Circular dichroism measurements with DNA-spermine complexes at 0.075 M NaCl and at 0.15 M NaCl reveal'+4' (type I) and -W (type II) CD spectra respectively. From small-angle X-ray scattering studles it could be shown that type I has a long-range order, short-range order supramolecular structure, while type II is of long-range disorder, short-range disorder structure. The secondary structure of the DNA in both types of condensates is B-like as concluded from wide-angle X-ray scattering diagrams of the condensates and from a comparison with the wide-angle X-ray curves of DNA and RNA in solution.

The modification of the triple helical structure in aqueous gelatin solutions by changing pH and adding alkyl sulphates at 298 K and after rechilling the solution to 283 K was investigated by CD-measurement. At 298 K the triple helical content at the IEP of the gelatin has its maximum value. It is only weakly affected by adding sodium dodecyl sulphate (SDDS) at concentrations < 10 -4 M/dm 3. The unfolding of the triple helix affected by pH and SDDS is reversible by rechilling the solution. The triple helical content of gelatin solutions decreases at SDDS concentrations higher than 10 -4 M/dm 3. In all cases the decrease of the amount of triple helical structure is connected with an increase of the cis-configuration in single chains and leads to chain reversals. At sufficiently high SDDS concentrations 3-sheets are formed. These changes are thermally irreversible. Sodium decyl sulphate (SDS) has a more minor influence than SDDS except in the range of the c.m.c, of SDS. At sufficiently high SDS concentrations, 3-turns appear.

The structural behaviour of gelatins from different raw materials and manufacturing processes at thermal denaturation and isothermal dehydration and rehydration is investigated by CD. At both thermal denaturation and isothermal dehydration with all gelatins examined, the triple helix content decreases. Simultaneously, the appearance of cis peptide bonds is observed. At rehydration, a structural hysteresis occurs, the reconstitution of the triple helix structure being correlated with a decrease in the content of cis peptide bonds. The possibility of the formation of chain reversals upon destruction of the triple helix is discussed.

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Using CD-measurements the influence of nonionic surfactants (dodecyl polyoxyethylene, ethoxylated para tert. octyl phenol, commercial nonionic WON 100 and octyl diethyl phosphinoxid) on the secondary structure of gelatin in aqueous solutions was investigated. At surfactant concentrations smaller than the c.m.c, the triple helical content of the gelatin is increased. At concentrations exceeding the c.m.c, the triple helical content decreases. Chain reversals of the peptide chains after the destruction of the triple helical structure were shown to appear in acidic environment at 298 K. This destruction is reversible by rechilling the solution.