Turning “On” and “Off” a Pyridoxal 5′-Phosphate Mimic Using Light

Photoresponsive Small Molecule Enzyme Mimics

Enzyme mimics emulate the catalytic activities of their natural counterparts. Light-responsive enzyme mimics are an emerging branch of biomimetic chemistry where the catalytic activities can be controlled reversibly by light. These light-responsive systems are constructed by incorporating a suitable photoswitchable unit around the active-site mimic. As these systems are addressable by light, they do not leave back any undesired side products, and their activation-deactivation can be easily controlled. Naturally, these systems have enormous potential in the field of on-demand catalysis. The synthetic light-responsive enzyme mimics are robust and stable under harsh conditions. They do not require special handling protocols like those for real enzymes and can be tailor-made for improved solubility in a variety of solvents. How the introduction of the light-responsive systems has offered a new-edge to the field of small-molecule enzyme mimic has been elaborated in this Mini-review. Recent breakthroughs in light-responsive enzyme-like systems have been highlighted. Finally, the current obstacles and future prospects of this field have been discussed.

A Photoregulated Racemase Mimic

The racemase enzymes convert L-amino acids to their D-isomer. The reaction proceeds through a stepwise deprotonation-reprotonation mechanism that is assisted by a pyridoxal phosphate (PLP) coenzyme. This work reports a PLP-photoswitch-imidazole triad where the racemization reaction can be controlled by light by tweaking the distance between the basic residue and the reaction centre.

Efficient Light-Induced pKa Modulation Coupled to Base-Catalyzed Photochromism

Photoswitchable acid-base pairs, whose pK_a values can be reversibly altered, are attractive molecular tools to control chemical and biological processes with light. A significant, light-induced pK_a change of three units in aqueous medium has been realized for two thermally stable states, which can be interconverted using UV and green light. The light-induced pK_a modulation is based on incorporating a 3-H-thiazol-2-one moiety into the framework of a diarylethene photoswitch, which loses the heteroaromatic stabilization of the negatively charged conjugate base upon photochemical ring closure, and hence becomes significantly less acidic. In addition, the efficiency of the photoreactions is drastically increased in the deprotonated state, giving rise to catalytically enhanced photochromism. It appears that protonation has a significant influence on the shape of the ground- and excited-state potential energy surfaces, as indicated by quantum-chemical calculations.

Light-Controlled Reversible Modulation of Frontier Molecular Orbital Energy Levels in Trifluoromethylated Diarylethenes

Among bistable photochromic molecules, diarylethenes (DAEs) possess the distinct feature that upon photoisomerization they undergo a large modulation of their π-electronic system, accompanied by a marked shift of the HOMO/LUMO energies and hence oxidation/reduction potentials. The electronic modulation can be utilized to remote-control charge- as well as energy-transfer processes and it can be transduced to functional entities adjacent to the DAE core, thereby regulating their properties. In order to exploit such photoswitchable systems it is important to precisely adjust the absolute position of their HOMO and LUMO levels and to maximize the extent of the photoinduced shifts of these energy levels. Here, we present a comprehensive study detailing how variation of the substitution pattern of DAE compounds, in particular using strongly electron-accepting and chemically stable trifluoromethyl groups either in the periphery or at the reactive carbon atoms, allows for the precise tuning of frontier molecular orbital levels over a broad energy range and the generation of photoinduced shifts of more than 1 eV. Furthermore, the effect of different DAE architectures on the transduction of these shifts to an adjacent functional group is discussed. Whereas substitution in the periphery of the DAE motif has only minor implications on the photochemistry, trifluoromethylation at the reactive carbon atoms strongly disturbs the isomerization efficiency. However, this can be overcome by using a nonsymmetrical substitution pattern or by combination with donor groups, rendering the resulting photoswitches attractive candidates for the construction of remote-controlled functional systems.

Light-Activated Sensitive Probes for Amine Detection

Our new, simple, and accurate colorimetric method is based on diarylethenes (DAEs) for the rapid detection of a wide range of primary and secondary amines. The probes consist of aldehyde- or ketone-substituted diarylethenes, which undergo an amine-induced decoloration reaction, selectively to give the ring-closed isomer. Thus, these probes can be activated at the desired moment by light irradiation, with a sensitivity that allows the detection of amines at concentrations as low as 10^-6  m in solution. In addition, the practical immobilization of DAEs on paper makes it possible to detect biogenic amines, such as cadaverine, in the gas phase above a threshold of 12 ppbv within 30 seconds.

Bifunctional Molecular Photoswitches Based on Overcrowded Alkenes for Dynamic Control of Catalytic Activity in Michael Addition Reactions

The emerging field of artificial photoswitchable catalysis has recently shown striking examples of functional light-responsive systems allowing for dynamic control of activity and selectivity in organocatalysis and metal-catalysed transformations. While our group has already disclosed systems featuring first generation molecular motors as the switchable central core, a design based on second generation molecular motors is lacking. Here, the syntheses of two bifunctionalised molecular switches based on a photoresponsive tetrasubstituted alkene core are reported. They feature a thiourea substituent as hydrogen-donor moiety in the upper half and a basic dimethylamine group in the lower half. This combination of functional groups offers the possibility for application of these molecules in photoswitchable catalytic processes. The light-responsive central cores were synthesized by a Barton-Kellogg coupling of the prefunctionalized upper and lower halves. Derivatization using Buchwald-Hartwig amination and subsequent introduction of the thiourea substituent afforded the target compounds. Control of catalytic activity in the Michael addition reaction between (E)-3-bromo-β-nitrostyrene and 2,4-pentanedione is achieved upon irradiation of stable-(E) and stable-(Z) isomers of the bifunctional catalyst 1. Both isomers display a decrease in catalytic activity upon irradiation to the metastable state, providing systems with the potential to be applied as ON/OFF catalytic photoswitches.

Control of Imine Exchange Kinetics with Photoswitches to Modulate Self-Healing in Polysiloxane Networks by Light Illumination

Various aldehyde-containing photoswitches have been developed whose reactivity toward amines can be controlled externally. A thermally stable bifunctional diarylethene, which in its ring-closed form exhibits imine formation accelerated by one order of magnitude, was used as a photoswitchable crosslinker and mixed with a commercially available amino-functionalized polysiloxane to yield a rubbery material with viscoelastic and self-healing properties that can be reversibly tuned by irradiation.

Photodynamic chiral molecular switches with thermal stability: from reflection wavelength tuning to handedness inversion of self-organized helical superstructures

A good turn: Three compounds that bear two axially chiral bridged binaphthyl units were developed as photodynamic chiral dopants for nematic liquid crystals. For compounds with suitable bridge lengths, a change in the dihedral angle induced a switch of the binaphthyl units from the cisoid to the transoid form upon UV irradiation, which led to an inversion of the handedness of the helices.

An advance in the chemical synthesis of homogeneous N-linked glycopolypeptides by convergent aspartylation

We describe a useful advance in glycopeptide synthesis. We have developed a one-flask aspartylation/deprotection method, wherein long peptide fragments, bearing proximal pseudoproline functionality are merged with complex glycan domains. Following aspartylation, acidmediated global deprotection reveals the elaborated glycopeptide. The temporary pseudoproline functionality serves to suppress formation of aspartimide side products during solid phase peptide synthesis and aspartylation.

Amidation of aldehydes and alcohols through α-iminonitriles and a sequential oxidative three-component Strecker reaction/thio-Michael addition/alumina-promoted hydrolysis process to access β-mercaptoamides from aldehydes, amines, and thiols

Mild and general alumina-promoted hydrolysis conditions for converting α-iminonitriles into carboxamides have been developed. In combination with the oxidative three-component Strecker reaction, the one-pot direct amidation of aldehydes and alcohols is reported. Subsequently, an Yb(OTf)(3)-catalyzed Michael addition of thiols to α,β-unsaturated α-iminonitriles is reported for the synthesis of β-mercapto-α-iminonitriles. The successful integration of an oxidative Strecker reaction, thio-Michael addition, and neutral-alumina-promoted hydrolysis of β-mercapto-α-iminonitriles into a three-component one-pot process allowed us to develop the direct conversion of amines, aldehydes, and thiols into β-mercaptoamides. All of these procedures were applicable to aromatic and aliphatic amines and aldehydes.