# Electronic Transition Moments of 6-methyl Isoxanthopterin—A Fluorescent Analogue of the Nucleic Acid Base Guanine

 Title: Electronic Transition Moments of 6-methyl Isoxanthopterin—A Fluorescent Analogue of the Nucleic Acid Base Guanine Author: Widom, Julia; Rappoport, Dmitrij; Perdomo-Ortiz, Alejandro; Thomsen, Hanna; Johnson, Neil P.; von Hippel, Peter H.; Aspuru-Guzik, Alan; Marcus, Andrew H. Note: Order does not necessarily reflect citation order of authors. Citation: Widom, Julia R., Dmitrij Rappoport, Alejandro Perdomo-Ortiz, Hanna Thomsen, Neil P. Johnson, Peter H. von Hippel, Alan Aspuru-Guzik, and Andrew Marcus. 2012. Electronic transition moments of 6-methyl isoxanthopterin—a fluorescent analogue of the nucleic acid base guanine. Nucleic Acids Research 41(2): 995-1004. Full Text & Related Files: Nucl. Acids Res.-2013-Widom-995-1004.pdf (435.8Kb; PDF) Abstract: Fluorescent nucleic acid base analogues are important spectroscopic tools for understanding local structure and dynamics of DNA and RNA. We studied the orientations and magnitudes of the electric dipole transition moments (EDTMs) of 6-methyl isoxanthopterin (6-MI), a fluorescent analogue of guanine that has been particularly useful in biological studies. Using a combination of absorption spectroscopy, linear dichroism (LD) and quantum chemical calculations, we identified six electronic transitions that occur within the 25 000–50 000 $$cm^{−1}$$ spectral range. Our results indicate that the two experimentally observed lowest-energy transitions, which occur at 29 687 $$cm^{−1}$$ (337 nm) and 34 596 $$cm^{−1}$$ (289 nm), are each polarized within the plane of the 6-MI base. A third in-plane polarized transition is experimentally observed at 47 547 $$cm^{−1}$$ (210 nm). The theoretically predicted orientation of the lowest-energy transition moment agrees well with experiment. Based on these results, we constructed an exciton model to describe the absorption spectra of a 6-MI dinucleotide–substituted double-stranded DNA construct. This model is in good agreement with the experimental data. The orientations and intensities of the low-energy electronic transitions of 6-MI reported here should be useful for studying local conformations of DNA and RNA in biologically important complexes. Published Version: doi:10.1093/nar/gks1148 Terms of Use: This article is made available under the terms and conditions applicable to Other Posted Material, as set forth at http://nrs.harvard.edu/urn-3:HUL.InstRepos:dash.current.terms-of-use#LAA Citable link to this page: http://nrs.harvard.edu/urn-3:HUL.InstRepos:10382939 Downloads of this work: