased gel strength. On the other hand, a reduced length of the helical strands weakens the cross-links between the different chains and, consequently, the gel., (Copyright 2002 Wiley Periodicals, Inc. Biopolymers 65: 299-312, 2002)

Conformational transitions of holo-alpha-lactalbumin in a hydro-ethanolic cosolvent system was studied by spectrofluorescence, CD in near- and far-uv regions, and high-sensitivity differential scanning calorimetry. Experimental results allow us to propose that in isothermal conditions alpha-lactalbumin undergoes a number of conformational transitions with increasing ethanol concentration: N<=>I<=>D<=>H. The existence of I-state was deduced from spectrofluorometric and near-uv CD data. In this state the aromatic chromophores of the amino acid side chains are more accessible to the solvent displaying higher local mobility. The H-state was detected from far-uv CD spectra as a state corresponding to the content of alpha-helices higher than originally found in native protein. However, calorimetric measurements provide data revealing only the two-state mechanism of alpha-lactalbumin unfolding in both water and in aqueous ethanol solutions. This indicates that the energy levels of N- and I-states as well as of D- and H-states are similar. Thermodynamics of the unfolding of alpha-lactalbumin in hydroethanolic solutions was analyzed with the help of the linear model of solvent denaturation. Unfolding increments of enthalpy, entropy, and Gibbs energy of transfer of the protein from a reference aqueous solution to hydro-ethanolic solutions of different concentrations were determined from the calorimetric data. They are linear functions of molar ethanol fraction. The slope of the unfolding increment of Gibbs energy of transfer was calculated from data on transfer of amino acid residues taking into account the average solvent accessibility of amino acid residues in the native structure of small globular proteins, using the additive group contribution method.