Photoresponsive Formation of an Intermolecular Minimal G‐Quadruplex Motif

The ability of three different bifunctional azobenzene linkers to enable the photoreversible formation of a defined intermolecular two‐tetrad G‐quadruplex upon UV/Vis irradiation was investigated. Circular dichroism and NMR spectroscopic data showed the formation of G‐quadruplexes with K⁺ ions at room temperature in all three cases with the corresponding azobenzene linker in an E conformation. However, only the para–para‐substituted azobenzene derivative enables photoswitching between a nonpolymorphic, stacked, tetramolecular G‐quadruplex and an unstructured state after E–Z isomerization.     

Construction of photoresponsive RNA for photoswitching RNA hybridization

By introducing azobenzenes into RNA using d-threoninol as a scaffold, a photoresponsive RNA was constructed for efficiently photoswitching the formation and dissociation of RNA/RNA duplexes. The difference in melting temperature (T(m)) between the trans and cis forms was so large that efficient photoregulation of RNA hybridization became possible, irrespective of the sequence adjacent to the introduced azobenzene. Compared to the corresponding photoresponsive DNA, the photoregulatory efficiency of azobenzene-modified RNA was even higher due to the drastic destabilization by cis-azobenzene. Structural analysis by NMR and molecular modeling indicated that the planar trans-azobenzene could not stabilize the RNA/RNA duplex with a rigid A-form structure by base pair stacking. However, the large steric hindrance caused by nonplanar cis-azobenzene was quite effective at distorting and destabilizing the duplex structure. We also discuss the effect of methylation of azobenzene at the ortho positions on photoregulation of RNA/RNA duplex formation. This newly constructed photoresponsive RNA has promising applications such as photoswitching of RNA functions.     

Molecular Design for Reversing the Photoswitching Mode of Turning ON and OFF DNA Hybridization

A new photoswitch for DNA hybridization involving para‐substituted azobenzenes (such as isopropyl‐ or tert‐butyl‐substituted derivatives) with L ‐threoninol as a linker was synthesized. Irradiation of the modified DNA with visible light led to dissociation of the duplex owing to the destabilization effect of the bulky substituent on the trans‐azobenzene. In contrast, trans‐to‐cis isomerization (UV light irradiation) facilitated duplex formation. The direction of this photoswitching mode was entirely reversed relative to the previous system with an unmodified azobenzene on D ‐threoninol whose trans form turned on the hybridization, and cis form turned it off. Such reversed and reversible photoswitching of DNA hybridization was directly demonstrated by using fluorophore‐ and quencher‐attached oligonucleotides. Furthermore, it was revealed that the cis‐to‐trans thermal isomerization was greatly suppressed in the presence of the complementary strand owing to the formation of the more‐stable duplex in the cis form.     

A visible light responsive azobenzene‐functionalized polymer: Synthesis,self‐assembly,and photoresponsive properties

A novel visible light responsive random copolymer consisting of hydrophobic azobenzene‐containing acrylate units and hydrophilic acrylic acid units has been prepared. The azobenzene molecule bearing methoxy groups at all four ortho positions is readily synthesized by one‐step conversion of diazotization. The as‐prepared polymer can self‐assemble into nanoparticles in water due to its amphiphilic nature. The tetra‐o‐methoxy‐substituted azobenzene‐functionalized polymer can exhibit the trans‐to‐cis photoswitching under the irradiation with green light of 520 nm and the cis‐to‐trans photoswitching under the irradiation with blue light of 420 nm in both solution and aggregate state. The morphologies of the self‐assembled nanoparticles are revealed by TEM and DLS. The controlled release of loaded molecules from the nanoparticles can be realized by adjusting pH value since the copolymer possesses pH responsive acrylic acid groups. The fluorescence of loaded Nile Red in the nanoparticles can be tuned upon the visible light irradiation. The reversible photoswitching of the azobenzene‐functionalized polymer under visible light may endow the polymer with wide applications without using ultraviolet light at all. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 2768–2775     

Host–Guest Chemistry of a Bis‐Calix[4]pyrrole Derivative Containing a trans/cis‐Switchable Azobenzene Unit with Several Aliphatic Bis‐Carboxylates

The azobenzene unit used as a photochemically and thermally switchable linker in the assembly of a bis‐calix[4]pyrrole receptor provides a means to modulate the binding of bis‐carboxylates of significant biological importance in cancer research. Conversely, the complexation of different bis‐anionic guests has significant kinetic effects on both the photochemical and thermal trans/cis isomerization of the azobenzene unit.     

Observation of Collective Photoswitching in Free‐Standing TATA‐Based Azobenzenes on Au(111)

Light‐induced transitions between the trans and cis isomer of triazatriangulenium‐based azobenzene derivatives on Au(111) surfaces were observed directly by scanning tunneling microscopy, allowing atomic‐scale studies of the photoisomerization kinetics. Although the azobenzene units in these adlayers are free‐standing and spaced at uniform distances of 1.26 nm, their photoswitching depends on the isomeric state of the surrounding molecules and, specifically, is accelerated by neighboring cis isomers. These collective effects are supported by ab initio calculations indicating that the electronic excitation preferably localizes on the n–π* state of trans isomers with neighboring cis azobenzenes.     

Photoswitchable Adhesives Using Azobenzene‐Containing Materials

Photoinduced reversible solid‐to‐liquid transitions of azobenzene‐containing materials can control adhesion. Photoswitchable adhesives based on azobenzene‐containing small molecules and polymers are under intense investigation. The melting points or glass transition temperatures of such azobenzene‐containing materials in trans and cis forms are above and below room temperature, respectively. Photoswitching of these materials results in reversible trans‐cis isomerization and solid‐to‐liquid transitions. The solid trans azobenzene‐containing materials have strong adhesion and the liquid cis azobenzene‐containing materials have weaker adhesion. In this Minireview, we introduce adhesives based on azobenzene‐containing small molecules and polymers. The remaining challenges and perspectives in the field of photoswitchable adhesives using azobenzene‐containing materials are also discussed.     

Photoswitched Cell Adhesion on Azobenzene‐Containing Self‐Assembled Films

Stimuli‐responsive surfaces that can regulate and control cell adhesion have attracted much attention for their great potential in diverse biomedical applications. Unlike for pH‐ and temperature‐responsive surfaces, the process of photoswitching requires no additional input of chemicals or thermal energy. In this work, two different photoresponsive azobenzene films are synthesized by chemisorption and electrostatic layer‐by‐layer (LbL) assembly techniques. The LbL film exhibits a relatively loose packing of azobenzene chromophores compared with the chemisorbed film. The changes in trans/cis isomer ratio of the azobenzene moiety and the corresponding wettability of the LbL films are larger than those of the chemisorbed films under UV light irradiation. The tendency for cell adhesion on the LbL films decreases markedly after UV light irradiation, whereas adhesion on the chemisorbed films decreases only slightly, because the azobenzene chromophores stay densely packed. Interestingly, the tendency for cell adhesion can be considerably increased on rough substrates, the roughness being introduced by use of photolithography and inductively coupled plasma deep etching techniques. For the chemisorbed films on rough substrates, the amount of cells that adhere also changes slightly after UV light irradiation, whereas, the amount of cells that adhere to LbL films on rough substrates decreases significantly.     

Highly Stable Duplex Formation by Artificial Nucleic Acids Acyclic Threoninol Nucleic Acid (aTNA) and Serinol Nucleic Acid (SNA) with Acyclic Scaffolds

The stabilities of duplexes formed by strands of novel artificial nucleic acids composed of acyclic threoninol nucleic acid (aTNA) and serinol nucleic acid (SNA) building blocks were compared with duplexes formed by the acyclic glycol nucleic acid (GNA), peptide nucleic acid (PNA), and native DNA and RNA. All acyclic nucleic acid homoduplexes examined in this study had significantly higher thermal stability than DNA and RNA duplexes. Melting temperatures of homoduplexes were in the order of aTNA>PNA≈GNA≥SNA?RNA>DNA. Thermodynamic analyses revealed that high stabilities of duplexes formed by aTNA and SNA were due to large enthalpy changes upon formation of duplexes compared with DNA and RNA duplexes. The higher stability of the aTNA homoduplex than the SNA duplex was attributed to the less flexible backbone due to the methyl group of D ‐threoninol on aTNA, which induced clockwise winding. Unlike aTNA, the more flexible SNA was able to cross‐hybridize with RNA and DNA. Similarly, the SNA/PNA heteroduplex was more stable than the aTNA/PNA duplex. A 15‐mer SNA/RNA was more stable than an RNA/DNA duplex of the same sequence.     

Effect of the ortho Modification of Azobenzene on the Photoregulatory Efficiency of DNA Hybridization and the Thermal Stability of its cis Form

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 (ΔT_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 ΔT_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 ΔT_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 π orbital of the nitrogen atom and the methyl group on the distal benzene ring.     

Observation of Collective Photoswitching in Free-Standing TATA-Based Azobenzenes on Au(111)

Light-induced transitions between the trans and cis isomer of triazatriangulenium-based azobenzene derivatives on Au(111) surfaces were observed directly by scanning tunneling microscopy, allowing atomic-scale studies of the photoisomerization kinetics. Although the azobenzene units in these adlayers are free-standing and spaced at uniform distances of 1.26 nm, their photoswitching depends on the isomeric state of the surrounding molecules and, specifically, is accelerated by neighboring cis isomers. These collective effects are supported by ab initio calculations indicating that the electronic excitation preferably localizes on the n–π* state of trans isomers with neighboring cis azobenzenes.     

Photo-Induced Switchable Binding Behavior of Bridged Bis(β-cyclodextrin) with an Azobenzene Dicarboxylate Linker

A bridged bis(β-cyclodextrin) (1) with an azobenzene dicarboxylate linker was synthesized, and its binding behavior with a fluorescent dye, acridine red (AR), was investigated by means of fluorescence spectroscopy. Due to the photo-induced conversion of the azobenzene dicarboxylate linker from the trans-conformer to the cis-conformer, 1 exhibits a different binding behavior before and after UV light irradiation, giving a stronger binding ability towards AR in the cis form. This switchable binding behavior of 1 may open a new channel to the design of azobenzene-linked dimeric receptors as photo-induced molecular devices.     

Modulation of Electronic Mobility of a One‐Dimensional Coordination Polymeric Molecular Wire with Light

Metal ions often influence the photoswitching efficiency of a photochromic system. This article reports a one‐dimensional polymer having cyclic azobenzenes coordinated to silver ions that are bridged by nitrates. The coordination polymer (CP‐ 2 ) displays a photoresponsive behavior. The switching ability in the polymer form was faster compared to the parent azobenzene ligand without the metal ions. Azobenzenes are reported to be poorly conducting. Here, although the azobenzene ligand does not show significant electronic mobility, the coordination polymer (CP‐ 2 ) displays a modest conductivity. The conductance in the cis form of the polymer is significantly higher compared to the trans form. Upon exposure to visible light, the cis form undergoes photoisomerization to the trans form with a drastic drop in the electronic mobility. The trans form can be reverted to the cis form thermally or by using UV light. Thus, this system offers a reversible control of the conductivity using light.     

Major‐Groove‐Halogenated DNA: The Effects of Bromo and Iodo Substituents Replacing H−C(7) of 8‐Aza‐7‐deazapurine‐2,6‐diamine or H−C(5) of Uracil Residues

Oligonucleotides containing halogenated `purine' and pyrimidine bases were synthesized. Bromo and iodo substituents were introduced at the 7‐position of 8‐aza‐7‐deazapurine‐2,6‐diamine (see 2b , c ) or at the 5‐position of uracil residues (see 3b , c ). Phosphoramidites were synthesized after protection of 2b with the isobutyryl residue and of 2c with the benzoyl group. Duplexes containing the residues 2b or 2c gave always higher T<sub>m</sub> values than those of the nonmodified counterparts containing 2′‐deoxyadenosine, the purine‐2,6‐diamine 2′‐deoxyribonucleoside ( 1 ), or 2a at the same positions. Six 2b residues replacing dA in the duplex 5′‐d(TAGGTCAATACT)‐3′ ( 11 )⋅5′‐d(AGTATTGACCTA)‐3′ ( 12 ) raised the T<sub>m</sub> value from 48 to 75° (4.5° per modification (Table 3)). Contrary to this, incorporation of the 5‐halogenated 2′‐deoxyuridines 3b or 3c into oligonucleotide duplexes showed very little influence on the thermal stability, regardless of which `purine' nucleoside was located opposite to them (Tables 4 and 5). The positive effects on the thermal stability of duplexes observed in DNA were also found in DNA⋅RNA hybrids or in DNA with parallel chain orientation (Tables 8 and 9, resp.).     

The synthesis and photoactive properties of cyclic amphiphilic polymers with azobenzene in main chain

The cyclic amphiphilic polymers with azobenzene in main chain, cyclic azobenzene tetraethylene glycol polystyrene (cyclic‐Azo‐TEG‐PS) with different molecular weights, were successfully synthesized by combining atom transfer radical polymerization (ATRP) and Cu (I)‐catalyzed azide/alkyne cycloaddition (CuAAC). Gel permeation chromatography (GPC), proton nuclear resonance (¹H NMR), Fourier transform‐infrared (FT‐IR), and matrix‐assisted laser desorption/ionization time of flight (MALDI‐TOF) mass spectrometry were used to prove the complete conversion from linear polymers to cyclic ones. The thermal properties and photoisomerization behaviors of obtained cyclic polymers have been investigated by comparison with the linear analogues. The cyclic polymer displayed a higher glass transition temperature compared with the linear one, measured by differential scanning calorimetry (DSC). It was found that the trans‐to‐cis and cis‐to‐trans isomerization of cyclic polymers was both slower than that of their respective linear counterparts upon irradiation by UV/visible light. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1834–1841     

Photophysical and Duplex‐DNA‐Binding Properties of Distamycin Dimers Based on 4,4′‐ and 2,2′‐Dialkoxyazobenzenes as the Core

Distamycin‐based tetrapeptide ( 1 ) was covalently tethered to both ends of the central dihydroxyazobenzene moiety at either the 2,2′ or 4,4′ positions. This afforded two isomeric, distamycin–azobenzene–distamycin systems, 2 (para) and 3 (ortho), both of them being photoisomerizable. Illumination of these conjugates in solution at approximately 360 nm induced photoisomerization and the time course of the process was followed by UV/Vis and ¹H NMR spectroscopy. The kinetics of the thermal reversion at various temperatures of cis to trans isomers of the conjugates obtained after photoillumination were also examined. This afforded the respective thermal‐activation parameters. Both the molecular architecture and the location of the substituent around the core azobenzene determined the rate and activation‐energy barrier for the cis‐to‐trans back‐isomerization of these conjugates in solution. Duplex–DNA binding of the conjugates and the changes in DNA‐binding efficiency upon photoisomerization was also examined by CD spectroscopy, thermal denaturation studies, and a Hoechst displacement assay. The conjugate 2 showed higher DNA‐binding affinity and a greater change in the DNA‐binding efficiency upon photoisomerization compared with its 2,2′‐disubstituted counterpart. The experimental findings were substantiated by using molecular‐docking studies involving each conjugate with a model duplex d[(GC(AT)₁₀CG)]₂ DNA molecule.     

Thermodynamically Stable,Photoreversible Pharmacology in Neurons with One‐ and Two‐Photon Excitation

Photoswitchable bioprobes enable bidirectional control of cell physiology with different wavelengths of light. Many current photoswitches use cytotoxic UV light and are limited by the need for constant illumination owing to thermal relaxation in the dark. Now a photoswitchable tetrafluoroazobenzene(4FAB)‐based ion channel antagonist has been developed that can be efficiently isomerized in two separate optical channels with visible light. Importantly, the metastable cis configuration showed very high stability in the dark over the course of days at room temperature. In neurons, the 4FAB antagonist reversibly blocks voltage‐gated ion channels with violet and green light. Furthermore, photoswitching could also be achieved with two‐photon excitation yielding high spatial resolution. 4FAB probes have the potential to enable long‐term biological studies where both ON and OFF states can be maintained in the absence of irradiation.     

Thermodynamically Stable,Photoreversible Pharmacology in Neurons with One‐ and Two‐Photon Excitation

Photoswitchable bioprobes enable bidirectional control of cell physiology with different wavelengths of light. Many current photoswitches use cytotoxic UV light and are limited by the need for constant illumination owing to thermal relaxation in the dark. Now a photoswitchable tetrafluoroazobenzene(4FAB)‐based ion channel antagonist has been developed that can be efficiently isomerized in two separate optical channels with visible light. Importantly, the metastable cis configuration showed very high stability in the dark over the course of days at room temperature. In neurons, the 4FAB antagonist reversibly blocks voltage‐gated ion channels with violet and green light. Furthermore, photoswitching could also be achieved with two‐photon excitation yielding high spatial resolution. 4FAB probes have the potential to enable long‐term biological studies where both ON and OFF states can be maintained in the absence of irradiation.     

Azobenzene‐Functionalized Cage Silsesquioxanes as Inorganic–Organic Hybrid,Photoresponsive, Nanoscale,Building Blocks

Mono‐ and octa‐azobenzene‐functionalized cage silsesquioxanes were easily synthesized by the reaction of 4‐bromoazobenzene with monovinyl‐substituted octasilsesquioxane and cubic octavinylsilsesquioxane through the Heck coupling reaction. Excited‐state energies obtained from time‐dependent density functional theory (TDDFT) and the CAM‐B3LYP functional correlate very well with experimental trans–cis photoisomerization results from UV/Vis spectroscopy. These azobenzene‐functionalized cages exhibit good thermal stability and are fluorescent with maximum emission at approximately 400 nm, making them potential materials for blue‐light emission.     

(2′‐O‐Methyl‐RNA)‐3′‐PNA Chimeras: A New Class of Mixed Backbone Oligonucleotide Analogues with High Binding Affinity to RNA

The automated on‐line synthesis of DNA‐3′‐PNA chimeras 1 – 4 and (2′‐O‐methyl‐RNA)‐3′‐PNA chimeras 5 – 8 is described, in which the 3′‐terminal part of the oligonucleotide is linked to the N‐terminal part of the PNA via N‐(ω‐hydroxyalkyl)‐N‐[(thymin‐1‐yl)acetyl]glycine units (alkyl=Et, Ph, Bu, and pentyl). By means of UV thermal denaturation, the binding affinities of all chimeras were directly compared by determining their T_m values in the duplex with complementary DNA and RNA. All investigated DNA‐3′‐PNA chimeras and (2′‐O‐methyl‐RNA)‐3′‐PNA chimeras form more‐stable duplexes with complementary DNA and RNA than the corresponding unmodified DNA. Interestingly, a N‐(3‐hydroxypropyl)glycine linker resulted in the highest binding affinity for DNA‐3′‐PNA chimeras, whereas the (2′‐O‐methyl‐RNA)‐3′‐PNA chimeras showed optimal binding with the homologous N‐(4‐hydroxybutyl)glycine linker. The duplexes of (2′‐O‐methyl‐RNA)‐3′‐PNA chimeras and RNA were significantly more stable than those containing the corresponding DNA‐3′‐PNA chimeras. Surprisingly, we found that the charged (2′‐O‐methyl‐RNA)‐3′‐PNA chimera with a N‐(4‐hydroxybutyl)glycine‐based unit at the junction to the PNA part shows the same binding affinity to RNA as uncharged PNA. Potential applications of (2′‐O‐methyl‐RNA)‐3′‐PNA chimeras include their use as antisense agents acting by a RNase‐independent mechanism of action, a prerequisite for antisense‐oligonucleotide‐mediated correction of aberrant splicing of pre‐mRNA.