Figure 3.

DNA force curves with different amine counterions are fit to double exponential functions with λ and λ/2 decay lengths. All counterion concentrations are large enough that forces depend only slightly on ionic strength. Spermidine³⁺ and spermine⁴⁺ spontaneously precipitate DNA. The amplitude of the λ decay length exponential changes from repulsive to attractive between putrescine²⁺ and spermidine³⁺. The best fitting decay length λ varies between 4.2 – 5.0 Å. The λ/2 decay length exponential is repulsive for all the charged amines shown. The amplitudes of the λ/2 decay length exponential for DNA interactions in NH₄⁺, putrescine²⁺, spermidine³⁺, and spermine⁴⁺ are closely comparable. A similar result for the λ/2 decay length force was found for DNA with an extended set of homologous arginine peptides ranging from +1 to +6 charges. The triangle symbols with dots and crosses are for DNA in 1.2 M TMA⁺ at 5° and 50° C, respectively. The square symbols with dots and crosses are for DNA in 30 mM putrescine²⁺ at 5° and 50° C. In spite of the ~25% decrease in the dielectric constant of water between 5° and 50° C, forces change negligibly.