DNA condensation by polyamines: A laser light scattering study of structural effects

Polyamines such as spermidine and spermine are abundant in living cells and are believed to aid in the dense packaging of cellular DNA. DNA condensati on is a prerequisite for the transport of gene vectors in living cells. To elucidate the structural features of polyamines governing DNA condensation, we studied the collapse of lambda -DNA by spermine and a series of its hom ologues, H2N(CH2)(3)NH(CH2)(n=2-12)NH(CH2)(3)NH2 (n = 4 for spermine), usin g static and dynamic light scattering techniques. All polyamines provoked D NA condensation; however, their efficacy varied with the structural geometr y of the polyamine. In 10 mM sodium cacodylate buffer, the EC50 values for DNA condensation were comparable (4 I muM) for spermine homologues with n = 4-8, whereas the lower and higher homologues provoked DNA condensation at higher EC50 values. The EC50 values increased with an increase in the monov alent ion (Na+) concentration in the buffer. The slope of a plot of log [EC 50(polyamine(4+))] against log [Na+] was similar to1.5 for polyamines with even number values of n, whereas the slope value was similar to1 for compou nds with odd number values of n. Dynamic light scattering measurements show ed the presence of compact particles with hydrodynamic radii (Rh) of about 40-50 nm for compounds with n = 3-6. Rh increased with further increase in methylene chain length separating the secondary amino groups of the polyami nes (Rh = 60-70 am for n = 7-10 and > 100 nm for n = 11 and 12). Determinat ion of the relative binding affinity of polyamines to DNA using an ethidium bromide displacement assay showed that homologues with n = 2 and 3 as well as those with n > 7 had significantly lower DNA binding affinity compared to spermine and homologues with n = 5 and 6. These data suggest that the ch emical structure of isovalent polyamines exerts a profound influence on the ir ability to recognize and condense DNA, and on the size of the DNA conden sates formed in aqueous solution.