title: Re-evaluation of the conformational structure of sulfadiazine species using NMR and ab initio DFT studies and its implication on sorption and degradation.
abstract: In the environment, the sorption and the degradation of organic pollutants are of increasing interest. The investigation of the chemical structures provides a basis for the development of a suitable binding model approach and for the mechanistic understanding of the chemical fate processes. The aim of this study was the identification of different species of the antibiotic compound sulfadiazine (SDZ) using (1)H and (13)C NMR experiments and ab initio density functional theory (DFT) calculations. In the neutral, aprotic solvent dimethylsulfoxide-d(6) (DMSO-d(6)), a new sulfadiazine structure containing an O-H-N hydrogen bond was identified. In the protic solvent water-d(2) and in dependence on pH and the position of the amidogen hydrogen atom nine possible SDZ conformations were analyzed and five structures were identified. Good conformity between theory and calculation of (1)H NMR was observed. Unfortunately, (13)C NMR is not sensitive enough for comparison and differentiation. In order to verify the identified structures, additional NBO/NLMO (natural localized molecular orbital) analyses were conducted (calculation of net atomic charges, bond polarity, atomic valence, and electron delocalization). Finally, conformation optimizations were performed in order to investigate the stability of the SDZ species. We showed that SDZ contains no S=O double bond, but that it has two S-O single bonds. Surprisingly, negative charges were observed at the pyrimidine nitrogen atom. With these results, the known structure of SDZ was revised. Studies of the geometrical structure and the torsion angles showed that SDZ is very flexible and can be easily fitted to the sorbent. These observations would explain the strong sorbance and hence the rapid formation of non-extractable residues in the environment because SDZ acts as a strong ligand. These results show that that the sulfonamide hydrogen is important for the biological activity but the pyrimidine nitrogen and the sulfonamide oxygen is responsible for the sorbance in environment.
authors.affiliation: Institute of Ecology, Technische Universität Berlin, Salzufer 12, D-10587 Berlin, Germany; Agrosphere Institute, ICG-4, Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany. Electronic address: g_huschek@igv-gmbh.de.
authors.forename: Gerd
authors.initials: G
authors.lastname: Huschek
authors.affiliation: Leibniz-Institut für Katalyse e.V. an der Universität Rostock, Albert-Einstein-Strasse 29a, D-18059 Rostock, Mecklenburg-Vorpommern, Germany. Electronic address: dirk.hollmann@catalysis.de.
authors.forename: Dirk
authors.initials: D
authors.lastname: Hollmann
authors.affiliation: Institute of Ecology, Technische Universität Berlin, Salzufer 12, D-10587 Berlin, Germany.
authors.forename: Nadine
authors.initials: N
authors.lastname: Kurowski
authors.affiliation: Institute of Ecology, Technische Universität Berlin, Salzufer 12, D-10587 Berlin, Germany.
authors.forename: Martin
authors.initials: M
authors.lastname: Kaupenjohann
authors.affiliation: Agrosphere Institute, ICG-4, Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany.
authors.forename: Harry
authors.initials: H
authors.lastname: Vereecken
mesh_terms: D000042:Absorption; D006863:Hydrogen-Ion Concentration; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D008958:Models, Molecular; D015394:Molecular Structure; D013411:Sulfadiazine / Q000737:chemistry*
chemical_list: D013411:Sulfadiazine