Synthesis and Photophysical Characterization of a New, Highly Hydrophilic Caging Group

Two-photon uncaging of bioactive thiols in live cells at wavelengths above 800 nm

Photoactivatable protecting groups (PPGs) are useful for a broad range of applications ranging from biology to materials science. In chemical biology, induction of biological processes via photoactivation is a powerful strategy for achieving spatiotemporal control. The importance of cysteine, glutathione, and other bioactive thiols in regulating protein structure/activity and cell redox homeostasis makes modulation of thiol activity particularly useful. One major objective for enhancing the utility of photoactivatable protecting groups (PPGs) in living systems is creating PPGs with longer wavelength absorption maxima and efficient two-photon (TP) absorption. Toward these objectives, we developed a carboxyl- and dimethylamine-functionalized nitrodibenzofuran PPG scaffold (cDMA-NDBF) for thiol photoactivation, which has a bathochromic shift in the one-photon absorption maximum from λmax = 315 nm with the unfunctionalized NDBF scaffold to λmax = 445 nm. While cDMA-NDBF-protected thiols are stable in the presence of UV irradiation, they undergo efficient broad-spectrum TP photolysis at wavelengths as long as 900 nm. To demonstrate the wavelength orthogonality of cDMA-NDBF and NDBF photolysis in a biological setting, caged farnesyltransferase enzyme inhibitors (FTI) were prepared and selectively photoactivated in live cells using 850-900 nm TP light for cDMA-NDBF-FTI and 300 nm UV light for NDBF-FTI. These experiments represent the first demonstration of thiol photoactivation at wavelengths above 800 nm. Consequently, cDMA-NDBF-caged thiols should have broad applicability in a wide range of experiments in chemical biology and materials science.

Effect of Photocaged Isopropyl β-d-1-thiogalactopyranoside Solubility on the Light Responsiveness of LacI-controlled Expression Systems in Different Bacteria

Photolabile protecting groups play a significant role in controlling biological functions and cellular processes in living cells and tissues, as light offers high spatiotemporal control, is non‐invasive as well as easily tuneable. In the recent past, photo‐responsive inducer molecules such as 6‐nitropiperonyl‐caged IPTG (NP‐cIPTG) have been used as optochemical tools for Lac repressor‐controlled microbial expression systems. To further expand the applicability of the versatile optochemical on‐switch, we have investigated whether the modulation of cIPTG water solubility can improve the light responsiveness of appropriate expression systems in bacteria. To this end, we developed two new cIPTG derivatives with different hydrophobicity and demonstrated both an easy applicability for the light‐mediated control of gene expression and a simple transferability of this optochemical toolbox to the biotechnologically relevant bacteria Pseudomonas putida and Bacillus subtilis. Notably, the more water‐soluble cIPTG derivative proved to be particularly suitable for light‐mediated gene expression in these alternative expression hosts.

Does the photoredox reaction affect the photorelease of anthraquinone protected benzaldehyde? A time-resolved spectroscopic study

Time-resolved spectroscopy studies coupled with the results from density functional theory (DFT) computations were utilized to unravel the photodeprotection reaction mechanism of Aqe-diol-PPG and the photoredox of photoproduct Aqe-diol.

Photocleavable antimicrobial peptide mimics for precluding antibiotic resistance

UV-cleavable antimicrobial peptide mimics were synthesized to address environmental accumulation issues.

Alpha-carboxy-6-nitroveratryl: a photolabile protecting group for carboxylic acids

The synthesis of a new photolabile protecting group for carboxylic acids, alpha-carboxy-6-nitroveratryl (alphaCNV), is described. Bromide 3, prepared in four steps from 3,4-dimethoxyphenylacetic acid, was used to alkylate carboxylic acids under mild conditions in good yield. Palladium-catalyzed deallylation afforded the acids 4a-h, which underwent rapid and quantitative photolysis at wavelengths longer than 300 nm to liberate the carboxylic acid in good to quantitative yield. The rate of photolysis and quantum yield were determined to be 325 s(-1) and 0.17.

The α,5-dicarboxy-2-nitrobenzyl caging group, a tool for biophysical applications with improved hydrophilicity: synthesis, photochemical properties and biological characterization

Earlier we reported on the synthesis of α,4-dicarboxy-2-nitrobenyzl caged compounds (Schaper, K. et al. [2002] Eur. J. Org. Chem., 1037-1046). These compounds have the advantage of an increased hydrophilicity compared with the well-established α-carboxy-2-nitrobenzyl caged compounds; however, the release of the active compound becomes slower due to the introduction of the additional carboxy group. Based upon theoretical calculations we predicted that the release would become faster when the additional carboxy group is moved to the 5-position. Here we describe the synthesis and the photochemical and biological characterization of an α,5-dicarboxy-2-nitrobenyzl caged compound. The high hydrophilicity of the new caging group is maintained due to the fact that the additional carboxy moiety is preserved, while the release of the active species from the new derivative is even faster than for the reference, an α-CNB caged compound.

NMR-spectroscopic investigation of o-nitrosobenzoic acid

The synthesis of o-nitrosobenzoic acid 2 has been known for more than 100 years, and the photochemical preparation from o-nitrobenzaldehyde 1 became a textbook example for [1,5]-hydrogen shifts. However, neither the (1)H-NMR spectra nor the (13)C--{(1)H}-NMR of this compound have been reported so far. This fact can most likely be attributed to the monomer-dimer equilibrium of the nitrosobenzoic acid, which leads to rather complex, concentration-dependent NMR spectra. In this paper, we report a thorough investigation of these spectra. In the (13)C-{(1)H}-NMR spectra, all 21 lines could be assigned to the monomeric form, the E-dimer, and the Z-dimer.

Photoprocesses of molecules with 2-nitrobenzyl protecting groups and caged organic acids

The 308 nm photoinduced formation of the nitroso product and the intermediacy of the aci-nitro form(s) were studied for a series of 2-nitrobenzyl alkyl and aryl esters (1a-4e) and bis-(nitrophenyl)methyl acetates (5a-6b) by time-resolved UV-vis spectroscopy. A triplet state appears as major transient, when 2-nitrobenzyl derivatives 1 are substituted by 4,5-dimethoxy (2) and 4,5-methylenedioxy (3/4) groups. This triplet of charge transfer character is, however, not part of the route via the aci-nitro into the 2-nitroso form. The activation energy and preexponential factor of the longest lifetime component (tau(aci)), i.e. the major part of the aci-nitro decay, were determined. The carboxylic acids as leaving groups have rather small effects on tau(aci). An additional nitrated phenyl ring in alpha-position (5) leads generally to shorter tau(aci) value. Otherwise, the photogeneration of nitroso products is similar. The quantum yield (Phi(d)) varies only moderately with structure, the yield of the aci-nitro form and Phi(d) are correlated and little affected by solvent properties.

Characterisation of a series of acetals/ketals of bis(2-nitrophenyl) ethanediol and bis(4,5-dimethoxy-2-nitrophenyl) ethanediol under APCI mass spectrometric conditions

A series acetals/ketals of aldehydes and ketones formed by the reaction of two photolabile protecting groups, bis(2-nitrophenyl) ethanediol and bis(4,5-dimethoxy-2-nitrophenyl) ethanediol (I and II, respectively), were analysed under EI, LSIMS, ESI and APCI conditions to obtain molecular weights as well as structural information. The EI and LSIMS techniques failed to give molecular weight information. The positive ESI yielded [M + H](+) ions only for I; however, with added Na(+) both I and II formed [M + Na](+) adducts. But upon decomposition, the [M + Na](+) ions yielded Na(+) ion as the only product ion. Similarly, under negative ion ESI conditions both I and II gave molecular weight information by forming adduct ions with halide anions (F(-), Cl(-), Br(-) and I(-)); however, they did not give structural information as they resulted in only the halide anion as the abundant fragment ion upon dissociation. All the compounds formed abundant M(-*) ions under negative ion APCI conditions, and their MS/MS spectra showed characteristic fragment ions; hence the acetals/ketals of I and II could be successfully characterized under negative ion APCI conditions.

Formulation of photocleavable liposomes and the mechanism of their content release

In pursuit of designing photocleavable liposomes as drug delivery vehicles, we synthesized several amphiphilic lipids by connecting stearyl amine (as the non-polar tail) and charged amino acids (as polar heads) via the o-nitrobenzyl derivatives. The lipids containing Glu, Asp, and Lys amino acids were subjected to photocleavage reaction by UV light, and the overall spectral changes of the chromophoric o-nitrobenzyl conjugates were determined as a function of time. The experimental data revealed that the feasibility of the cleavage reaction, nature and magnitude of the spectral changes during the course of the cleavage reaction, and their overall kinetic profiles were dictated by the type of amino acid constituting the polar head groups. The cleavage reactions of the Asp and Glu containing lipids were found to be more facile than that of the lysine-containing lipid. Using these lipids, we formulated photocleavable liposomes, and investigated the photo-triggered release of an encapsulated (within the liposomal lumen) dye as a function of time. The kinetic data revealed that the release of the liposomal content conformed to a two-step mechanism, of which the first (fast) step involved the photocleavage of lipids followed by the slow release of the liposomal content during the second step. The overall mechanistic features intrinsic to the photocleavage of Asp, Glu and Lys containing o-nitrobenzyl conjugated lipids, and their potential applications in formulating liposomes (whose contents can be "unloaded" by the UV light) as drug delivery vehicles are discussed.

Design of photocleavable lipids and their application in liposomal "uncorking"

The design of o-nitrobenzyl containing photocleavable lipid-amino acid conjugates, and their application in liposomal uncorking are described.

Effects of 4,5-dimethoxy groups on the time-resolved photoconversion of 2-nitrobenzyl alcohols and 2-nitrobenzaldehyde into nitroso derivatives

The photoinduced conversion of the aci-nitro in the nitroso form was studied with four compounds containing the o-nitrobenzyl moiety in solution at ambient temperature using time-resolved UV-vis spectroscopy. For 4,5-dimethoxy-2-nitrobenzyl alcohol (2) and 4,5-methylenedioxy-2-nitrobenzyl alcohol (3) the absorption spectra are red-shifted and, in contrast to the parent 2-nitrobenzyl alcohol (1), a triplet state with CT character was detected after the 308 nm laser pulse. The other photochemical properties of 1-3 are similar. The aci-nitro form of 1-3 in acetonitrile or ethanol is quenched by water, the rate constant is (0.3-1.7) x 10(5) M(-1) s(-1). A CT triplet state and the nitroso product but no aci-nitro form were observed for 4,5-methylenedioxy-2-nitrobenzaldehyde (4). The conversion of the aci-nitro into the nitroso monomer and eventual dimer formation were studied by FTIR spectroscopy. The common features and specific differences in the photoreaction mechanisms of 1-4 are discussed.