Nowdays, the study of the structure of molecular layers on surfaces is an relevant issue, because atomic arrangement determines the mechanical properties, electronic behaviour and reactivity of surfaces. Peptide nucleic acid (PNA) is an achiral and uncharged DNA mimic of high biological and chemical stability that could have preceded RNA at the first stages of molecular evolution. Previous studies have reported the unique properties of single stranded PNA oligomers for self-assembly on gold surfaces as well as their capability for recognizing complementary DNA in solution. Therefore, we have investigated these interactions between ssPNA and PNA-DNA oligomers on pyrite surfaces. Pyrite is one of the most ubiquitous metal sulphides on Earth; to comprehend its role as absorbent and reactant in natural environments is a great value as a first approach for understanding the role played by surfaces in the origin and evolution of life.
Here, we report on the structural characterization of the ssPNA and PNA-DNA adlayers immobilized on natural pyrite surfaces. We have investigated the chemical interactions of ssPNA and PNA-DNA at different molecular coverage, and the role played by the surface has been analyzed in detail. These results were obtained by the combined use of surface characterization techniques: X-ray photoemission spectroscopy (XPS), X-ray absorption near-edge spectroscopy (XANES), Reflection Absorption Infrared Spectroscopy (RAIRS) and Atomic Force Microscopy (AFM). We have shown that whereas single stranded PNA forms partially-ordered self-assembled monolayers (SAMs) on gold surfaces, no preferential orientation of thiolated ssPNA is observed upon adsorption on natural pyrite (FeS2) surfaces. ssPNA interacts strongly with the Fe from the pyrite surface, and adsorbs without forming an unique covalent bonding to the surface through the S atom. Therefore, molecular orientation of PNA makes difficult the hybridization process with complementary DNA, as PNA nucleic bases are not available for hybridization process. Nevertheless, XPS and RAIRS analysis of PNA-DNA show detection of phosphorus, proving the presence of DNA on the pyrite surface, so even if the hybridization process does not take place, DNA interacts with pyrite surface.
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