Spontaneous formation of liquid crystalline phases and phase transitions in highly concentrated plasmid DNA

Abstract

The liquid crystalline (LC) properties of two supercoiled plasmid DNA samples, pBSK (2958 bp) and pGEM (3000 bp), have been studied using polarised light microscopy (PLM), circular dichroism (CD) and UV–Vis spectroscopy. The influence of methods of isolation on plasmid LC behaviour is described, and using PLM we have demonstrated the spontaneous formation of cholesteric fingerprint-like textures. Preliminary studies of LC phase transitions in pGEM show the irreversibility of LC phase formation, as a consequence of changes in the tertiary structure of supercoiled plasmids. Using UV–Vis spectroscopy a hyperchromic effect was observed with increasing temperature. The CD spectra clearly showed structural changes, and probably mismatching of DNA bases, during cooling. Finally, we have observed an irreversible phase transition in plasmid DNA which is very different from that previously reported in linear DNA.

Keywords:

• plasmid DNA
• liquid crystal
• cholesteric phase
• phase transition
• fingerprint-like texture
• supercoiling
• hyperchromic effect
• DNA isolation techniques
• circular dichroism

Acknowledgements

We gratefully acknowledge funding for this research from the Foundation for Polish Science, Programme “Welcome”, and the European Regional Development Fund and European Commission through the Human Potential Programme (Marie-Curie RTN BIMORE, MRTN–CT–2006–035859 and N50713231/3302).