Postcolumn HPLC detection of mono- and oligosaccharides with a chemosensor

Live imaging of the extracellular matrix with a glycan-binding fluorophore

All multicellular systems produce and dynamically regulate extracellular matrices (ECMs) that play essential roles in both biochemical and mechanical signaling. Though the spatial arrangement of these extracellular assemblies is critical to their biological functions, visualization of ECM structure is challenging, in part because the biomolecules that compose the ECM are difficult to fluorescently label individually and collectively. Here, we present a cell-impermeable small-molecule fluorophore, termed Rhobo6, that turns on and red shifts upon reversible binding to glycans. Given that most ECM components are densely glycosylated, the dye enables wash-free visualization of ECM, in systems ranging from in vitro substrates to in vivo mouse mammary tumors. Relative to existing techniques, Rhobo6 provides a broad substrate profile, superior tissue penetration, non-perturbative labeling, and negligible photobleaching. This work establishes a straightforward method for imaging the distribution of ECM in live tissues and organisms, lowering barriers for investigation of extracellular biology.

A melanin-like polymer bearing phenylboronic units as a suitable bioplatform for living cell display technology

Surface display of functional groups with specific reactivity around living cells is an emerging, low cost and highly eco-compatible technology that serves multiple applications, ranging from basic biochemical studies to biomedicine, therapeutics and environmental sciences. Conversely to classical methods exploiting hazardous organic synthesis of precursors or monovalent functionalization via genetics, here we perform functional decoration of individual living microalgae using suitable biocoatings based on polydopamine, a melanin-like synthetic polymer. Here we demonstrate the one-pot synthesis of a functional polydopamine bearing phenylboronic units which can decorate the living cell surfaces via a direct ester formation between boronic units and surface glycoproteins. Furthermore, biosorption of fluorescent sugars on functionalized cell membranes is triggered, demonstrating that these organic coatings act as biocompatible soft shells, still functional and reactive after cell engineering.

Imaging the extracellular matrix in live tissues and organisms with a glycan-binding fluorophore

All multicellular systems produce and dynamically regulate extracellular matrices (ECM) that play important roles in both biochemical and mechanical signaling. Though the spatial arrangement of these extracellular assemblies is critical to their biological functions, visualization of ECM structure is challenging, in part because the biomolecules that compose the ECM are difficult to fluorescently label individually and collectively. Here, we present a cell-impermeable small molecule fluorophore, termed Rhobo6, that turns on and red shifts upon reversible binding to glycans. Given that most ECM components are densely glycosylated, the dye enables wash-free visualization of ECM, in systems ranging from in vitro substrates to in vivo mouse mammary tumors. Relative to existing techniques, Rhobo6 provides a broad substrate profile, superior tissue penetration, nonperturbative labeling, and negligible photobleaching. This work establishes a straightforward method for imaging the distribution of ECM in live tissues and organisms, lowering barriers for investigation of extracellular biology.

High performance liquid chromatographic method with post-column detection for quantification of reducing sugars in foods

A novel liquid chromatographic analysis method with post-column detection for sugars was developed to improve existing methods in regard to operation time, selectivity, and sensitivity. This method involves separation of reducing sugars on HPLC column at 30 °C and 0.8 mL min^-1 flow rate, post-column reaction of sugars with Cu(II)-neocuproine (Nc) reagent at 80 °C and 0.3 mL min^-1 flow rate, and measurement of Cu(I)-Nc product at 450 nm. The proposed assay was applied to glucose, fructose, maltose, and lactose as reducing sugars. Non-reducing sucrose was determined indirectly, after conversion to its constitutive monomers glucose and fructose by hydrolysis, and analysis with a relative error from -2.41 to 2.09%. Honey, apple juice, and milk samples were evaluated as commercial products. The results obtained with the proposed assay compared to those of the alkaline Cu(II)-Nc reference method were found close to each other, and compatible with the label values of commercial products. The accuracy of the developed method was performed by spiking glucose to honey and lactose to milk samples using two different concentrations. The obtained recoveries with respect to the post-column HPLC method were between 97 and 105% for honey and 96-107% for milk. The method gave linear responses against sugar concentration with correlation coefficients greater than 0.996 for the four analytes (glucose, fructose, maltose and lactose) in a range of 9.0 - 342.3 mg L^-1 with LOD values ≤ 7.4 mg L^-1. With the developed method, it was possible to sensitively determine reducing sugars in various food samples at a lower temperature of post-column reaction (compared to literature values) with easy application of low cost reagents requiring minimal preliminary operation.

Multiwell Assay for the Analysis of Sugar Gut Permeability Markers: Discrimination of Sugar Alcohols with a Fluorescent Probe Array Based on Boronic Acid Appended Viologens

With the aim of discerning between different sugar and sugar alcohols of biomedical relevance, such as gut permeability, arrays of 2-component probes were assembled with up to six boronic acid-appended viologens (BBVs): 4,4'-o-BBV, 3,3'-o-BBV, 3,4'-o-BBV, 4,4'-o,m-BBV, 4,7'-o-PBBV, and pBoB, each coupled to the fluorophore 8-hydroxypyrene, 1,3,6-trisulfonic acid trisodium salt (HPTS). These probes were screened for their ability to discriminate between lactulose, l-rhamnose, 3-O-methyl-d-glucose, and xylose. Binding studies of sugar alcohols mannitol, sorbitol, erythritol, adonitol, arabitol, galactitol, and xylitol revealed that diols containing threo-1,2-diol units have higher affinity for BBVs relative diols containing erythro-1,2 units. Those containing both threo-1,2- and 1,3-syn diol motifs showed high affinity for boronic acid binding. Fluorescence from the arrays were examined by principle component analysis (PCA) and linear discriminant analysis (LDA). Arrays with only three BBVs sufficed to discriminate between sugars (e.g., lactulose) and sugar alcohols (e.g., mannitol), establishing a differential probe. Compared with 4,4'-o-BBV, 2-fold reductions in lower limits of detection (LOD) and quantification (LOQ) were achieved for lactulose with 4,7-o-PBBV (LOD 41 μM, LOQ 72 μM). Using a combination of 4,4'-o-BBV, 4,7-o-PBBV, and pBoB, LDA statistically segregated lactulose/mannitol (L/M) ratios from 0.1 to 0.5, consistent with values encountered in small intestinal permeability tests. Another triad containing 3,3'-o-BBV, 4,4'-o-BBV, and 4,7-o-PBBV also discerned similar L/M ratios. This proof-of-concept demonstrates the potential for BBV arrays as an attractive alternate to HPLC to analyze mixtures of sugars and sugar alcohols in biomedical applications and sheds light on structural motifs that make this possible.

Boronic acids for fluorescence imaging of carbohydrates

Carbohydrate biomarkers are particularly important targets for fluorescence imaging given their pivotal role in numerous important biological events. This review highlights the development of fluorescence imaging agents based on boronic acids.

Advances in chemical labeling of proteins in living cells

The pursuit of quantitative biological information via imaging requires robust labeling approaches that can be used in multiple applications and with a variety of detectable colors and properties. In addition to conventional fluorescent proteins, chemists and biologists have come together to provide a range of approaches that combine dye chemistry with the convenience of genetic targeting. This hybrid-tagging approach amalgamates the rational design of properties available through synthetic dye chemistry with the robust biological targeting available with genetic encoding. In this review, we discuss the current range of approaches that have been exploited for dye targeting or for targeting and activation and some of the recent applications that are uniquely permitted by these hybrid-tagging approaches.

Progress toward red and near-infrared (NIR) emitting saccharide sensors

Red-shifted and near-infrared (NIR)-active rhodamine analogs and their boronic acid derivatives were synthesized and studied. These latter compounds function as fluorogenic NIR active substrates for sugar sensing. The effects of benzannulation and boronic acid functionalization on fluorophore optical and sensing properties are described.

Detecting specific saccharides via a single indicator

An improved synthesis of a rhodamine boronic acid indicator is reported. This compound is used in an optimized data collection protocol for wavelength- and time-dependent selectivity of sugars such as fructose and ribose derivatives. One indicator is thus used to selectively distinguish structurally related sugar analytes.

Selective recognition of protein tetraserine motifs with a cell-permeable, pro-fluorescent bis-boronic acid

There is considerable interest in novel cell imaging tools that avoid the use of fluorescent proteins. One widely used class of such reagents are "pro-fluorescent" biarsenical dyes such as FlAsH, ReAsH, CrAsH, and Cy3As. Despite their utility, biarsenicals are plagued by high background labeling and cytotoxicity and are challenging to apply in oxidizing cellular locale. Here we demonstrate that [(3-oxospiro[isobenzofuran-1(3H),9'-[9H]xanthene]-3',6'-diyl)bis(iminomethylene-2,1-phenylene)]bis-(9CI), a rhodamine-derived bisboronic acid (RhoBo) described initially as a monosaccharide sensor, functions as a cell-permeable, turn-on fluorescent sensor for tetraserine motifs in recombinant proteins. RhoBo binds peptides or proteins containing Ser-Ser-Pro-Gly-Ser-Ser with affinities in the nanomolar concentration range and prefers this sequence to simple monosaccharides by >10,000-fold. RhoBo fails to form fluorescent complexes with constituents of the mammalian cell surface, as judged by epifluorescent, confocal, and TIRF microscopy, but fluoresces brightly within the Ser-Ser-Pro-Gly-Ser-Ser-rich cell interior. These results suggest that current efforts to identify optimal serine-rich sequences for RhoBo will allow it to function effectively as a selective small-molecule label for appropriately tagged proteins either upon or within living cells.

Bioorthogonal chemistry: fishing for selectivity in a sea of functionality

The study of biomolecules in their native environments is a challenging task because of the vast complexity of cellular systems. Technologies developed in the last few years for the selective modification of biological species in living systems have yielded new insights into cellular processes. Key to these new techniques are bioorthogonal chemical reactions, whose components must react rapidly and selectively with each other under physiological conditions in the presence of the plethora of functionality necessary to sustain life. Herein we describe the bioorthogonal chemical reactions developed to date and how they can be used to study biomolecules.

Stereochemical and regiochemical trends in the selective detection of saccharides

Several discreet sugar-boronate complexes exist in solution. This is due to the complex equilibria between isomeric species of even the simplest monosaccharides. In the current investigation, we determine the regio- and stereochemical features of the various equilibrating sugar isomers that induce signal transduction in boronic acid chemosensors such as 1 as well as 2 and 3. We present a unique example of a chemosensor (1) that is selective for ribose, adenosine, nucleotides, nucleosides, and congeners. As a result of this study, we are able to predict and achieve selective fluorescence and colorimetric responses to specific disaccharides as a consequence of their terminal sugar residue linkage patterns and configurations. We also find that the combined use of chemosensors exhibiting complementary reactivities may be used cooperatively to obtain enhanced selectivity for ribose and rare saccharides.

Sialic acid: a novel marker of cardiovascular disease?

The global burden posed by cardiovascular disease (CVD), due to a rising incidence of known risk factors, underlines an urgent need to identify other potential risk factors. Sialic acid (SA), an abundant terminal monosaccharide of glycoconjugates, is a possible risk factor for CVD. Although large-scale epidemiological surveys have shown that serum total sialic acid (TSA) is positively associated with mortality from coronary artery disease (CAD) and stroke, studies investigating the correlation between serum TSA and the severity of atherosclerosis are conflicting. Clinical and epidemiological studies indicate that serum TSA is a marker of a sustained inflammatory response in CVD, rather than causal in nature. Data also indicates ethnic variation in baseline TSA. This article reviews current methods for determining serum TSA and evidence supporting serum TSA as a risk factor for CVD. Potential mechanisms for this role are examined. The use of serum TSA as a marker of atherosclerotic disease is evaluated.

Detection of homocysteine and cysteine

At elevated levels, homocysteine (Hcy, 1) is a risk factor for cardiovascular diseases, Alzheimer's disease, neural tube defects, and osteoporosis. Both 1 and cysteine (Cys, 3) are linked to neurotoxicity. The biochemical mechanisms by which 1 and 3 are involved in disease states are relatively unclear. Herein, we describe simple methods for detecting either Hcy or Cys in the visible spectral region with the highest selectivity reported to date without using biochemical techniques or preparative separations. Simple methods and readily available reagents allow for the detection of Cys and Hcy in the range of their physiologically relevant levels. New HPLC postcolumn detection methods for biological thiols are reported. The potential biomedical relevance of the chemical mechanisms involved in the detection of 1 is described.

Direct detection of homocysteine

Homocysteine (Hcy) is a biomarker for significant disorders including Alzheimer's and cardiovascular disease. The monitoring of Hcy levels in plasma is of current concern. We describe highly selective colorimetric methods for the direct detection of Hcy. Inexpensive, commercially available materials are employed. The results show potential application for the detection of Hcy in human blood plasma.