Spiropyran Photoswitches in the Context of DNA: Synthesis and Photochromic Properties

A new design is presented for the incorporation of spiropyran photoswitches into nucleic acids by oligonucleotide solid phase synthesis. This design enables interaction of the 6‐nitrospirobenzopyran (NitroBIPS) photoswitch with the adjacent nucleobases and leaves the photochemical properties of NitroBIPS intact. UV/Vis spectroscopy and HPLC revealed that NitroBIPS incorporated into DNA consists of up to 40 % merocyanine in its thermal equilibrium and undergoes reversible switching between the photoisomeric spiropyran (SP) and merocyanin (MC) state by alternating excitation using visible light or heat for at least fifteen switching cycles. Exchanging the chromene part of NitroBIPS on the DNA level gives access to differently substituted spiropyran derivatives allowing the screening for spiropyrans with suitable properties in a straightforward manner. Thus, by incorporating the highly hydrolysis‐stable pyrido‐spiropyran derivative PyBIPS pure light‐triggered reversible switching of a spiropyran in DNA has been realized for the first time. Therefore, this design represents a new useful platform for investigating the photochromic behavior of different spiropyran photoswitches in a nucleic acid environment and for using spiropyrans to induce light‐ or heat‐triggered changes in conformations or in fluorescence quenching properties of oligonucleotides.     

cis/trans photoisomerization of secondary thiopeptide bonds

The reversible cis/trans photoisomerization of secondary thiopeptide bonds has been systematically studied with UV-visible absorption, capillary electrophoresis, 1H NMR spectroscopy, and circular dichroism methods. It was found that the concentration of the cis conformers could be increased from less than 1 % in the thermal equilibrated solution to up to 20 % in the photostationary state. The rotational barriers of the thiopeptide bond and the pH dependence of the isomerization rates were also studied. The quantum yields of the trans-->cis and the cis-->trans processes were determined from photokinetic analysis.     

Light‐Induced Bistability in the 2 D Coordination Network {[Fe(bbtr)3][BF4]2}∞: Wavelength‐Selective Addressing of Molecular Spin States

Whereas the neat polymeric Fe^II compound {[Fe(bbtr)₃][ClO₄]₂}_∞ (bbtr=1,4‐di(1,2,3‐triazol‐1‐yl)butane) shows an abrupt spin transition centered at 107 K facilitated by a crystallographic symmetry breaking, in the covalently linked 2D coordination network of {[Fe(bbtr)₃][BF₄]₂}_∞, Fe^II stays in the high‐spin state down to 10 K. However, strong cooperative effects of elastic origin result in reversible, persistent, and wavelength‐selective photoswitching between the low‐spin and high‐spin manifolds. This compound thus shows true light‐induced bistability below 100 K. The persistent bidirectional optical switching behavior is discussed as a function of temperature, irradiation time, and intensity. Crystallographic studies reveal a photoinduced symmetry breaking and serve to establish the correlation between structure and cooperative effects. The static and kinetic behavior is explicated within the framework of the mean‐field approximation.     

Photoregulation of α‐Chymotrypsin Activity by Spiropyran‐Based Inhibitors in Solution and Attached to an Optical Fiber

Here the synthesis and characterization of a new class of spiropyran‐based protease inhibitor is reported that can be reversibly photoswitched between an active spiropyran (SP) isomer and a less active merocyanine (MC) isomer upon irradiation with UV and visible light, respectively, both in solution and on a surface of a microstructured optical fiber (MOF). The most potent inhibitor in the series ( SP ‐ 3 b ) has a C‐terminal phenylalanyl‐based α‐ketoester group and inhibits α‐chymotrypsin with a K_i of 115 nM . An analogue containing a C‐terminal Weinreb amide ( SP‐2 d ) demonstrated excellent stability and photoswitching in solution and was attached to the surface of a MOF. The SP isomer of Weinreb amide 2 d is a competitive reversible inhibitor in solution and also on fiber, while the corresponding MC isomer was significantly less active in both media. The ability of this new class of spiropyran‐based protease inhibitor to modulate enzyme activity on a MOF paves the way for sensing applications.     

Modulation of unconventional fluorescence of novel photochromic perimidine spirodimers

Fluorescence modulation by a class of photochromic perimidine spirodimers, which exhibit a characteristic fluorescence associated with their photochromic reactions, has been described. Upon irradiation using 365 nm light, these non-fluorescent spiro molecules undergo a thermally-reversible ring opening at their spiro junction resulting in the generation of strong fluorescence. The fluorescing species is distinctly different from both the stable ring-closed and the ring-opened compounds, though it appears to have been formed from and remains in equilibrium with the photochemically generated ring-opened form. While the fluorescing species possesses a narrow absorption band with its maximum centered at 500 nm, the ring-opened form exhibits a broad absorption across the visible region with two maxima centered at 410 and 650 nm, respectively. After initiating the photochromic reactions in these molecules using 365 nm light, purely photochemically-controlled fluorescence modulation can be carried out using two wavelengths in the visible region, that is, 500 and 700 nm, while the equilibrium concentration of the ring-opened form and the fluorescing species is controlled. Fluorescence modulation is attained also by controlling the ratio of the ring-closed and ring-opened forms by photochemical ring-opening and thermal ring-closing reactions. The study on the effect of substitution of these molecules suggests that by extending the conjugation of the perimidine core in the ring-opened form the molecule is rendered non-fluorescent and hence it can be assumed that the perimidine core forms the fluorescing entity of the molecule.     

Giant Amplification of Photoswitching by a Few Photons in Fluorescent Photochromic Organic Nanoparticles

Controlling or switching the optical signal from a large collection of molecules with the minimum of photons represents an extremely attractive concept. Promising fundamental and practical applications may be derived from such a photon‐saving principle. With this aim in mind, we have prepared fluorescent photochromic organic nanoparticles (NPs), showing bright red emission, complete ON–OFF contrast with full reversibility, and excellent fatigue resistance. Most interestingly, upon successive UV and visible light irradiation, the NPs exhibit a complete fluorescence quenching and recovery at very low photochromic conversion levels (<5 %), leading to the fluorescence photoswitching of 420±20 molecules for only one converted photochromic molecule. This “giant amplification of fluorescence photoswitching” originates from efficient intermolecular energy‐transfer processes within the NPs.     

Enlightened enzymes: strategies to create novel photoresponsive proteins

The photocontrol of protein functions, enzymatic reactions, and concomitantly biological activities in living cells and organisms represents a rapidly developing and interdisciplinary field of research at the interface of chemistry and biology. The nature of light allows for a non-invasive gearing with unprecedented spatiotemporal resolution to exert control at the molecular and cellular level. This concept article aims to illustrate some of the classic as well as recently developed strategies employing protein-engineering and chemical-biological hybrid methods to achieve photocontrol of biological processes. Recent cell biological examples will be highlighted and possible biotechnological applications will be presented.