A versatile solid-phase approach based on peptide chemistry was used to construct four classes of structurally diverse polyamines with modified backbones: linear, partially constrained, branched, and cyclic. Their effects on DNA duplex stability and structure were examined. The polyamines showed distinct activities, thus highlighting the importance of polyamine backbone structure. Interestingly, the rank order of polyamine ability for DNA compaction was different to that for their effects on circular dichroism and melting temperature, thus indicating that these polyamines have distinct effects on secondary and higher-order structures of DNA.

The introduction of hexahistidine (His tag) is widely used as a tool for affinity purification of recombinant proteins, since the His tag binds selectively to nickel–nitrilotriacetic acid (Ni²⁺–NTA) complex. To develop efficient “turn-on” fluorescent probes for His-tagged proteins, we adopted a fluorophore displacement strategy, that is, we designed probes in which a hydroxycoumarin fluorophore is joined via a linker to a metal–NTA moiety, with which it forms a weak intramolecular complex, thereby quenching the fluorescence. In the presence of a His tag, with which the metal–NTA moiety binds strongly, the fluorophore is displaced, which results in a dramatic enhancement of fluorescence. We synthesized a series of hydroxycoumarins that were modified by various linkers with NTA (NTAC ligands), and investigated the chemical and photophysical properties of the free ligands and their metal complexes. From the viewpoint of fluorescence quenching, Ni²⁺ and Co²⁺ were the best metals. Fluorescence spectroscopy revealed a 1:1 binding stoichiometry for the Ni²⁺ and Co²⁺ complexes of NTACs in pH 7.4 aqueous buffer. As anticipated, these complexes showed weak intrinsic fluorescence, but addition of a His-tagged peptide (H-(His)₆-Tyr-NH₂; Tyr was included to allow convenient concentration measurement) in pH 7.4 aqueous buffer resulted in up to a 22-fold increase in the fluorescence quantum yield. We found that the Co²⁺ complexes showed superior properties. No fluorescence enhancement was seen in the presence of angiotensin I, which contains two nonadjacent histidine residues; this suggests that the probes are selective for the polyhistidine peptide.