Buffer exchange

Buffers with different tris concentrations were prepared and exchanged with the DNA folding buffer through spin filtration.³⁵ Sixty microliters of DNA solution was introduced inside a 100 kDa Amicon filter with additional 440 μL of the desired buffer. The solution was then spun at 7000 rpm for 4 min. About 50–60 μL were left inside the filter after filtration. A second and third filtration cycle was done with additional 440 μL of the desired buffer added after each cycle. The volume of the resulting solution was then measured and subsequently adjusted to meet the desired DNA concentration (20–30 ng μL⁻¹). The approximate concentrations of tris and Mg²⁺ were estimated from the remaining volume after the filtration cycles and the additional buffer solution added during the concentration adjustments. After exchange with pure water, the average DNA concentration is 5.5 ± 0.9 nM with estimated average Mg²⁺ and tris concentrations of 10 ± 3 μM and 32 ± 8 μM, respectively.