title: Effect of the ortho modification of azobenzene on the photoregulatory efficiency of DNA hybridization and the thermal stability of its cis form.
abstract: We synthesized various azobenzenes methylated at their ortho positions with respect to the azo bond for more effective photoregulation of DNA hybridization. Photoregulatory efficiency, evaluated from the change of T(m) (DeltaT(m)) induced by trans-cis isomerization, was significantly improved for all ortho-modified azobenzenes compared with non-modified azobenzene due to the more stabilized trans form and the more destabilized cis form. Among the synthesized azobenzenes, 4-carboxy-2',6'-dimethylazobenzene (2',6'-Me-Azo), in which two ortho positions of the distal benzene ring with respect to carboxyl group were methylated, exhibited the largest DeltaT(m), whereas the newly synthesized 2,6-Me-Azo (4-carboxy-2,6-dimethylazobenzene), which possesses two methyl groups on the two ortho positions of the other benzene ring, showed moderate improvement of DeltaT(m). Both NMR spectroscopic analysis and computer modeling revealed that the two methyl groups on 2',6'-Me-Azo were located near the imino protons of adjacent base pairs; these stabilized the DNA duplex by stacking interactions in the trans form and destabilized the DNA duplex by steric hindrance in the cis form. In addition, the thermal stability of cis-2',6'-Me-Azo was also greatly improved, but not that of cis-2,6-Me-Azo. Solvent effects on the half-life of the cis form demonstrated that cis-to-trans isomerization of all the modified azobenzenes proceeded through an inversion route. Improved thermal stability of 2',6'-Me-Azo but not 2,6-Me-Azo in the cis form was attributed to the retardation of the inversion process due to steric hindrance between lone pair electrons of the pi orbital of the nitrogen atom and the methyl group on the distal benzene ring.
authors.affiliation: Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan.
authors.forename: Hidenori
authors.initials: H
authors.lastname: Nishioka
authors.forename: Xingguo
authors.initials: X
authors.lastname: Liang
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Asanuma
mesh_terms: D001391:Azo Compounds / Q000737:chemistry*; D020029:Base Pairing; D003198:Computer Simulation; D003432:Cross-Linking Reagents; D004247:DNA / Q000737:chemistry*; D015342:DNA Probes; D007536:Isomerism; D008027:Light; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009690:Nucleic Acid Conformation; D009693:Nucleic Acid Hybridization; D010777:Photochemistry; D013696:Temperature; D013816:Thermodynamics
chemical_list: D001391:Azo Compounds; D003432:Cross-Linking Reagents; D015342:DNA Probes; D004247:DNA; C009850:azobenzene