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 condensation 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 λ-DNA by spermine and a series of its homologues, H₂N(CH₂)₃NH (CH₂)_n=_2-12NH (CH₂)₃NH₂ (_n = 4 for spermine), using static and dynamic light scattering techniques. All polyamines provoked DNA condensation; however, their efficacy varied with the structural geometry of the polyamine. In 10 mM sodium cacodylate buffer, the EC₅₀ values for DNA condensation were comparable (4 ± 1 μM) 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 monovalent ion (Na⁺) concentration in the buffer. The slope of a plot of log EC₅₀(polyamine⁴⁺)] against log [Na⁺] was ∼ 1.5 for polyamines with even number values of n, whereas the slope value was ̃ 1 for compounds with odd number values of n. Dynamic light scattering measurements showed the presence of compact particles with hydrodynamic radii (R_h) of about 40-50 nm for compounds with n = 3-6. R_h increased with further increase in methylene chain length separating the secondary amino groups of the polyamines (R_h = 60-70 nm for n = 7 - 10 and > 100 nm for n = 11 and 12). Determination 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 chemical structure of isovalent polyamines exerts a profound influence on their ability to recognize and condense DNA, and on the size of the DNA condensates formed in aqueous solution.