RESUMEN
Deoxyribonucleoprotamine (DNPn) from sonicated nuclei of sturgeon sperm heads was studied by means of ring dichroism. A derivative analysis of DNA and DNPn melting curves in 1 mM Tris. HCl pH 8.0 revealed the fraction of protein-free DNA being about 30% and suggested the preferable binding of protamine molecules with AT-rich DNA regions. The latter is also confirmed by the data on ring dichroism of protein-poor soluble DNAPn fraction in 0,14 M NaCl. Ring dichroism of DNA and DNPn in 1 mM Tris coinsides at the wavelength of 310-240 nm at concentrations of 500-50 mkg/ml. Dilution of DNPn to 5 mkg-ml resulted in the decrease of the ellipticity at 275 nm and produced no effect at 260-210 nm. The effect observed is suggested to be due to a partial transition of DNA in DNPn into C-form under the dilution as a result of a higher molecule hydration and a destruction of some hydrogen bonds between guanidine residues of arginine and oxygen of phosphate groups, stabilyzing DNA in the B-form. Ring dichroism spectrum of protamine, calculated by the subtraction of DNA spectrum from DNPn spectrum at the region of 240-210 nm coinsides with that of free protamine and indicates the absence of an ordered structure in protamine molecules in DNPn.