College of Chemistry and Molecular Sciences, Key Laboratory of Biomedical Polymers, Ministry of Education Wuhan University, Hubei, Wuhan 430072 (P.R. China)

Hydrogen bonding between Nucleobases In DNA Watson-Crick base pairs Wobles and Hoogsteen base pairs Hoogsteen base pair is a variation of base paring in the nucleic acids. Hoogsteen pairs have different properties from Watson-Crick base pairs Angle between glycocidic bonds Smaller distance between the chains

Non –Watson-Crick pair allows the winding of third strand on a duplex formed by Watson-Crick pairing This paring also allows the formation of secondary structures of the DNA and RNA

Nucleic acid sequences which are rich in guanine are capable of forming four-stranded structures called G-quadruplexes. A telomere is a region of repetitive DNA at the end of a chromosome, which protects the end of the chromosome from deterioration.

Thermal stability of Telomere DNA (d-(TTAGGG)4) CD signal at 265 nm is monitored Melting experiments suggest that the stability depends on the configuration of the azobenzene In a solution of trans-azobenzene, d(TTAGGG)4 showed a large variation in absorbance over a wide range of temperatures (Tm=55.8ºC). UV-titration the binding constant of trans-azobenzene and the telomere DNA was about (3.9 0.5)x106m-1 UV-vis absorption titration spectra of 10μM tans azobenzene and 10μM cis – azobenzene with tel DNA.

Binding affinity of the azobenzene UV-vis absorption titration spectra of 10μM tans - azobenzene and 10μM cis – azobenzene with tel DNA.

Switch for exonuclease I UV/Vis light could act as a switch for exonuclease I, which catalyzes hydrolysis reactions, on the basis of the conversion of a folded G quadruplex into an unfolded conformation by photoillumination.

Modeling studies using Gaussian 03 The trans isomer displayed a planar azobenzene core, which participated in - stacking with the guanine quartets The positively charged side chains linked to the azobenzene core displayed a high degree of relaxation and approached the phosphate backbone. The cis isomer formed a v-shaped groove. This configuration reduced - stacking and the rotary flexibility of the side chain. cis isomer and telomere DNA could not interact sufficiently to effect a conformational change

Summary Azobenzene derivative have been synthesized and characterized using UV/Vis and 1H NMR spectroscopy. The photo regulation of G-quadruplex DNA by cis-trans isomerization of the azobenzene was demonstrated using CD spectroscopy, thermal denaturation studies, a UV/Vis-absorption titration experiment, and an exonuclease I hydrolysis assay. Theoretical modeling studies showed that at the trans state of the azobenzene core assumes a planar geometry to participate in - stacking with the guanine quartets. The V-shaped groove conformation of the cis-azobenzene reduced the - stacking and the rotary flexibility of the telomere DNA. Compound 1 is the first small molecule that has been used to induce reversible stretching and folding in a G quadruplex on the basis of photoirradiation. This nanodevice directly converts light into mechanical work and could be applied as a nanounit that moves or works by photoregulation.