A new fluorogenic transformation: development of an optical probe for coenzyme Q

Visible-to-NIR-Light Activated Release: From Small Molecules to Nanomaterials

Photoactivatable (alternatively, photoremovable, photoreleasable, or photocleavable) protecting groups (PPGs), also known as caged or photocaged compounds, are used to enable non-invasive spatiotemporal photochemical control over the release of species of interest. Recent years have seen the development of PPGs activatable by biologically and chemically benign visible and near-infrared (NIR) light. These long-wavelength-absorbing moieties expand the applicability of this powerful method and its accessibility to non-specialist users. This review comprehensively covers organic and transition metal-containing photoactivatable compounds (complexes) that absorb in the visible- and NIR-range to release various leaving groups and gasotransmitters (carbon monoxide, nitric oxide, and hydrogen sulfide). The text also covers visible- and NIR-light-induced photosensitized release using molecular sensitizers, quantum dots, and upconversion and second-harmonic nanoparticles, as well as release via photodynamic (photooxygenation by singlet oxygen) and photothermal effects. Release from photoactivatable polymers, micelles, vesicles, and photoswitches, along with the related emerging field of photopharmacology, is discussed at the end of the review.

Molecular probe designviathe “covalent-assembly” principle

Fluorescent probes are useful molecular tools. We summarize the recent progress with the “covalent-assembly” design principle, which warrants high-performance fluorescence probes exhibiting a highly sensitive turn-on signal from the dark background.

A general strategy for visible-light decaging based on the quinone cis-alkenyl lock

Combining the fast thermal cyclization of o-coumaric acid derivatives with the intramolecular photoreduction of quinones gives new visible-light photoremovable protecting groups absorbing well above 450 nm.

Concise Total Synthesis of (±)-Deguelin and (±)-Tephrosin Using a Vinyl Iodide as a Key Building Block

A concise and protecting-group-free total synthesis of the antiproliferative natural product (±)-deguelin (2) was accomplished in four steps and 62% overall yield from commercially available precursors. The key transformation employed a vinyl iodide as the pivotal building block to construct the 4-acylchromene substructure present in deguelin. Subsequent Cu₂O-mediated α-hydroxylation of deguelin (2) afforded tephrosin (3) in 90% yield.

Studies toward the Development of Antiproliferative Neoclerodanes from Salvinorin A

The success rate for central nervous system (CNS) drug candidates in the clinic is relatively low compared to the industry average across other therapeutic areas. Penetration through the blood-brain barrier (BBB) to reach the therapeutic target is a major obstacle in development. The rapid CNS penetration of salvinorin A has suggested that the neoclerodane nucleus offers an excellent scaffold for developing antiproliferative compounds that enter the CNS. The Liebeskind-Srogl reaction was used as the main carbon-carbon bond-forming step toward the synthesis of quinone-containing salvinorin A analogues. Quinone-containing salvinorin A analogues were shown to have antiproliferative activity against the MCF7 breast cancer cell line, but show no significant activity at the κ-opioid receptors. In an in vitro model of BBB penetration, quinone-containing salvinorin A analogues were shown to passively diffuse across the cell monolayer. The analogues, however, are substrates of P-glycoprotein, and thus further modification of the molecules is needed to reduce the affinity for the efflux transporter.

Design, synthesis, and biological evaluation of the analogues of glaziovianin A, a potent antitumor isoflavone

Various analogues of glaziovianin A, an antitumor isoflavone, were synthesized, and their biological activities were evaluated. O(7)-modified glaziovianin A showed strong cytotoxicity against HeLa S(3) cells. Compared to glaziovianin A, the O(7)-benzyl and O(7)-propargyl analogues were more cytotoxic against HeLa S(3) cells and more potent M-phase inhibitors. Furthermore, O(7)-modified molecular probes of glaziovianin A were synthesized for biological studies.

A Green and Efficient Synthesis of 1-Chloromethyl -2,3,4,5-Tetramethoxy-6-Methylbenzene

The title compound, a key intermediate for preparing coenzyme Q analogues, was prepared in excellent yield by a reaction sequence starting from the commercially available 3,4,5-trimethoxybenzadehyde via Wolff–Kishner reduction, selective bromination, methoxylation and Blanc chloromethylation reaction.

Design and synthesis of novel quinone inhibitors targeted to the redox function of apurinic/apyrimidinic endonuclease 1/redox enhancing factor-1 (Ape1/ref-1)

The multifunctional enzyme apurinic endonuclease 1/redox enhancing factor 1 (Ape1/ref-1) maintains genetic fidelity through the repair of apurinic sites and regulates transcription through redox-dependent activation of transcription factors. Ape1 can therefore serve as a therapeutic target in either a DNA repair or transcriptional context. Inhibitors of the redox function can be used as either therapeutics or novel tools for separating the two functions for in vitro study. Presently there exist only a few compounds that have been reported to inhibit Ape1 redox activity; here we describe a series of quinones that exhibit micromolar inhibition of the redox function of Ape1. Benzoquinone and naphthoquinone analogues of the Ape1-inhibitor E3330 were designed and synthesized to explore structural effects on redox function and inhibition of cell growth. Most of the naphthoquinones were low micromolar inhibitors of Ape1 redox activity, and the most potent analogues inhibited tumor cell growth with IC(50) values in the 10-20 microM range.

Role of the multifunctional DNA repair and redox signaling protein Ape1/Ref-1 in cancer and endothelial cells: small-molecule inhibition of the redox function of Ape1

The DNA base excision-repair pathway is responsible for the repair of DNA damage caused by oxidation/alkylation and protects cells against the effects of endogenous and exogenous agents. Removal of the damaged base creates a baseless (AP) site. AP endonuclease1 (Ape1) acts on this site to continue the BER-pathway repair. Failure to repair baseless sites leads to DNA strand breaks and cytotoxicity. In addition to the repair role of Ape1, it also functions as a major redox-signaling factor to reduce and activate transcription factors such as AP1, p53, HIF-1alpha, and others that control the expression of genes important for cell survival and cancer promotion and progression. Thus, the Ape1 protein interacts with proteins involved in DNA repair, growth-signaling pathways, and pathways involved in tumor promotion and progression. Although knockdown studies with siRNA have been informative in studying the role of Ape1 in both normal and cancer cells, knocking down Ape1 does not reveal the individual role of the redox or repair functions of Ape1. The identification of small-molecule inhibitors of specific Ape1 functions is critical for mechanistic studies and translational applications. Here we discuss small-molecule inhibition of Ape1 redox and its effect on both cancer and endothelial cells.

Total Synthesis of (±)-Frondosin B

An expeditious reaction sequence featuring a microwave-assisted tandem 5-exo cyclization-Claisen rearrangement process was used to assemble the A/B ring system of frondosin B. Completion of the target natural product was achieved in 38.2% yield over an eight-step linear sequence.

Cocktails of Tb3+ and Eu3+ Complexes: A General Platform for the Design of Ratiometric Optical Probes

Fluorescent and luminescent reporters that signal molecular events of interest by modulating the ratio of peaks in their emission profile have advantages over reporters that simply modulate their emission intensity, since ratiometric measurement is concentration-independent and allows them to be effective in complex contexts, such as living cells or sensor microarrays. We herein describe a general platform for the design of ratiometric probes based on a heterometallic Tb(3+)/Eu(3+) bis-lanthanide ensemble, consisting of a mixture, or "cocktail", of otherwise identical heterometalated chelates. The chelate contains an organic photon antenna that sensitizes the Tb(3+)/Eu(3+) luminescence. The contributions of the two metals to the composite luminescence spectrum can be tuned to the same relative scale by adjusting the stoichiometry of the cocktail, allowing subtle changes in their ratio to be accurately measured. Importantly, the ratio responds to chemical and environmental changes experienced by the photon antenna, making the system an ideal platform for the design of chemical and enzymatic probes. As proofs of concept, we describe a ratiometric probe for esterase activity and a polarity-responsive ratiometric sensor.

Remote Position Substituents as Modulators of Conformational and Reactive Properties of Quinones. Relevance of the π/π Intramolecular Interaction

Several studies have described that quinoid rings with electron-rich olefins at remote position experience changes in their redox potential. Since the original description of these changes, different approaches have been developed to describe the properties of the binding sites of ubiquinones. The origin of this phenomenon has been attributed to lateral chain flexibility and its effect on the recognition between proteins and substrates associated with their important biological activity. The use of electrochemical-electron spin resonance (EC-ESR) assays and theoretical calculations at MP2/6-31G(d,p) and MP2/6-31++G(d,p)//MP2/6-31G(d,p) levels of several conformers of perezone [(2-(1,5-dimethyl-4-hexenyl)-3-hydroxy-5-methyl-1,4-benzoquinone] established that a weak pi-pi interaction controls not only the molecular conformation but also its diffusion coefficient and electrochemical properties. An analogous interaction can be suggested as the origin of similar properties of ubiquinone Q10. The use of nuclear magnetic resonance rendered, for the first time, direct evidence of the participation of different perezone conformers in solution and explained the cycloaddition process observed when the aforementioned quinone is heated to form pipitzols, sesquiterpenes with a cedrene skeleton. The fact that biological systems can modulate the redox potential of this type of quinones depending on the conformer recognized by an enzyme during a biological transformation is of great relevance.

Phosphorylation State-Responsive Lanthanide Peptide Conjugates: A Luminescence Switch Based on Reversible Complex Reorganization

[reaction: see text] A luminogenic probe for peptide dephosphorylation has been developed. It consists of a serine-/tyrosine-containing peptide modified on the N-terminus with a tryptophan residue and a DTPA chelate capable of binding Tb(3+). We propose a mechanistic model for the luminescence enhancement based on the interconversion of monomeric and dimeric lanthanide species, which is affected by the phosphorylation state of the serine or tyrosine residue. The optical switch reports effectively on phosphatase-catalyzed dephosphorylation in vitro.

Synthesis of luminescent heterometallic bis-lanthanide complexes via selective, sequential metallation

A modular synthetic method for the differential incorporation of two lanthanide ions into a single molecular scaffold is reported; the mixed bimetallic Tb/Eu complex displays an interesting solvent polarity-dependent ratiometric luminescence.