Contribution of Hydrophobicity to Thermodynamics of Ligand-DNA Binding and DNA Collapse

The importance of understanding the dynamics of DNA condensation is inherent in the biological significance of DNA packaging in cell nuclei, as well as for gene therapy applications. Specifically, the role of ligand hydrophobicity in DNA condensation has received little attention. Considering that only multivalent cations can induce true DNA condensation, previous studies exploring monovalent lipids have been unable to address this question. In this study we have elucidated the contribution of the hydrophobic effect to multivalent cation- and cationic lipid-DNA binding and DNA collapse by studying the thermodynamics of cobalt hexammine-, spermine-, and lipospermine-plasmid DNA binding at different temperatures. Comparable molar heat capacity changes (ΔCp) associated with cobalt hexammine- and spermine-DNA binding (−23.39 cal/mol K and −17.98 cal/mol K, respectively) suggest that upon binding to DNA, there are insignificant changes in the hydration state of the methylene groups in spermine. In contrast, the acyl chain contribution to the ΔCp of lipospermine-DNA binding (ΔCp ϕ = ΔCp lipospermine − ΔCp spermine) is significant (−220.94 cal/mol K). Although lipopermine induces DNA ordering into “tubular” suprastructures, such structures do not assume toroidal dimensions as observed for spermine-DNA complexes. We postulate that a steric barrier posed by the acyl chains in lipospermine precludes packaging of DNA into dimensions comparable to those found in nature.