Chromatic models. Interactions between DNA and polypeptides containing L-lysine L-valine: circular dichroism and thermal denaturation studies.

The interaction of calf thymus DNA with statistical copolymers of L-lysine and L-valine [poly(L-Lys100f-Lvalf)] and block copolymers [poly(L-Lys)100f-poly(L-Val)f] were investigated as a function of ionic strength using circular dichroism (CD) spectroscopy. It was found that valine suppresses the ability of the copolymer-DNA complexes to yield a psi-type CD spectra as found for poly(L-Lys)-DNA [Jordan, C.F., Lerman, L.S., and Venable, J.N. (1972), Nature (london), New Biol. 236, 67] and lowers the ionic strength at which CD distortion occurs. Thermal denaturation, simultaneously monitoring 280-nm ellipticity, [theta]280, and hyperchromicity, h280, was carried out on annealed complexes of poly(L-Lys)-DNA, poly(L-Lys84.5-L-Val15.5)-DNA, poly(L-Lys)87.2-poly(L-Val)12.8-DNA, and directly mixed complexes of poly(L-Lys)-DNA, IN 2.5 X 10(-4) MEDTA, pH 7.0 solution. The CD denaturation of uncomplexed DNA at several ionic strengths was also determined to examine pre-melting. Despite the inability of both statistical and block copolymers of L-Lys and L-Val to form psi-type complexes with DNA, they bind as well to DNA as does poly(L-Lys) and give rise to a thermal denaturation pattern showing bound peaks between 90 and 100 degrees C, seen clearly with CD denaturation. The thermal denaturation of mixed and annealed complexes of poly(L-Lys)-DNA shows similar patterns in hyperchromicity changes as a function of temperature but very different CD melts. From the CD melt of annealed poly(L-Lys)-DNA, it appears that aggregation and long-range order of the complex are significant in low salt (2.5 X 10(-4) MEDTA) as well as in 1.0 M NaCl. These studies further illustrate the importance of the nature of nonionic interactions (hydrophobic) between polypeptides and DNA in determining the behavior of their complexes, such as causing condensation into higher order asymmetric structures. In light of these observations, the possible significance to the CD melting of chromatin and the validity of identification of C-form DNA by CD spectroscopy are discussed.