Porphyrin self-assembly as template for RNA?
Abstract
The self-assembly of chiral porphyrin molecules HpD (hematoporphyrin IX derivative) has been shown to form helical fibers in low salt aqueous conditions. The spectroscopic (UV and circular dichroism (CD)), thermodynamic (Tm, differential scanning calorimetry (DSC)) and microscopic (light and scanning force microscopy (SFM)) examinations of the HpD properties were performed individually and in the presence of nucleic acid double strands (15–60 °C, 0–50 mM NaCl). The asymmetric HpD molecules themselves at room temperature show sharp positive or negative CD signals, which increase enormously with HpD concentration. The data show strong evidence for the external self-stacking interaction of HpD, pure and in the presence of polynucleotides. At low salt concentration (<40 mM NaCl, pH 7) the spectra change completely by increasing the temperature. At 35 to 40 °C RNA-similar spectra of the pure HpD self-assemblies (without nucleic acids) occur. At higher temperatures the aggregates become unstable and break off. At room temperature the helical structure of the fibers could be visualized by SFM investigations. Molecular modeling analysis offers dynamic arrangements of the self-assemblies from stacks to spiral-like superstructures with increasing temperature. Hydrogen bonding, electron transferring and electrostatic interactions determine the shape of the proposed highly flexible arrangements. Moreover, the interrelation between the HpD stacks and the helix of the polynucleotides was studied. The calculated low transition energies indicate the importance of these structures as a crossing link. All data are discussed in favor of a hypothetical evolutionary matrix role in porphyrin self-assembly for RNA.
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