Patterned surface derivatization using Diels-Alder photoclick reaction

The utility of photochemically induced hetero-Diels-Alder reaction for the light-directed surface derivatization and patterning was demonstrated. Glass slides functionalized with vinyl ether moieties are covered with aqueous solution of substrates conjugated to 3-(hydroxymethyl)-2-naphthol (NQMP). Subsequent irradiation via shadow mask results in an efficient conversion of the latter functionality into reactive 2-napthoquinone-3-methide (oNQM) in the exposed areas. oNQM undergoes very facile hetero Diels-Alder addition (k(D-A) ≈ 4 × 10(4) M(-1)s(-1)) to immobilized vinyl ether molecules resulting in a photochemically stable covalent link between a substrate and a surface. Unreacted oNQM groups are rapidly hydrated to regenerate NQMP. The click chemistry based on the addition of photochemically generated oNQM to vinyl ether works well in aqueous solution, proceeds at high rate under ambient conditions, and does not require catalyst or additional reagents. This photoclick strategy represents an unusual paradigm in photopatterning: the surface itself is photochemically inert, while photoreactive component is present in the solution. The short lifetime (τ ≈ 7 ms in H(2)O) of the active form of a photoclick reagent in aqueous solution prevents its migration from the site of irradiation, thus allowing for the spatial control of surface derivatization. Both o-napthoquinone methide precursors and vinyl ethers are stable in dark and the reaction is orthogonal to other derivatization techniques, such as acetylene-azide click reaction.     

Light-induced hetero-Diels-Alder cycloaddition: a facile and selective photoclick reaction

2-Napthoquinone-3-methides (oNQMs) generated by efficient photodehydration (Φ=0.2) of 3-(hydroxymethyl)-2-naphthol undergo facile hetero-Diels-Alder addition (k(D-A)～ 4×10(4) M(-1) s(-1)) to electron-rich polarized olefins in an aqueous solution. The resulting photostable benzo[g]chromans are produced in high to quantitative yield. The unreacted oNQM is rapidly hydrated (k(H2O) ～145 s(-1)) to regenerate the starting diol. This competition between hydration and cycloaddition makes oNQMs highly selective, since only vinyl ethers and enamines are reactive enough to form the Diels-Alder adduct in an aqueous solution; no cycloaddition was observed with other types of alkenes. To achieve photolabeling or photoligation of two substrates, one is derivatized with a vinyl ether moiety, while 3-(hydroxymethyl)-2-naphthol is attached to the other via an appropriate linker. The light-induced Diels-Alder "click" strategy permits the formation of either a permanent or hydrolytically labile linkage. Rapid kinetics of this photoclick reaction (k=4×10(4) M(-1) s(-1)) is useful for time-resolved applications. The short lifetime (τ ～7 ms in H(2)O) of the active form of the photoclick reagent prevents its migration from the site of irradiation, thus, allowing for spatial control of the ligation or labeling.     

Triplet-Triplet Energy Transfer: A Simple Strategy for an Efficient Visible Light-Induced Photoclick Reaction

Photoclick reactions combine the advantages offered by light-driven processes and classical click chemistry and have found applications ranging from surface functionalization, polymer conjugation, photo-crosslinking, and protein labeling. Despite these advances, the dependency of most of the photoclick reactions on UV light poses a severe obstacle for their general implementation, as this light can be absorbed by other molecules in the system resulting in their degradation or unwanted reactivity. However, the development of a simple and efficient system to achieve bathochromically shifted photoclick transformations remains challenging. Here, we introduce triplet-triplet energy transfer as a fast and selective way to enable visible light-induced photoclick reactions. Specifically, we show that 9,10-phenanthrenequinones ( PQ s) can efficiently react with electron-rich alkenes ( ERA s) in the presence of a catalytic amount (as little as 5 mol %) of photosensitizers. The photocycloaddition reaction can be achieved under green (530 nm) or orange (590 nm) light irradiation, representing a bathochromic shift of over 100 nm as compared to the classical PQ-ERA s system. Furthermore, by combining appropriate reactants, we establish an orthogonal, blue and green light-induced photoclick reaction system in which the product distribution can be precisely controlled by the choice of the color of light.     

Diels-Alder chemistry on alkene functionalized films

A substrate-independent method has been devised for ring formation at solid surfaces. This entails the aminolysis reaction of allylamine with maleic anhydride pulsed plasma polymer films to yield terminal alkene groups at the surface. Subsequent exposure to 1,3-cyclohexadiene leads to a Diels-Alder type (4 + 2) cycloaddition reaction to give a mixture of endo- and exo-bicyclo[2.2.2]oct-2-ene rings.     

Molecular Engineering To Enhance Reactivity and Selectivity in an Ultrafast Photoclick Reaction

Light-induced 9,10-phenanthrenequinone-electron-rich alkene ( PQ-ERA ) photocycloadditions are an attractive new type of photoclick reaction, featuring fast conversions and high biocompatibility. However, the tunability of the reaction was hardly investigated up to now. To this end, we explored the influence of substituents on both reaction partners and the reaction rate between the PQs and ERAs . We identified new handles for functionalization and discovered that using enamines as ERAs leads to drastically enhanced rates (>5400 times faster), high photoreaction quantum yields ( Φ_P , up to 65 %), and multicolor emission output as well as a high fluorescence quantum yield of the adducts ( Φ_F , up to 97 %). Further investigation of the photophysical and photochemical properties provided insights to design orthogonal reaction systems both in solution and on nanoparticle surfaces for ultrafast chemoselective functionalization by photoclick reactions.     

Controlling silica surfaces using responsive coupling agents

The surface chemistry of silica was modified using coupling agents capable of participating in oxidation or in the Diels-Alder (and retro Diels-Alder) reactions. The synthesis of the latter coupling agents, using trialkoxysilane groups linked to a cyclopentadiene structure, was achieved by the condensation of sodium cyclopentadienolide with 1-chlorodimethylsilyl-3-triethoxysilylpropane to give 1-cyclopentadienyldimethylsilyl-3-trialkoxysilylpropane. The cyclopentadiene ring in this structure was shown to undergo normal Diels-Alder chemistry with maleimides or maleic anhydride to give 7-(dimethylsilylpropyltrialkoxysilane)-5-norbornene-2,3-dicarboxylic acid anhydride. The retro Diels-Alder reaction of 7-(dimethylsilylpropyltrialkoxysilane)-5-norbornene-2,3-dicarboxylic acid anhydride in solution was not very efficient: the adduct is very stable and only undergoes the retro Diels-Alder reaction at temperatures in excess of 200 °C. Once grafted to the surface, however, the retro Diels-Alder reaction was achieved at a level greater than 90% by use of thermolysis in the presence of free cyclopentadiene. In addition, a polyene-based coupling agent derived from squalene was prepared by hydrosilylation using HMe₂SiOSiMe₂CH₂CH₂Si(OEt)₃. Once grafted to the surface, oxidation by ozone led to ozonides that could be reduced to ketone/aldehyde groups. These in turn could be trapped by functional groups such as hydrazines to make surface-bound hydrazones. With both types of coupling agents, the surface energy and nature of the functional groups bound to the surface could be changed on demand in response to an external stimulus.     

In-plane enyne metathesis and subsequent Diels-Alder reactions on self-assembled monolayers

We report in-plane enyne metathesis and subsequent Diels-Alder reactions on self-assembled monolayers (SAMs) terminating in vinyl and acetylenyl groups on gold. After the formation of SAMs of vinyl and acetylenyl group-containing dithiols on gold, in-plane enyne metathesis of the vinyl and acetylenyl groups, leading to the formation of 1,3-diene, was achieved on the SAMs, and Diels-Alder reactions were then successfully performed with tetracyanoethylene, maleic anhydride, and maleimide. The reactions were confirmed by FT-IR spectroscopy, X-ray photoelectron spectroscopy, and time-of-flight secondary-ion mass spectrometry. In-plane enyne metathesis developed herein would offer a versatile platform for the functionalization of surfaces with mild reaction conditions and a high compatibility in functional groups.     

A Photoclick-Based High-Throughput Screening for the Directed Evolution of Decarboxylase OleT

Enzymatic oxidative decarboxylation is an up-and-coming reaction yet lacking efficient screening methods for the directed evolution of decarboxylases. Here, we describe a simple photoclick assay for the detection of decarboxylation products and its application in a proof-of-principle directed evolution study on the decarboxylase OleT. The assay was compatible with two frequently used OleT operation modes (directly using hydrogen peroxide as the enzyme's co-substrate or using a reductase partner) and the screening of saturation mutagenesis libraries identified two enzyme variants shifting the enzyme's substrate preference from long chain fatty acids toward styrene derivatives. Overall, this photoclick assay holds promise to speed-up the directed evolution of OleT and other decarboxylases.     

Lewis碱稳定的硼代苯与亲二烯体的Diels-Alder反应的理论研究

采用密度泛函理论方法在B3LYP/6-31G(d)水平上研究了Lewis碱稳定的硼代苯与一些亲二烯体的两种可能的Diels-Alder反应的微观机理和势能剖面, 并研究了反应的溶剂效应和取代基效应. 计算结果表明, 一部分反应以直接的近同步的协同方式进行, 而在另一部分反应中, 两个反应物分子先形成分子间复合物, 然后再经过协同的过渡态生成产物. 与气相中相比, 二氯甲烷溶剂使所研究的大部分反应的活化能垒有所增加. 在乙炔或乙烯分子中分别引入吸电子基团CO2Me或CN能显著降低反应的活化能垒. 形成一个C—B键的杂Diels-Alder反应都比相应的Diels-Alder反应在热力学和动力学上容易进行, 这与实验结果一致.     

Solvent Effects on a Diels-Alder Reaction Involving a Cationic Diene: Consequences of the Absence of Hydrogen-Bond Interactions for Accelerations in Aqueous Media

In order to study the influence of hydrogen-bond interactions on the accelerations of Diels-Alder reactions in water and highly aqueous mixed solvent systems, second-order rate constants for the Diels-Alder reaction of acridizinium bromide (1a) with cyclopentadiene (CP) have been measured in aqueous media and organic solvents. Only modest rate accelerations were found in water-rich media. This is attributed to the absence of hydrogen-bonding groups in the reactants. Comparison with cycloadditions of CP with 9-carbomethoxyacridizinium bromide (1b), acrylonitrile (3), and methyl vinyl ketone (4), which do contain hydrogen-bond acceptors, reveals substantially larger aqueous accelerations. These results demonstrate that hydrogen bonding is a major factor in aqueous accelerations. Also rate constants for the cycloaddition of CP to 1a in surfactant solutions were determined. Micellar catalysis is observed in SDS solutions, due to binding of both the diene and the dienophile to the anionic micellar surface.     

Quantum Chemical Calculations and Experimental Validation of the Photoclick Reaction for Fluorescent Labeling of the 5’ cap of Eukaryotic mRNAs

Bioorthogonal click reactions are powerful tools to specifically label biomolecules in living cells. Considerable progress has been made in site-specific labeling of proteins and glycans in complex biological systems, but equivalent methods for mRNAs are rare. We present a chemo-enzymatic approach to label the 5’ cap of eukaryotic mRNAs using a bioorthogonal photoclick reaction. Herein, the N7-methylated guanosine of the 5’ cap is enzymatically equipped with an allyl group using a variant of the trimethylguanosine synthase 2 from Giardia lamblia (GlaTgs2). To elucidate whether the resulting N²-modified 5’ cap is a suitable dipolarophile for photoclick reactions, we used Kohn–Sham density functional theory (KS-DFT) and calculated the HOMO and LUMO energies of this molecule and nitrile imines. Our in silico studies suggested that combining enzymatic allylation of the cap with subsequent labeling in a photoclick reaction was feasible. This could be experimentally validated. Our approach generates a turn-on fluorophore site-specifically at the 5’ cap and therefore presents an important step towards labeling of eukaryotic mRNAs in a bioorthogonal manner.     

Controlling dispersion and migration of particulate additives with block copolymers and Diels-Alder chemistry

Reversible Diels-Alder chemistry was exploited to develop thermo-responsive polymer films. Here, low molecular weight poly(styrene) (PS) and poly(ethylene glycol) (PEG) were prepared with furyl and maleimido chain ends, respectively. These polymers were then tethered together to form a thiol-terminated PEG-b-PS diblock copolymer ligand via a Diels-Alder linkage and were employed to randomly disperse 10 nm diameter Au nanoparticles within a matrix of PEG. Thermal treatment caused the Diels-Alder linkages between the polymer blocks to be severed, resulting in controllable surface functionalization due to phase separation. Migration of the Au nanoparticles to the surface of the films was characterized by Rutherford backscattering spectroscopy, small-angle X-ray scattering, contact angle measurements, and atomic force microscopy.     

Diels-Alder chemistry at furan ring functionalized solid surfaces

A substrate-independent method for Diels-Alder chemistry at solid surfaces is described for the first time.     

Photochemical tuning of materials: A click chemistry perspective

Light driven reactions are perpetual tools for environment sustainability. As an external trigger, most of the photon driven reactions are stereoselective, precise, efficient and offer temporal control for biomolecules. The photoinduced reactions are key to unique molecular transformations that include click and unclick reactions. Since 2003, there has been an exponential rise research papers citing light driven reactions. This review considers the light promoted development and modification of reactions that fall under the criteria of ‘Click’ series of transformations. The review lays emphasis on the light induced biochemical, carbohydrate modification, surface labelling, bioconjugation, polymer modification, dendrimer synthesis, [4 + 2] and [2 + 2] reaction, thiol–ene/yne coupling, Cu assisted cycloaddition, strain-promoted azide alkyne cycloaddition (SPAAC), nitro photoclick and photounclick reactions published in last one and half decade. This series of photoclick reactions use short wavelength radiations and are instant, clean, and near to perfect for transforming reactants to the desirable products.     

Rapid, photoactivatable turn-on fluorescent probes based on an intramolecular photoclick reaction

Photoactivatable fluorescent probes are invaluable tools for the study of biological processes with high resolution in space and time. Numerous strategies have been developed in generating photoactivatable fluorescent probes, most of which rely on the photo-"uncaging" and photoisomerization reactions. To broaden photoactivation modalities, here we report a new strategy in which the fluorophore is generated in situ through an intramolecular tetrazole-alkene cycloaddition reaction ("photoclick chemistry"). By conjugating a specific microtubule-binding taxoid core to the tetrazole/alkene prefluorophores, robust photoactivatable fluorescent probes were obtained with fast photoactivation (～1 min) and high fluorescence turn-on ratio (up to 112-fold) in acetonitrile/PBS (1:1). Highly efficient photoactivation of the taxoid-tetrazoles inside the mammalian cells was also observed under a confocal fluorescence microscope when the treated cells were exposed to either a metal halide lamp light passing through a 300/395 filter or a 405 nm laser beam. Furthermore, a spatially controlled fluorescent labeling of microtubules in live CHO cells was demonstrated with a long-wavelength photoactivatable taxoid-tetrazole probe. Because of its modular design and tunability of the photoactivation efficiency and photophysical properties, this intramolecular photoclick reaction based approach should provide a versatile platform for designing photoactivatable fluorescent probes for various biological processes.     

Surface restructuring behavior of various types of poly(dimethylsiloxane) in water detected by SFG

Surface structures of several different poly(dimethylsiloxane) (PDMS) materials, tetraethoxysilane-cured hydroxy-terminated PDMS (TEOS-PDMS), platinum-cured vinyl-terminated PDMS (Pt-PDMS), platinum-cured vinyl-terminated poly(diphenylsiloxane)-co-poly(dimethylsiloxane) (PDPS-co-PDMS), and PDMS-co-polystyrene (PDMS-co-PS) copolymer in air and water have been investigated by sum frequency generation (SFG) vibrational spectroscopy. The SFG spectra collected from all PDMS surfaces in both air and water are dominated by methyl group stretches, indicating that all the surfaces are mainly covered by methyl groups. Other than surface-dominating methyl groups, some -Si-CH2-CH2- moieties on the Pt-PDMS surface have also been detected in air, which are present at cross-linking points. Information about the average orientation angle and angle distribution of the methyl groups on the PDMS surface has been evaluated. Surface restructuring of the methyl groups has been observed for all PDMS surfaces in water. Upon contacting water, the methyl groups on all PDMS surfaces tilt more toward the surface. The detailed restructuring behaviors of several PDMS surfaces in water and the effects of molecular weight on restructuring behaviors have been investigated. For comparison, in addition to air and water, surface structures of PDMS materials mentioned above in a nonpolar solvent, FC-75, have also been studied. By comparing the different response of phenyl groups to water on both PDPS-co-PDMS and PS-co-PDMS surfaces, we have demonstrated how the restructuring behaviors of surface phenyl groups are affected by the structural flexibility of the molecular chains where they are attached.     

Surface-grafting of phosphates onto a polymer for potential biomimetic functionalization of biomaterials

In the human body, phosphate groups play important roles in signaling and the biological functions of proteins and peptides. Despite the importance of phosphate groups, polymer surfaces have not been directly grafted with phosphate groups by chemical reactions because the usual organic solvents used to graft phosphate groups can dissolve or swell polymers. We focused this study on grafting phosphate groups onto a poly(ethylene-co-acrylic acid) (PEAA) surface in an aqueous solution. O-phospho L-serine and O-phosphoethanolamine were grafted on PEAA surfaces to introduce phosphate groups by activating carboxylic acid groups of PEAA using N-hydroxysuccinimide (NHS) and 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) in an aqueous environment. X-ray photoelectron spectroscopy (XPS) was used to elucidate the process by which surface grafting occurs and the process that the phosphate group is cleaved into a phosphate ion and a hydrolyzed molecule at high pH. It was found that under appropriate reaction conditions the phosphate groups could be successfully grafted on the polymer surfaces. The phosphate-grafted polymer surfaces showed lower water contact angles than the initial polymer surfaces likely due to their highly mobile and hydrophilic phosphate side groups. This work demonstrates a technique to successfully graft phosphate groups onto organic polymer surfaces in a biocompatible aqueous environment, which may open new avenues to functionalizing synthetic polymeric and natural macromolecule derived biomaterials.     

SURFACE OF GELATIN MODIFIED POLY(L-LACTIC ACID)FILM

In this paper, the surface structure of poly(L-lactic acid) (PLLA) film modified with gelatin was investigated. ThePLLA film specimens were treated directly with aqueous alkali solution to provide their surfaces with carboxyl groups, sothat these functional groups could become the reactive sites for gelatin immobilization. The functional groups of the PLLAfilms were identified by ATR-FTIR spectra and XPS spectra, the changes in surface morphology were observed by usingenvironmental scanning electron microscopy (ESEM), and the hydrophilicity of modified PLLA films was examined bywater contact angle measurement. Experimental results showed that the gelatin was immobilized with water-solublecarbodiimide (EDC) onto the PLLA film's surfaces, and the gelatin content on the polymer surface was related to carboxylicgroup formed in the controlled hydrolysis process. Rough surfaces caused by hydrolysis will predominantly favor the adhesion and growth of cell; and the hydrophilicity of these surfaces after the modification procedure is enhanced.     

Molecular packing of lysozyme,fibrinogen, and bovine serum albumin on hydrophilic and hydrophobic surfaces studied by infrared-visible sum frequency generation and fluorescence microscopy

Infrared-visible sum frequency generation (SFG) vibrational spectroscopy, in combination with fluorescence microscopy, was employed to investigate the surface structure of lysozyme, fibrinogen, and bovine serum albumin (BSA) adsorbed on hydrophilic silica and hydrophobic polystyrene as a function of protein concentration. Fluorescence microscopy shows that the relative amounts of protein adsorbed on hydrophilic and hydrophobic surfaces increase in proportion with the concentration of protein solutions. For a given bulk protein concentration, a larger amount of protein is adsorbed on hydrophobic polystyrene surfaces compared to hydrophilic silica surfaces. While lysozyme molecules adsorbed on silica surfaces yield relatively similar SFG spectra, regardless of the surface concentration, SFG spectra of fibrinogen and BSA adsorbed on silica surfaces exhibit concentration-dependent signal intensities and peak shapes. Quantitative SFG data analysis reveals that methyl groups in lysozyme adsorbed on hydrophilic surfaces show a concentration-independent orientation. However, methyl groups in BSA and fibrinogen become less tilted with respect to the surface normal with increasing protein concentration at the surface. On hydrophobic polystyrene surfaces, all proteins yield similar SFG spectra, which are different from those on hydrophilic surfaces. Although more protein molecules are present on hydrophobic surfaces, lower SFG signal intensity is observed, indicating that methyl groups in adsorbed proteins are more randomly oriented as compared to those on hydrophilic surfaces. SFG data also shows that the orientation and ordering of phenyl rings in the polystyrene surface is affected by protein adsorption, depending on the amount and type of proteins.