Binding Studies of G-Quadruplex DNA and Porphyrins: Cu(T4) vs Sterically Friendly Cu(tD4).

This investigation explores binding interactions involving G-quadruplex DNA and two copper(II)-containing porphyrins (5,10,15,20-tetra(N-methylpyridinium-4-yl)porphyrinato)copper(II) and the sterically friendlier analogue (trans-5,15-di(N-methylpyridinium-4-yl)porphyrinato)copper(II), or Cu(T4) and Cu(tD4), respectively. The study incorporates five different DNA sequences that support the formation of unimolecular and bimolecular G-quadruplex hosts capable of exhibiting at least nine different structures in toto. Absorbance and emission results establish that G-quadruplex DNA is more adept at sequestering Cu(tD4) compared with the bulkier Cu(T4) ligand, even though the predominant mode of uptake is by end-capping, irrespective of the porphyrin or DNA sequence employed. One of the more impressive observations is that the emission intensities exhibited by Cu(tD4) bound to G-quadruplex DNA are many-fold higher than corresponding signals obtained with single- or double-stranded DNA hosts. With human sequence DNA the Cu(tD4) system is also unusual in that it preferentially binds to structures containing antiparallel strands. Refining the binding properties of porphyrin ligands is of interest because work from many laboratories has established that stabilizing G-quadruplex structure is an effective way to inhibit telomerase, a key enzyme involved in the immortalization of most types of cancer cells.

Internal versus external binding of cationic porphyrins to single-stranded DNA.

Absorbance, induced circular dichroism, and emission studies establish that the tetrasubstituted cationic porphyrin Cu(T4) preferentially binds externally to single-stranded (ss) DNA sequences, except in a purine-rich system like 5'-(dA)10-3' where a degree of internalization occurs. On the other hand, the sterically friendly, disubstituted Cu(tD4) system exclusively binds to ss DNA by internalization, that is, pseudointercalation. By and large the results show that double-stranded DNA hosts decisively outcompete more flexible ss hosts for the uptake of a porphyrin, regardless of the binding motif. The findings are relevant because ss domains of DNA appear during replication, in different types of DNA-secondary structure, and as products of the disassembly of multistranded forms.

Cationic copper(II) porphyrins intercalate into domains of double-stranded RNA.

A cationic, copper(II)-containing ligand, derived from bulky 5,10,15,20-tetrakis(N-methylpyridinium-4-yl)porphyrin, Cu(T4), and two sterically friendlier forms, [trans-5,15-di(N-methylpyridinium-4-yl)porphyrinato]copper(II), Cu(tD4), and [cis-5,10-di(N-methylpyridinium-4-yl)porphyrinato]copper(II), Cu(cD4), bind to DNA and RNA hosts. Six hairpin-forming RNA 18-mer sequences and two previously studied DNA analogues serve as convenient binding platforms of programmable base composition. A crystal structure shows that the copper center of Cu(tD4) is four-coordinate, establishing compatibility with intercalative binding as well as susceptibility to solvent-induced emission quenching. From the hypochromic responses and the induced emission intensities obtained with all three porphyrins, it is clear that internalization into the RNA host occurs, irrespective of the base pair composition. Further analysis reveals that the porphyrins intercalate into the double-stranded stem domains. Subtle geometric and/or electronic aspects of the binding account for the signs of induced circular dichroic signals and splitting of the Soret band of Cu(tD4).

Experimental in vitro arterial reactivity and tissue culture solutions alter the time-dependent stability of anthocyanins from elderberry, chokeberry, and bilberry extracts.

We examined the effect of solutions commonly used for in vitro assessment of blood vessel physiology and pharmacology on the half-lives of monomeric anthocyanins contained in extracts from elderberry, chokeberry, and bilberry. We observed that monomeric anthocyanin degradation in all extracts was accelerated when they were solubilized in an in vitro vascular physiological salt solution (PSS) compared with extracts in purified water. Degradation was accelerated further by increasing the temperature of the PSS to 37 degrees C and bubbling it with 95% oxygen/5% carbon dioxide. A common, complex, tissue culture media yielded similar results to the physiological salt solution at 37 degrees C. We also observed that the percentage polymeric color estimated by bisulfite bleaching corresponded to monomeric degradation in PSS. These results suggest that exposure of anthocyanins to physiological conditions that mimic those in the human body may stimulate the conversion of monomeric anthocyanins to their polymeric forms. Such conversion would probably contribute to the effects of anthocyanins on physiological functions in in vitro experiments and needs to be considered when evaluating effects of these compounds on physiological processes.