Robert M. Strongin

NIR dyes for bioimaging applications

Fluorescent dyes based on small organic molecules that function in the near infrared (NIR) region are of great current interest in chemical biology. They allow for imaging with minimal autofluorescence from biological samples, reduced light scattering, and high tissue penetration. Herein, examples of ongoing NIR fluorophore design strategies as well as their properties and anticipated applications relevant to the bioimaging are presented.

Conjugate Addition/Cyclization Sequence Enables Selective and Simultaneous Fluorescence Detection of Cysteine and Homocysteine

Biological thiols are essential for maintaining the appropriate redox status of proteins, cells, and organisms.[1] Cysteine (Cys) is an essential amino acid that is involved in protein synthesis, detoxification, and metabolism. Elevated levels of Cys have been associated with neurotoxicity,[2] and Cys deficiency is involved in slowed growth rate, hair depigmentation, edema, lethargy, liver damage, muscle and fat loss, skin lesions, and weakness.[3] Homocysteine (Hcy) has been implicated in various types of vascular and renal diseases. Elevated Hcy in the blood (>12 µm) is a well-known risk factor for cardiovascular[4a] and Alzheimer’s disease,[4b] neutral tube defects, complications during pregnancy, inflammatory bowel disease, and osteoporosis.[4c] Because Cys and Hcy levels are associated with different diseases despite their similar structures, their discrimination is necessary.

Hidden Formaldehyde in E-Cigarette Aerosols

This letter reports a chemical analysis of vapor from electronic cigarettes that shows high levels of formaldehyde, a known carcinogen. The authors project that the associated incremental lifetime risk of cancer could be higher than that from long-term smoking.

Thiol Reactive Probes and Chemosensors

Thiols are important molecules in the environment and in biological processes. Cysteine (Cys), homocysteine (Hcy), glutathione (GSH) and hydrogen sulfide (H2S) play critical roles in a variety of physiological and pathological processes. The selective detection of thiols using reaction-based probes and sensors is very important in basic research and in disease diagnosis. This review focuses on the design of fluorescent and colorimetric probes and sensors for thiol detection. Thiol detection methods include probes and labeling agents based on nucleophilic addition and substitution, Michael addition, disulfide bond or Se-N bond cleavage, metal-sulfur interactions and more. Probes for H2S are based on nucleophilic cyclization, reduction and metal sulfide formation. Thiol probe and chemosensor design strategies and mechanism of action are discussed in this review.

Supramolecular Interactions in Chemomechanical Polymers

Molecular recognition is the basis for the operation of most biological functions; outside of nature, it has also been developed to a high degree of sophistication within the framework of supramolecular chemistry. More recently, selective noncovalent interactions, which constitute molecular recognition, are being used in intelligent new materials that transform chemical signals into actions, such as the release of drugs. The presence of supramolecular binding sites allows chemomechanical polymers to operate as sensors and actuators within a single unit without the need for any additional devices such as transducers or power supplies. A polymer can be designed so that a particular chemical substance, most often in aqueous surroundings, will trigger either a large expansion or a large contraction, depending on the mechanism. The translation of binding energy into mechanical motion can, with a suitable arrangement of the materials in tubes or on flexible films, be harnessed for unidirectional drives, flow control, the liberation of drugs, or the uptake of toxic compounds, among other applications. Miniaturization of the polymer particles allows one to enhance both the sensitivity and speed of the response, which is of particular importance in sensing. The basis for the selective response to external effector compounds, such as metal ions, amino acids, peptides, or nucleotides, is their noncovalent interaction with complementary functions covalently bound to the polymer network. With suitable polymers, selectivity between structural isomers, and even between enantiomers, as triggers can be achieved. As with supramolecular complexes in solution, the underlying interactions in polymers comprise a variety of noncovalent binding mechanisms, which are not easy to distinguish and quantify, and more so with polymers that are not monodisperse. In this Account, we present systematic comparisons of different polymers and effector classes that allow, for the first time, the characterization of these contributions in chemomechanical polymers: they comprise ion pairing, metal coordination, stacking, cation-pi, dispersive, and hydrophobic forces. In contrast, hydrogen bonding has a major role primarily in the hydrogel network structure itself. The fully reversible polymer volume changes are essentially determined by water uptake or release. In gels derived from boronic acid, glucose can serve as a cross-linking effector in promoting contractions via strong, reversible covalent bond formation in a highly distinctive manner. Cooperativity between two different effector compounds is more frequently seen with such polymers than in solution: it leads to logical AND gates by different motions of the particles, with a direct communication link to the outside world. For example, with a polymer that bears several recognition sites, triggering peptides induce motion only if Zn(2+) or Cu(2+) ions are simultaneously present. The molecular recognition mechanisms that cause volume changes in polymers share similarities with extensively studied supramolecular systems in solution, but there are also remarkable differences. In this Account, we bring the knowledge learned from solution studies to bear on our systematic analysis of polymeric systems in an effort to promote the effective harnessing of the forces involved in chemomechanical polymers and the smart materials that can be created with them.

Selective fluorescence detection of cysteine and N-terminal cysteine peptide residues.

A new fluorogenic fluorescein derivative containing an alpha,beta-unsaturated aldehyde moiety produces a selective fluorescent signal enhancement in the presence of cysteine or peptides containing N-terminal cysteine residues. The mechanism is based on synergistic covalent and supramolecular interactions.

Homocystamides promote free-radical and oxidative damage to proteins

Elevated levels of homocysteine are associated with several major diseases. However, it is not clear whether homocysteine is a marker or a causative agent. The majority (ca. 80%) of the homocysteine present in humans is protein bound. The study of the posttranslational modification of proteins by homocysteine and its cyclic congener, homocysteine thiolactone, is emerging as an area of great current interest for unraveling the ongoing “mediator/marker controversy” [Jacobsen DW (2009) Clin Chem 55:1–2]. Interestingly, many of the pathologies associated with homocysteine are also linked to oxidative stress. In the current study, chemical evidence for a causal relationship between homocysteine-bound proteins and oxidative damage is presented. For example, a reproducible increase in protein carbonyl functionality occurs as a consequence of the reaction of human serum albumin with homocysteine thiolactone. This occurs at physiological temperature upon exposure to air without any added oxidants or free-radical initiators. Alpha-amino acid carbon-centered radicals, well-known precursors of protein carbonyls, are shown to form via a hydrogen atom transfer process involving thiolactone-derived homocystamides. Model peptides in buffer as well as native proteins in human blood plasma additionally exhibit properties in keeping with the homocystamide-facilitated hydrogen atom transfer and resultant carbon-centered radicals.

Seminaphthofluorones are a family of water-soluble, low molecular weight, NIR-emitting fluorophores

A readily accessible new class of near infrared (NIR) molecular probes has been synthesized and evaluated. Specific fluorophores in this unique xanthene based regioisomeric seminaphthofluorone dye series exhibit a combination of desirable characteristics including (i) low molecular weight (339 amu), (ii) aqueous solubility, and (iii) dual excitation and emission from their fluorescent neutral and anionic forms. Importantly, systematic changes in the regiochemistry of benzannulation and the ionizable moieties afford (iv) tunable deep-red to NIR emission from anionic species and (v) enhanced Stokes shifts. Anionic SNAFR-6, exhibiting an unusually large Stokes shift of ≈200 nm (5,014 cm−1) in aqueous buffer, embodies an unprecedented fluorophore that emits NIR fluorescence when excited in the blue/green wavelength region. The successful use of SNAFR-6 in cellular imaging studies demonstrates proof-of-concept that this class of dyes possesses photophysical characteristics that allow their use in practical applications. Notably, each of the new fluorophores described is a minimal template structure for evaluation of their basic spectral properties, which may be further functionalized and optimized yielding concomitant improvements in their photophysical properties.

Tröger's Base Derivatives—New Life for Old Compounds

Synthetic Trögers base derivatives are reviewed, including their properties and applications. The rigid V-shaped Trögers base framework and its inherent chirality have promoted the preparation of diverse receptor systems. Heterocyclic Trögers base derivatives exhibit affinity for DNA. Metal complexes of Trögers base are used as catalysts. New applications continue to emerge as synthetic methods are developed.

A Fast Response Highly Selective Probe for the Detection of Glutathione in Human Blood Plasma

A fluorescent probe for glutathione (GSH) detection was developed. Our study indicates a possible mechanism which couples a conjugate addition and micelle-catalyzed large membered ring formation/elimination sequence. This method enables excellent selectivity towards GSH over other biological thiols such as cysteine (Cys) and homocysteine (Hcy). The proposed method is precise with a relative standard deviation (R.S.D) lower than 6% (n = 3) and has been successfully applied to determine GSH in human plasma with recoveries between 99.2% and 102.3%.