Jessie A. Key

Fluorescent small-molecule probes of biochemistry at the plasma membrane.

The function of cellular membranes remains a critical area of study. Advances in molecular biology and biochemistry have helped to define the plasma membrane as a dynamic and heterogeneous structure. Probes capable of resolving molecular interactions and biochemical changes within the membrane are becoming a necessary tool for cell biology studies. We review current examples that apply small molecule fluorophores to label either lipids or proteins to study the plasma membrane. We discuss probes of the lipid environment itself, as well as labeling strategies for membrane proteins and membrane receptors.

Detection of Cellular Sialic Acid Content Using Nitrobenzoxadiazole Carbonyl-Reactive Chromophores

The selective ligation of hydrazine and amino-oxy compounds with carbonyls has gained popularity as a detection strategy with the recognition of aniline catalysis as a way to accelerate the labeling reaction in water. Aldehydes are a convenient functional group choice since there are few native aldehydes found at the cell surface. Aldehydes can be selectively introduced into sialic acid containing glycoproteins by treatment with dilute sodium periodate. Thus, the combination of periodate oxidation with aniline-catalyzed ligation (PAL) has become a viable method for detection of glycoconjugates on live cells. Herein we examine two fluorescent nitrobenzoxadiazole dyes for labeling of glycoproteins and cell surface glycoconjugates. We introduce a novel 4-aminooxy-7-nitro-benz-[2,1,3-d]-oxadiazole (NBDAO) (5) fluorophore, and offer a comparison to commercial dyes including the known 4-hydrazino-7-nitro-benz-[2,1,3-d]-oxadiazole (NBDH) (2) and Bodipy FL hydrazide. We confirm specificity for sialic acid moieties and that both dyes are suitable for in vitro and in vivo labeling studies using PAL and fluorescence spectroscopy. The dyes examined here are attractive labeling agents for microscopy, as they can be excited by a 488 nm laser line and can be made in a few synthetic steps. These carbonyl-reactive chromophores provide a one step alternative to avidin-biotin labeling strategies and simplify the detection of sialic acid in cells and glycoproteins.

A Modular Synthesis of Alkynyl-Phosphocholine Headgroups for Labeling Sphingomyelin and Phosphatidylcholine

A general route to phospho- and sphingolipids that incorporate an alkyne in the phosphocholine headgroup is described. The strategy preserves the ammonium functionality of the phosphocholine and can be easily modified to introduce desired functional groups at the N-acyl chain. The targets accessible with this strategy provide a bioorthogonal handle for postsynthetic introduction of fluorophores or other labeling agents with aqueous phase chemistry. We report the synthesis of sphingomyelin derivatives that incorporate a fluorophore and an alkyne. The modified sphingolipids retain activity as substrates for sphingomyelinase, making these compounds viable probes of enzymatic activity. Importantly, the strategy allows modification of the lipid across the phosphodiester, making the alkyne a potential probe of sphingomyelinase activity.

Practical Labeling Methodology for Choline-Derived Lipids and Applications in Live Cell Fluorescence Imaging

Lipids of the plasma membrane participate in a variety of biological processes, and methods to probe their function and cellular location are essential to understanding biochemical mechanisms. Previous reports have established that phosphocholine‐containing lipids can be labeled by alkyne groups through metabolic incorporation. Herein, we have tested alkyne, azide and ketone‐containing derivatives of choline as metabolic labels of choline‐containing lipids in cells. We also show that 17‐octadecynoic acid can be used as a complementary metabolic label for lipid acyl chains. We provide methods for the synthesis of cyanine‐based dyes that are reactive with alkyne, azide and ketone metabolic labels. Using an improved method for fluorophore conjugation to azide or alkyne‐modified lipids by Cu(I)‐catalyzed azide‐alkyne cycloaddition (CuAAC), we apply this methodology in cells. Lipid‐labeled cell membranes were then interrogated using flow cytometry and fluorescence microscopy. Furthermore, we explored the utility of this labeling strategy for use in live cell experiments. We demonstrate measurements of lipid dynamics (lateral mobility) by fluorescence photobleaching recovery (FPR). In addition, we show that adhesion of cells to specific surfaces can be accomplished by chemically linking membrane lipids to a functionalized surface. The strategies described provide robust methods for introducing bioorthogonal labels into native lipids.

Substituted benzoxadiazoles as fluorogenic probes: a computational study of absorption and fluorescence.

General chemical strategies which provide controlled changes in the emission or absorption properties of biologically compatible fluorophores remain elusive. One strategy employed is the conversion of a fluorophore-attached alkyne (or azide) to a triazole through a copper-catalyzed azide-alkyne coupling (CuAAC) reaction. In this study, we have computationally examined a series of structurally related 2,1,3-benzoxadiazole (benzofurazan) fluorophores and evaluated changes in their photophysical properties upon conversion from alkyne (or azide) to triazole forms. We have also determined the photophysical properties for a known set of benzoxadiazole compounds. The absorption and emission energies have been determined computationally using time-dependent density functional theory (TD-DFT) with the Perdew, Burke, and Ernzerhof exchange-correlation density functional (PBE0) and the 6-31+G(d) basis set. The TD-DFT results consistently agreed with the experimentally determined absorption and emission wavelengths except for certain compounds where charge-transfer excited states occurred. In addition to determining the absorption and emission wavelengths, simple methods for predicting relative quantum yields previously derived from semiempirical calculations were reevaluated on the basis of the new TD-DFT results and shown to be deficient. These results provide a necessary framework for the design of new substituted benzoxadiazole fluorophores.

7,7'-(3,3'-Dibenzyl-3H,3'H-4,4'-bi-1,2,3-triazole-5,5'-di-yl)bis-(4-methyl-2H-chromen-2-one).

The title compound, a bis-5,5′-triazole, C38H28N6O4, was observed as a side-product from the Sharpless–Meldal click reaction of the corresponding coumarin alkyne and benzylazide. Although the compound was present as a minor component, it crystallized in preference to the major product. The two triazole rings are almost orthogonal to each other [dihedral angle = 83.8 (1)°]. However the 4 and 4′ coumarin systems are close to coplanar with their respective triazole rings [23.6 (1) and 15.1 (1)°]. Each of the benzene rings packs approximately face-to-face with the opposing coumarin ring systems, with interplanar angles of 7.7 (1) and 25.3 (1)° and distances of 3.567 (2) and 3.929 (2) Å between the respective centroids of the opposing rings.

1-[1-(2,1,3-Benzoxadiazol-5-ylmeth-yl)-1H-1,2,3-triazol-4-yl]hexan-1-one.

The title compound, C15H17N5O2, was synthesized as part of a series of benzoxadiazole analogs which were examined for fluorescent properties by Cu-catalysed azide–alkyne cycloaddition (CuAAC) of a 4-azidomethyl-benzoxadiazole substrate. The structure shows a nearly coplanar orientation of the hexanone keto group and the 1,2,3-triazole ring [dihedral angle = 4.3 (3)°], while the benzoxadiazole and triazole groups are much more severely inclined [dihedral angle = 70.87 (4)°]. In the crystal, weak C—H⋯N interactions connect translationally-related triazole rings, while another set of C—H⋯N interactions is formed between inversion-related benzoxadiazole units, forming a three-dimensional network. The crystal studied was a non-merohedral twin with refined value of the twin fraction of 0.2289 (16).

5-(4-Hexyl-1H-1,2,3-triazol-1-yl)-2,1,3-benzoxadiazole

The title compound, C14H17N5O, a 1,2,3-triazole derivative of benzoxadiazole (C14H17N5O), was synthesized via Cu-catalysed azide–alkyne cycloaddition (CuAAC) from the corresponding n-octyne and 4-azidobenzoxadiazole. The benzoxadiazole and triazole rings show a roughly planar orientation [dihedral angle between the ring planes = 12.18 (5)°]. The alkane chain adopts a zigzag conformation, which deviates from the central triazole ring by 20.89 (6)°. These two torsion angles result in an overall twist to the structure, with a dihedral angle of 32.86 (7)° between the benzoxadiazole group and the hexyl chain. The crystal structure features C—H⋯N hydrogen bonds leading to chains propagating along [2-10] and offset parallel stacking interactions of the triazole and benzoxadiazole rings. The centroid of the extended π-system formed by the benzoxadiazole and triazole rings (14 atoms total) was calculated; the centroid–centroid distance was 4.179 Å, interplanar separation was 3.243 Å, and the resulting offset was 2.636 Å.

5-(1-Benzyl-1H-1,2,3-triazol-4-yl)-2,1,3-benzoxadiazole

In the title compound, C15H11N5O, which was prepared as part of a study to identify fluorogenic substrates for the Cu-catalysed azide–alkyne cycloaddition (CuAAC) reaction, the benzoxadiazole unit and the triazole ring are much more closely coplanar [dihedral angle = 10.92 (7)°] than either is to the benzyl group [dihedral angles = 69.13 (3)° and 78.20 (4)°, respectively]. The crystal structure features two different sets of weak intermolecular C—H⋯N interactions between adjacent benzoxadiazole and triazole rings, forming a chain that propagates in the [-110] direction parallel to the ab plane.