Photolabile Protecting Groups Research Articles

Enhancing the Efficacy of the Benzoin Photolabile Protecting Group Using Electron-Donating Substituents

Enhancing the Efficacy of the Benzoin Photolabile Protecting Group Using Electron-Donating Substituents

2-(4-Nitrophenyl)-1H-indolyl-3-methyl Chromophore: A Versatile Photocage that Responds to Visible-light One-photon and Near-infrared-light Two-photon Excitations

Abstract Due to cell damage caused by UV light, photoremovable protecting groups (PPGs) that are removed using visible or near-infrared light are attracting attention. A 2-(4-nitrophenyl)-1H-indolyl-3-methyl chromophore (NPIM) was synthesized as a novel PPG. Various compounds were caged using this PPG and uncaged using visible or near-infrared light. Low cytotoxicity of NPIM indicates that it may be applied in physiological studies.

Inside Cover Picture

The cover picture shows a green and efficient photolabile protecting group (PPG)‐mediated glycosidation approach for the synthesis of 2‐deoxy‐glycosides. The remarkable feature of this new approach is the employment of ortho‐nitrobenzyl carbonate (oNBC) as PPG for the assembly of 2‐deoxy‐glycosides via the gold(I)‐promoted glycosidation with per‐oNBC‐protected 2‐deoxy‐glycosyl ynenoates as donors. In particular, photo‐cleavage of the multiple oNBC groups in the 2‐deoxy‐glycosides is effectively achieved by irradiation at 365 nm under mild and neutral conditions. More details are given in the article by Yang et al. on page 3309—3314.image

Photolabile Protecting Group‐Mediated Synthesis of 2‐Deoxy‐Glycosides

Main observation and conclusionA green and efficient photolabile protecting group (PPG)‐mediated glycosidation approach for the synthesis of 2‐deoxy‐glycosides is reported. By employing ortho‐nitrobenzyl carbonate (oNBC) as PPG, N,N‐dimethylformamide (DMF)‐modulated SPhosAuNTf2‐promoted glycosidation with per‐oNBC‐protected 2‐deoxy‐glycosyl ynenoates affords the 2‐deoxy‐glycosides with moderate to excellent α‐selectivities presumably depending on the reactivities of the acceptor alcohols. Based on the PPG‐mediated glycosidation approach, oligosaccharides with three to six oNBC groups are effectively achieved. The multiple oNBC groups in the 2‐deoxy‐glycosides are completely cleaved by irradiation at 365 nm, resulting in the desired 2‐deoxy‐glycosides in an efficient manner without affecting the common alkyne, alkene, azide functional groups and the traditional protecting groups on the aglycones.

8-[4-(2-Hydroxypropane-2-yl)phenyl]-1,3,4,4,5,7-hexamethyl-4-boron-3a,4a-diaza-S-indacene

During recent years, the BODIPY core became a popular scaffold for designing photoremovable protecting groups (PPG). In this paper, we report the synthesis of a new molecule—8-[4-(2-hydroxypropane-2-yl)phenyl]-1,3,4,4,5,7-hexamethyl-4-boron-3a,4a-diaza-S-indacene—by the treatment of meso-(4-CO2Me-phenyl)-BODIPY with excess of MeMgI. The product was characterized by 1H, 13C NMR and HRMS. The combination of BODIPY core with tertiary benzilyc alcohol might be promising for utilizing this molecule as visible light removable PPG.

Two-Photon Sensitive Coumarinyl Photoremovable Protecting Groups with Rigid Electron-Rich Cycles Obtained by Domino Reactions Initiated by a 5-exo-Dig Cyclocarbopalladation.

We herein report the design, synthesis, and photophysical characterization of extended and rigid coumarinyl derivatives showing large two-photon sensitivities (δaΦu ≤ 125 GM) at 740 and 800 nm. To efficiently synthesize these complex photoremovable protecting groups (PPGs), we used step-economic domino reactions. Moreover, those new coumarinyl PPGs display unique bathochromic shifts (≤100 nm) of the uncaging subproducts as a result of the formation of a more conjugated fulvene moiety.

Photoregulation of PRMT-1 Using a Photolabile Non-Canonical Amino Acid.

Protein methyltransferases are vital to the epigenetic modification of gene expression. Thus, obtaining a better understanding of and control over the regulation of these crucial proteins has significant implications for the study and treatment of numerous diseases. One ideal mechanism of protein regulation is the specific installation of a photolabile-protecting group through the use of photocaged non-canonical amino acids. Consequently, PRMT1 was caged at a key tyrosine residue with a nitrobenzyl-protected Schultz amino acid to modulate protein function. Subsequent irradiation with UV light removes the caging group and restores normal methyltransferase activity, facilitating the spatial and temporal control of PRMT1 activity. Ultimately, this caged PRMT1 affords the ability to better understand the protein’s mechanism of action and potentially regulate the epigenetic impacts of this vital protein.

Multiwavelength Control of Mixtures Using Visible Light-Absorbing Photocages.

Selective deprotection of functional groups using different wavelengths of light is attractive for materials synthesis as well as for achieving independent photocontrol over substrates in biological systems. Here, we show that mixtures of recently developed visible light-absorbing BODIPY-derived photoremovable protecting groups (PRPGs) and a coumarin-derived PRPG can undergo wavelength-selective activation, giving independent optical control over a mixture of photocaged substrates using biologically benign long-wavelength light.

Photolytical reactions for light induced biological effectors release: on the road to the phototherapeutic window

The use of caged compounds, defined as photolabile precursors of biological effectors, have proven to be attractive in various fields of biology. Photolytical reactions have been widely used to allow a rapid and efficient concentration jump of various biological effectors within organized biological systems. During the last two decades, the challenge was to overcome the difficulty that only high energy UV light was used to induce photochemical reactions on photoremovable protecting groups (PPGs). Infrared-sensitive PPGs should be able to improve in vivo applications of caged compounds. The present review is focused on recent strategies enabling the use of excitation wavelength inside the photo-therapeutical window (600–1000 nm) to efficiently and specifically cleave a chemical bond.

A Photolabile Carboxyl Protecting Group for Solid Phase Peptide Synthesis.

A new kind of photolabile protecting group (PLPG) for carboxyl moieties was designed and synthesized as the linker between resin and peptide. This group can be used for the protection of amino acid carboxyl groups. The peptide was synthesized on Nph (2‐hydroxy‐3‐(2‐nitrophenyl)‐heptanoic acid)‐derivatized resins and could be cleaved under UV exposure, thus avoiding the necessity for harsh acid‐mediated resin cleavage. The PLPG has been successfully used for solid‐phase synthesis of peptides.

Transformation of COUPY Fluorophores into a Novel Class of Visible-Light-Cleavable Photolabile Protecting Groups.

Although photolabile protecting groups (PPGs) have found widespread applications in several fields of chemistry, biology and materials science, there is a growing interest in expanding the photochemical toolbox to overcome some of the limitations of classical caging groups. In this work, the synthesis of a new class of visible-light-sensitive PPGs based on low-molecular weight COUPY fluorophores with several attractive properties, including long-wavelength absorption, is reported. Besides being stable to spontaneous hydrolysis in the dark, COUPY-based PPGs can be efficiently photoactivated with yellow (560 nm) and red light (620 nm) under physiological-like conditions, thereby offering the possibility of unmasking functional groups from COUPY photocages under irradiation conditions in which other PPGs remain stable. Additionally, COUPY photocages exhibit excellent cellular uptake and accumulate selectively in mitochondria, opening the door to the delivery of caged analogues of biologically active compounds into these organelles.

Synthesis and Characterisation of Photolabile SPhNPPOC-Protected (R)-MiniPEG Containing Chiral γ-Peptide Nucleic Acid Monomers

Peptide nucleic acid (PNA) microarrays are expected to be developed as a new generation of gene detection tools. However, poor water solubility and the limitation of the sequence design of achiral PNA probes mainly hinder their application. Accordingly, (R)-diethylene glycol containing a chiral PNA (miniPEG-γPNA) has been developed to solve these problems. Light-directed synthesis is an effective method to fabricate high-density microarrays. Thiophenyl-2-(2-nitrophenyl)propoxycarbonyl (SPhNPPOC) is a newly synthesised photolabile protective group with high photolytic efficiency. Protecting the PNA monomers with SPhNPPOC may improve the preparation process of PNA microarrays by light-directed synthesis in terms of shortening the deprotection time and restraining side reactions. In this article, SPhNPPOC/carbobenzoxy (Cbz)-protected chiral miniPEG-γPNA monomers are synthesised, and the photo-deprotection rate is approximately twice that of a 2-(2-nitrophenyl)propyloxycarbonyl (NPPOC)-protected monomer. The monomers are expected to be used for the efficient and rapid fabrication of chiral miniPEG-γPNA microarrays through a photolithographic strategy.

Monitoring of uncaging processes by designing photolytical reactions.

The use of photolabile protecting groups (PPGs) has been growing in emphasis for decades, and nowadays they enable cutting-edge results in numerous fields ranging from organic synthesis to neurosciences. PPGs are chemical entities that can be conjugated to a biomolecule to hide its biological activity, forming a stable so called "caged compound". This conjugate can be simply cleaved by light and therefore, the functionality of the biomolecule is restored with the formation of a PPG by-product. However, there is a sizeable need for PPGs that are able to quantify the "uncaging" process. In this review, we will discuss several strategies leading to an acute quantification of the uncaging events by fluorescence. In particular, we will focus on how molecular engineering of PPG could open new opportunities by providing easy access to photoactivation protocols.

Photorelease of Pyridines Using a Metal-Free Photoremovable Protecting Group.

The photorelease of bioactive molecules has emerged as a valuable tool in biochemistry. Nevertheless, many important bioactive molecules, such as pyridine derivatives, cannot benefit from currently available organic photoremovable protecting groups (PPGs). We found that the inefficient photorelease of pyridines is attributed to intramolecular photoinduced electron transfer (PET) from PPGs to pyridinium ions. To alleviate PET, we rationally designed a strategy to drive the excited state of PPG from S1 to T1 with a heavy atom, and synthesized a new PPG by substitution of the H atom at the 3-position of 7-dietheylamino-coumarin-4-methyl (DEACM) with Br or I. This resulted in an improved photolytic efficiency of the pyridinium ion by hundreds-fold in aqueous solution. The PPG can be applied to various pyridine derivatives. The successful photorelease of a microtubule inhibitor, indibulin, in living cells was demonstrated for the potential application of this strategy in biochemical research.

Photorelease of Pyridines Using a Metal‐Free Photoremovable Protecting Group

AbstractThe photorelease of bioactive molecules has emerged as a valuable tool in biochemistry. Nevertheless, many important bioactive molecules, such as pyridine derivatives, cannot benefit from currently available organic photoremovable protecting groups (PPGs). We found that the inefficient photorelease of pyridines is attributed to intramolecular photoinduced electron transfer (PET) from PPGs to pyridinium ions. To alleviate PET, we rationally designed a strategy to drive the excited state of PPG from S1 to T1 with a heavy atom, and synthesized a new PPG by substitution of the H atom at the 3‐position of 7‐dietheylamino‐coumarin‐4‐methyl (DEACM) with Br or I. This resulted in an improved photolytic efficiency of the pyridinium ion by hundreds‐fold in aqueous solution. The PPG can be applied to various pyridine derivatives. The successful photorelease of a microtubule inhibitor, indibulin, in living cells was demonstrated for the potential application of this strategy in biochemical research.

Synthesis and biological evaluation of a novel photo-activated histone deacetylase inhibitor

Hydroxamic acid-based histone deacetylase inhibitors (HDACi) are a class of epigenetic agents with potentially broad therapeutic application to several disease states including post angioplasty mediated neointimal hyperplasia (NIH). Precise spatiotemporal control over the release of HDACi at the target blood vessel site is required for the safe and successful therapeutic use of HDACi in the setting of drug eluting balloon catheter (DEBc) angioplasty treatment of NIH. We aimed to develop and characterise a novel photoactive HDACi, as a potential coating agent for DEBc.Metacept-3 1 was caged with a photo-labile protecting group (PPG) to synthesise a novel UV365nm active HDACi, caged metacept-3 15. Conversion of caged metacept-3 15 to active/native metacept-3 1 by UV365nm was achievable in significant quantities and at UV365nm power levels in the milliwatt (mW) range.In vitro evaluation of the biological activity of pre and post UV365nm activation of caged metacept-3 15 identified significant HDACi activity in samples exposed to short duration, mW range UV365nm. Toxicity studies performed in human umbilical vein endothelial cells (HUVEC’s) identified significantly reduced toxicity of caged metacept-3 15 pre UV365nm exposure compared with native metacept-3 1 and paclitaxel (PTX).Taken together these findings identify a novel photo-activated HDACi, caged metacept-3 15, with pharmacokinetic activation characteristics and biological properties which may make it suitable for evaluation as a novel coating for targeted DEBc angioplasty interventions.

A Sunscreen-Based Photocage for Carbonyl Groups.

Photolabile protecting groups (PPGs) have been exploited in a wide range of chemical and biological applications, due to their ability to provide spatial and temporal control over light-triggered activation. In this work, we explore the concept of a new photocage compound based on the commercial UVA/UVB filter oxybenzone (OB; 2-hydroxy-4-methoxybenzophenone) for photoprotection and controlled release of carbonyl groups. The point here is that oxybenzone not only acts as a mere PPG, but also provides, once released, UV photoprotection to the carbonyl derivative. This design points to a possible therapeutic approach to reduce the severe photoadverse effects of drugs containing a carbonyl chromophore.

Understanding the Role of Aromaticity and Conformational Changes in Bond Dissociation Processes of Photo-Protecting Groups.

Photoremovable protecting groups (PPGs) provide spatial and temporal control over the release of various chemicals. Using surface hopping studies with multireference electronic structure methods we have unravelled the nuclear and the electronic events at play. Furthermore, the electronic changes along the reaction path were probed using excited state aromaticity quantifiers and orbital analysis. We find that upon irradiation with light of appropriate wavelength on the substituted coumarin system a π-π* electronic excitation occurs which is followed by an electron loss from the aromatic ring on gaining proper alignment between the π* and the C-LG (LG = leaving group) σ*. This alignment is brought about by a critical dihedral angle change in the molecule, which subsequently triggers C-LG bond cleavage. The sequence of events is indicative of an intramolecular electron catalyzed process which is established through investigations of changes in aromaticity of the phenyl ring which acts as an electron reservoir.

A chemoselective photolabile protecting group for aldehydes

A new and high-efficiency photolabile protecting group (PLPG) for aldehydes is described. The PLPG was introduced to aldehydes by using a Lewis acid. Results showed that the PLPG can be released rapidly and smoothly under ultraviolet (UV) irradiation with high efficiency and low cost. This PLPG can easily synthesized and also be selectively protect aldehydes in the presence of ketones.

Synthesis of Deoxypseudouridine 5'-Triphosphate Bearing the Photoremovable Protecting Group at the N1 Position Capable of Enzymatic Incorporation to DNA.

Enzymatic incorporation of deoxynucleoside 5'-triphosphate bearing the photocleavable protecting group is a useful method for the preparation of photocaged oligodeoxynucleotides. Here, we describe the synthesis of new photocaged deoxynucleoside triphosphates N1-(2-nitrobenzyl)-deoxypseudouridine triphosphate (dNBΨTP) and N1-(6-nitropiperonyloxymethyl)-deoxypseudouridine triphosphate (dNPOMΨTP). We successfully synthesized dNBΨTP and dNPOMΨTP and applied them to enzymatic synthesis of photocaged oligonucleotides. In addition, we also synthesized phosphoramidites of N1-(2-nitrobenzyl)- and N1-(6-nitropiperonyloxymethyl)-deoxypseudouridine to enable chemical synthesis of photocaged oligonucleotides incorporating them. The photocleavable 2-nitrobenzyl and 6-nitropiperonyloxymethyl in oligonucleotides were cleaved by irradiation at 365 nm for 30 and 10 s, respectively. We also studied the enzymatic incorporation of dNBΨTP and dNPOMΨTP using the Klenow fragment exo-. As a result, it was clarified that dNPOMΨTP could be incorporated to oligonucleotide 193 times more efficiently than dNBΨTP, as judged by Vmax/Km. We also performed the incorporation of at least eight dNPOMΨ residues in a 35-mer oligodeoxynucleotide. It has also been revealed that the oligodeoxynucleotides incorporating photocaged deoxypseudouridine were useful for photocontrol of DNA triplex formation.