A DNA nanodevice for mapping sodium at single-organelle resolution.

Cellular sodium ion (Na+) homeostasis is integral to organism physiology. Our current understanding of Na+ homeostasis is largely limited to Na+ transport at the plasma membrane. Organelles may also contribute to Na+ homeostasis; however, the direction of Na+ flow across organelle membranes is unknown because organellar Na+ cannot be imaged. Here we report a pH-independent, organelle-targetable, ratiometric probe that reports lumenal Na+. It is a DNA nanodevice containing a Na+-sensitive fluorophore, a reference dye and an organelle-targeting domain. By measuring Na+ at single endosome resolution in mammalian cells and Caenorhabditis elegans, we discovered that lumenal Na+ levels in each stage of the endolysosomal pathway exceed cytosolic levels and decrease as endosomes mature. Further, we find that lysosomal Na+ levels in nematodes are modulated by the Na+/H+ exchanger NHX-5 in response to salt stress. The ability to image subcellular Na+ will unveil mechanisms of Na+ homeostasis at an increased level of cellular detail.

A DNA-based voltmeter for organelles.

The role of membrane potential in most intracellular organelles remains unexplored because of the lack of suitable tools. Here, we describe Voltair, a fluorescent DNA nanodevice that reports the absolute membrane potential and can be targeted to organelles in live cells. Voltair consists of a voltage-sensitive fluorophore and a reference fluorophore for ratiometry, and acts as an endocytic tracer. Using Voltair, we could measure the membrane potential of different organelles in situ in live cells. Voltair can potentially guide the rational design of biocompatible electronics and enhance our understanding of how membrane potential regulates organelle biology.

Carbon-carbon bond cleavage in fluorescent pyronin analogues induced by yellow light.

A novel class of pyronin analogues, which undergoes a photochemically induced cleavage of the C-C bond in the presence of water in both solution and on a silica gel surface upon direct irradiation with visible light, is reported. The reaction course can be monitored by characteristic fluorescence of both the starting compound and the final product. This system could find useful applications in the field of photoremovable protecting groups or caged fluorophores.

Photochemistry of S-phenacyl xanthates.

Various synthetically readily accessible S-phenacyl xanthates are shown to undergo photoinitiated homolytic scission of the C-S bond in the primary step. The resultant fragments, phenacyl and xanthic acid radicals, recombine to form symmetrical 1,4-diketones and xanthogen disulfides, respectively, in high to moderate chemical yields in chemically inert solvents. They can also be efficiently trapped by a hydrogen-atom-donating solvent to give acetophenone and xanthic acid derivatives. The latter compound is in situ thermally converted to the corresponding alcohol in high chemical yields. S-Phenacyl xanthates could thus be utilized as synthetic precursors to the above-mentioned compounds or as photoremovable protecting groups for alcohols in which the xanthate moiety represents a photolabile linker. The photochemically released phenacyl radical fragments efficiently but reversibly add to the thiocarbonyl group of the parent xanthate molecule. The kinetics of this degenerative reversible addition-fragmentation transfer (RAFT)/macromolecular design via the interchange of xanthates (MADIX) mechanism was studied using laser flash photolysis (LFP) and density functional theory (DFT) calculations. The rate constants of the RAFT addition step, k(add) ∼ 7 × 10(8) M(-1) s(-1), and phenacyl radical addition to a double bond of 1,1-diphenylethylene, k(add) ∼ 10(8) M(-1) s(-1), in acetonitrile were experimentally determined by LFP. In addition, photoinitiation of the methyl methacrylate polymerization by S-phenacyl xanthate is demonstrated. The polydispersity index of the resulting poly(methyl methacrylate) was found to be ∼1.4. We conclude that S-phenacyl xanthates can serve simultaneously as photoinitiators as well as RAFT/MADIX agents in polymerization reactions.

Photochemical synthesis of substituted indan-1-ones related to donepezil.

A photoenolization reaction is shown to be the key reaction step in the preparation of substituted indan-1-ones as convenient precursors for the synthesis of donepezil, a well-known acetylcholinesterase inhibitor. Model 2,5-dialkylphenacyl chlorides, differently substituted in the alpha-carbon position, were found to produce indan-1-ones upon irradiation in non-nucleophilic solvents in high chemical yields via hydrogen chloride release. While direct excitation of 4,5-dimethoxy-2-methylphenacyl chloride led to a complex mixture of photoproducts, photolysis of the corresponding benzoate was found to form 5,6-dimethoxyindan-1-one in 62-72% chemical yields and a relatively low quantum efficiency (Phi approximately 0.02). This compound can then be easily converted to donepezil by standard synthetic steps described in the literature. Isotopic exchange and quenching experiments revealed that the product is obtained by the photoenolization process via the triplet excited state, while minor side-photoproducts originate from the singlet excited state. Irradiation of the reactant in neat acetone, used both as a triplet sensitizer and solvent at the same time, was found to form 5,6-dimethoxyindan-1-one exclusively in high (90%) chemical yield.