Porphyrin Self-Assembly as Template for RNA?
Self-assembly in biological systems is increasingly being recognized as an important phenomenon. As well as the presence of porphyrins is absolutely indispensable for biological occurrences. They exist in all living organisms of fauna and flora (e.g. plants, bacteria, yeast, insects, etc.) controlling oxidation and reduction processes. Furthermore, chiral porphyrins are able to form self-assemblies with left and/or right handed helical structures as basis for superhelical arrangements . In medical treatment, porphyrins often were applied for anti-viral or cytotoxic therapies (e.g. photodynamic therapy). For instance, the chiral nonmetalloporphyrin HpD is used as a photosensitizing agent that selectively kills tumor cells while leaving normal tissues relatively unaffected .
In this study, we investigate the properties of HpD. Destabilizing DNA interactions until DNA damage, concomitant with molecular oxygen production at the nucleus membrane, were found as relevant reactions for its cytotoxic effect . Spectroscopic and thermodynamic investigations were performed to probe the nucleic acid/HpD complex. Decreased Tm-values of the nucleic acid duplexes, as well as reduced CD signal in the presence of HpD were measured. Surprisingly, at room temperature (and 0 - 50 mM NaCl) the racemic HpD molecules themselves show a CD signal at 370 nm (positive or negative both found at concentrations above 0.15 µM HpD), which increases enormously with drug. This is indicative of aggregation of the porphyrin ring system forming right handed (positive) or left handed (negative) helices. The nature of the helix seems to be variable until the concentration is high enough. Before the chiral effect could be observed, the direction seems to be committed and all further aggregation has to follow the stationary course. At temperatures above 25 °C, the spectrum changes completely by decreasing the intensity at 370 nm and forming a characteristic (always positive) signal around 420 nm. At 35 to 40 °C (depending on the ionic strength, 20 to 50 mM NaCl) RNA-similar spectra of the HpD self-assembly occur. At higher temperatures (and lower salt concentration) the aggregate becomes unstable and breaks off.
In the presence of nucleic acids the left-handed aggregation seems to be preferred, because in the case of positive CD-signals (at 370 nm) all spectra turn into the negative direction, which spectra similarly could be measured starting with negative CD-signals (HpD pure 370 nm). In a former study, the formation of coaxially stacked RNA helices under the influence of (tentacle) porphyrins is described monitoring the folding of RNA . Furthermore, an aptamer-hemin complex was found, which could be regarded as a "prototype for redox-catalyzing ribozymes in a primordial 'RNA world'" . These, compared with our data, should now be discussed in favor of a hypothetical evolutionary matrix role of porphyrin self-assembly for RNA.
 H. Engelkamp et al., Science 284, 785-787 (1999); R.J.M. Nolte et al., Polymer preprints 40, 517-518 (1999); F. Venema et al., Chem. Eur. J. 4, 2237-2250 (1998)
G. Bischoff, S. Hoffmann
Martin Luther University Halle-Wittenberg, Institute of Biochemistry,