title: X-ray spectroscopy of heterocyclic biochemicals: xanthine, hypoxanthine, and caffeine.
abstract: The electronic structures of the purine derivatives xanthine, hypoxanthine and caffeine have been investigated in the gas phase using C, N, and O 1s X-ray photoemission (XPS) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The results have been interpreted by means of ab initio calculations using the third-order algebraic-diagrammatic construction (ADC(3)) method for the one-particle Green's function and the second-order ADC method (ADC(2)) for the polarization propagator. The carbon, nitrogen and oxygen K-edge NEXAFS spectra of xanthine and caffeine are very similar, since the molecules differ only by substitution of three hydrogen atoms by methyl groups. For hypoxanthine, the electronic structure and spectra differ considerably from xanthine as the purine ring is more highly conjugated, and there is one less oxo group. Effects due to oxo-hydroxy tautomerism were not observed. However, the two oxo tautomeric forms of hypoxanthine oxo-N(9)-H and oxo-N(7)-H are populated in the gas phase, and the C 1s spectra can be simulated only by taking account of these two tautomers, with appropriate Boltzmann population ratios which we have also calculated. For xanthine and caffeine, single tautomeric forms were observed.
authors.affiliation: Sincrotrone Trieste, Area Science Park, I-34149 Basovizza (Trieste), Italy.
authors.forename: Oksana
authors.initials: O
authors.lastname: Plekan
authors.forename: Vitaliy
authors.initials: V
authors.lastname: Feyer
authors.forename: Robert
authors.initials: R
authors.lastname: Richter
authors.forename: Angelica
authors.initials: A
authors.lastname: Moise
authors.forename: Marcello
authors.initials: M
authors.lastname: Coreno
authors.forename: Kevin C
authors.initials: KC
authors.lastname: Prince
authors.forename: Irina L
authors.initials: IL
authors.lastname: Zaytseva
authors.forename: Tatyana E
authors.initials: TE
authors.lastname: Moskovskaya
authors.forename: Dmitriy Yu
authors.initials: DY
authors.lastname: Soshnikov
authors.forename: Alexander B
authors.initials: AB
authors.lastname: Trofimov
mesh_terms: D002110:Caffeine / Q000737:chemistry*; D002244:Carbon / Q000737:chemistry; D005740:Gases; D019271:Hypoxanthine / Q000737:chemistry*; D008956:Models, Chemical; D015394:Molecular Structure; D009584:Nitrogen / Q000737:chemistry; D010100:Oxygen / Q000737:chemistry; D011789:Quantum Theory; D013052:Spectrometry, X-Ray Emission / Q000379:methods; D056928:X-Ray Absorption Spectroscopy / Q000379:methods; D019820:Xanthine / Q000737:chemistry*
chemical_list: D005740:Gases; D019820:Xanthine; D019271:Hypoxanthine; D002110:Caffeine; D002244:Carbon; D009584:Nitrogen; D010100:Oxygen