Proton concentration (pH) switches the higher-order structure of DNA in the presence of spermine.

Single-chain observations on the conformational change of giant DNA (T4 DNA) molecules were performed using fluorescence microscopy at different values of pH in the presence of spermine. Individual DNA molecules undergo a large discrete change, or all-or-none transition, in conformation from a folded compact state to an unfolded coil state with an increase in pH. This abrupt unfolding of DNA with an increase in pH is attributed to a decrease in the concentration of the tetravalent form in spermine [SPM(4+)]. We propose a scheme for the folding transition of single DNAs, where the manner of spermine binding changes dramatically from weak loose binding in the elongated coil state to strong tight binding in the folded compact state. We discuss the hierarchical nature of the transition, i.e. cooperative continuous change on the ensemble vs. all-or-none switching on individual DNAs.