Photoswitches are a subtype of molecular switch that undergo a reversible structural change upon irradiation with light to adopt a different configuration. Activation of a molecular switch by light offers several advantages. In addition to being traceless, light can be precisely controlled in its intensity (dosage control) and can be focused with sub-micron accuracy with a high temporal and spatial resolution. Consequently, photoswitches ideally lend themselves to the construction of light-responsive pharmaceutical compounds, a field that is described by the term photopharmacology.

Azobenzene (Ph-N=N-Ph), initially used as colorant, is a well-known and extensively investigated azo dye. It exists in a cis and in a trans form. The transition between these two stereoisomers may be induced by light and is reversible. Concomitantly, the absorption spectra of the isomers are different, thus the prevalence of the two geometric species may easily be controlled with strong monochromatic light sources like LEDs or lasers. The cis form of unsubstituted azobenzene absorbs at about 450 nm, while the trans form has a maximum in the range of 340 - 370 nm. The latter is thermodynamically favored by 42‑50 kJ mol^‑1 and can be regenerated 100% by thermal relaxation. However, quantitative conversion of one or the other isomer by irradiation with light is nearly impossible due to a substantial overlap of the absorption spectra of both isomers.

Azobenzene is the most common motif for photoswitches. Besides its optical characteristics, the unique feature of this cis-trans isomerism is the different spatial requirement of the photoisomers: while the trans isomer is almost planar and has no dipole moment, the phenyl rings are twisted about 55°against each other in the cis form, with a rather large dipole moment of about 3 D. This reaction is very fast, it takes place within picoseconds. Besides, azobenzenes can undergo approximately 100 switching cycles without detectable photodegradation or loss of responsiveness.

Our azobenzene product Fmoc-AMPB-OH (FAA9180) comes with a Fmoc protected amino function and a free carboxy group. Thus, it can be used like any amino acid building block in solid phase peptide synthesis (SPPS), to generate a light controllable peptide backbone.

Photo switchable cis-trans isomerism in a peptide backbone upon incorporation of an azobenzene moiety modifies the spatial requirements of the peptide which can be harnessed, e.g., for controlling peptide activity or drug release.

Besides, we are offering azobenzene-modified amino acids bearing the switchable unit within their side chain. These building blocks can also be used to generate peptides with switchable steric requirements upon light-induced isomerization.

The conformational changes at the double bond between the two nitrogens are quite powerful: Macrostructures such as liquid crystal elastomers (LCE), capable of light controlled self-sustained snapping locomotion, have been prepared by incorporating azobenzene into the polymer chains. When azo dyes are used for stapling (intramolecular crosslinking), peptides may be photodynamically switched between helical and non-helical conformations. This can lead to interesting applications in artificial light switchable proteins which naturally occur in a helix-turn-helix configuration, like, e.g., transcription factors.

Furthermore, azobenzene may be incorporated into peptide-based drug loaded micelles which release their payload upon irradiation with light or to control the liquefaction properties of „smart” emulsions. The light dependent azo-bond isomerization also has been utilized in the activation of proteolysis targeting chimeras (PROTACs) where it offers novel strategies for tumor therapy.

Another fascinating property of azo-dyes is that the cis-trans isomerization also may be controlled by two-photon absorption, aided by high energy lasers at infrared wavelengths. This allows the deep penetration of tissue, the precise spatial control of photoswitching, and simultaneously avoids the need for UV light, which may be harmful to living tissue due to its potential to damage DNA.

The absorption wavelengths and the two-photon absorption cross section of azobenzenes may be modulated by introducing and varying substituents at the benzene rings or by exchanging one of the benzene rings by another aromatic ring system, e.g. pyrazole.

→ Interested in other building blocks which may be activated with light? Explore our Photochemistry booklet! 

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