Exploring the energetics of histone H1.1 and H1.4 duplex DNA interactions

- H1.1 and H1.4 bind to DNA oligomers and to CT-DNA with favorable entropy changes.
- The 9 °C increase in Tm for DNA in the H1.4/DNA complex indicates tight binding.
- The H1.4 binding site size was found to be 36.9 bp in titrations of DNA oligomers.
- The H1.1 binding site size was found to be 32 bp in titrations of CT-DNA.
- Loss of water upon H1/DNA complex formation results in large negative ΔCp values.

H1.1 and H1.4 bind tightly to both short DNA oligomers and to CT-DNA (Ka ≈ 1 × 107). Binding is accompanied by an unfavorable enthalpy change (∆H ≈ + 22 kcal/mol) and a favorable entropy change (− T∆S ≈ − 30 kcal/mol). The Tm for the H1.4/CT-DNA complex is increased by 9 °C over the Tm for the free DNA. H1.4 titrations of the DNA oligomers yield stoichiometries (H1/DNA) of 0.64, 0.96, 1.29, and 2.04 for 24, 36, 48, and 72-bp DNA oligomers. The stoichiometries are consistent with a binding site size of 37 ± 1 bp. CT-DNA titration data are consistent with binding site sizes of 32 bp for H1.1 and 36 bp for H1.4. The heat capacity changes, ΔCp, for formation of the H1.1 and H1.4/CT-DNA complexes are − 160 cal mol− 1 K− 1 and − 192 cal mol− 1 K− 1 respectively. The large negative ΔCp values indicate the loss of water from the protein DNA interface in the complex.