Photon-Catalytic Coupling Reaction Study Of Adenine And Its Derivatives Using Surface-Enhanced Raman Spectroscopy

Adenine is not only one of the five bases in DNA and RNA but also an integral part of energetic adenosine5’-triphosphate (ATP) as well as several coenzymes such as nicotinamide adenine dinucleotide (NAD+) and flavin adenine dinucleotide (FAD)O There are a large number of researches on it and its derivatives. Surface-enhanced Raman spectroscopy (SERS) has been used to investigate adenine and its derivatives since1980s for it can provide the fingerprint information. But owing to the complexity of SERS and the unstability of silver nanostructures, the reported SERS spectra of adenine and its derivatives showed diversity. There are mainly two kinds of SERS spectra, one exhibits two strong peaks around730and1330cm-1, the other dose several strong peaks among1000to1500cm-1, such as1038,1240,1417,and1476cm-1which is definitely different from the former one. People used to attributed the second kind of spectra to the change of orientation, deprotonation, or some unsure reactions. In this thesis, by combining experiment and theory, a new surface plasmon-enhanced catalytic reaction of adenine to6,6’-azobis-9H-purine (azopurine) was proposed to explain the unusual SERS spectra.The amine group of adenine was first selectively oxidized to azopurine in the presence of silver ions or silver nanostructures in basic aqueous solution under visible light. The possible mechanism was also studied here. In the silver colloids, the hot spot from the aggregation of nanoparticles can enhance the reaction occourring greatly. The reaction product azopurine is responsible for the reported unusual signals of adenine and its derivatives. Moreover, we assigned all the peaks on the basis of calculated results of azopurine.The SERS spectra of azopurine are sensitive to the pH value. When the pH was turned to less than5, the SERS spectra changed obviously. Our DFT calculations showed azopurine was changed to6,6’-azobis-lH-purine. The protonation at N1leaded to a significant change to the structure, especially the N-N and C-N bond distances. While the vibrations of corresponding to these bonds were related to the strong peaks in the spectra. In addition, isotrope-labeling15N-15N azopurine synthesized were used to collect the SERS spectra on silver surfaces in basic and acidic aqueous solutions. The SERS peaks related to the N-N bond further demonstrated that the unusual SERS signal came from azopurine.In summary, we get the SERS spectra of azopurine from adenine containing important biomolecules such as adenine, adenosine, adenosine monophosphate (AMP), NAD+, and poly(dA)8,the results show that they can also undergo the similar surface catalytic azo coupling reaction to azopurine on silver nanostructures.