Paramagnetic fluorinated nanoemulsions for sensitive cellular fluorine-19 magnetic resonance imaging.

Fluorine-19 magnetic resonance imaging ((19)F MRI) probes enable quantitative in vivo detection of cell therapies and inflammatory cells. Here, we describe the formulation of perfluorocarbon-based nanoemulsions with improved sensitivity for cellular MRI. Reduction of the (19)F spin-lattice relaxation time (T1) enables rapid imaging and an improved signal-to-noise ratio, thereby improving cell detection sensitivity. We synthesized metal-binding ß-diketones conjugated to linear perfluoropolyether (PFPE), formulated these fluorinated ligands as aqueous nanoemulsions, and then metallated them with various transition and lanthanide ions in the fluorous phase. Iron(III) tris-ß-diketonate ('FETRIS') nanoemulsions with PFPE have low cytotoxicity (<20%) and superior MRI properties. Moreover, the (19)F T1 can readily be reduced by an order of magnitude and tuned by stoichiometric modulation of the iron concentration. The resulting (19)F MRI detection sensitivity is enhanced by three- to fivefold over previously used tracers at 11.7 T, and is predicted to increase by at least eightfold at the clinical field strength of 3 T.

Relative performance of alkynes in copper-catalyzed azide-alkyne cycloaddition.

Copper-catalyzed azide-alkyne cycloaddition (CuAAC) has found numerous applications in a variety of fields. We report here only modest differences in the reactivity of various classes of terminal alkynes under typical bioconjugative and preparative organic conditions. Propargyl compounds represent an excellent combination of azide reactivity, ease of installation, and cost. Electronically activated propiolamides are slightly more reactive, at the expense of increased propensity for Michael addition. Certain alkynes, including tertiary propargyl carbamates, are not suitable for bioconjugation due to copper-induced fragmentation. A fluorogenic probe based on such reactivity is available in one step from rhodamine 110 and can be useful for optimization of CuAAC conditions.

Solvent-Free Synthesis and Fluorescence of a Thiol-Reactive Sensor for Undergraduate Organic Laboratories.

A green organic laboratory experiment was developed in which students synthesize a sensor for thiols using a microscale, solventless Diels-Alder reaction at room temperature or 37 °C. The molecular probe is easily purified by column chromatography in a Pasteur pipet and characterized by thin-layer chromatography and NMR spectroscopy. The thiol-reactive sensor becomes intensely fluorescent upon exposure to thiols from N-acetylcysteine, bovine serum albumin, or human hair (pretreated with a reducing agent to reveal cysteine thiols in α-keratin). This fluorescence is observable even with micrograms of probe.

Engineered mutations change the structure and stability of a virus-like particle.

The single-coat protein (CP) of bacteriophage Qß self-assembles into T = 3 icosahedral virus-like particles (VLPs), of interest for a wide range of applications. These VLPs are very stable, but identification of the specific molecular determinants of this stability is lacking. To investigate these determinants along with manipulations that confer more capabilities to our VLP material, we manipulated the CP primary structure to test the importance of various putative stabilizing interactions. Optimization of a procedure to incorporate fused CP subunits allowed for good control over the average number of covalent dimers in each VLP. We confirmed that the disulfide linkages are the most important stabilizing elements for the capsid and that acidic conditions significantly enhance the resistance of VLPs to thermal degradation. Interdimer interactions were found to be less important for VLP assembly than intradimer interactions. Finally, a single point mutation in the CP resulted in a population of smaller VLPs in three distinct structural forms.

Degradable conjugates from oxanorbornadiene reagents.

Oxanorbornadienedicarboxylate (OND) reagents were explored for purposes of binding and releasing drugs from serum albumins as representative macromolecular carriers. Being highly reactive Michael acceptors, ONDs form adducts with thiols and amines, which then undergo retro-Diels-Alder fragmentation. A study of more than 30 model adducts revealed a number of modifications that can be used to influence adduct stability. For the most reactive OND linkers, the labeling of the single available bovine serum albumin (BSA) cysteine residue was complete within minutes at a mid-micromolar concentration of reactants. While a selectivity of greater than 1000-fold for thiol over amine was observed with model amino acids, the labeling of protein amines with ONDs is fast enough to be practical, as demonstrated by the reaction with thiol-depleted BSA. The OND-amine adducts were found to be up to 15 times more stable than OND-thiol adducts, and to be sensitive to acid by virtue of a stereochemically dependent acceleration of cycloreversion. The release rate of fluorescent cargo from serum albumins was tuned by selecting the coupling partners: the available half-lives ranged from 40 min to 7 days at 37 °C. Such versatility of release profiles from protein carriers, controlled by the nature of the OND linkage, is a useful addition to the drug delivery toolbox.

Aqueous-phase deactivation and intramolecular [2 + 2 + 2] cycloaddition of oxanorbornadiene esters.

Both inter- and intramolecular degradation pathways were identified for the aqueous phase deactivation of oxanorbornadiene (OND) electrophiles, and propargylic OND esters were found to undergo facile intramolecular [2 + 2 + 2] homo-Diels-Alder cycloaddition in polar media.

Thiol-selective fluorogenic probes for labeling and release.

Thiol alkylation is a powerful technique for the labeling of proteins. We report a new class of highly reactive, selective, and fluorogenic probes for thiols in aqueous solution at neutral pH, based on the 7-oxanorbornadiene (OND) framework. The maleate moiety in 7-oxabicyclo[2.2.1]hept-2,5-diene-2,3-dicarboxylic acid esters serves as both a tunable electrophile and an intramolecular quencher of an attached dansyl fluorophore. Thiols have been found to add with high rates (second-order rate constants of 40-200 M(-1) s(-1)) to give adducts that exhibit enhancements of fluorescence intensity up to 180-fold. The resulting adducts are also versatile with respect to cleavage (release) reactions by two mechanisms. First, retro-Diels-Alder fragmentation occurs with half-lives from days to weeks at room temperature, and an epoxide derivative is also reported that is incapable of cycloreversion cleavage. Second, monoamide OND derivatives undergo rapid closure to succinimides upon thiol addition, providing a thiol-triggered mechanism for immediate alcohol release. Peptides and proteins containing free thiol groups were labeled with OND electrophiles with high chemoselectivity. Since the system is so easily assembled from readily accessible modules, various functional groups can be added to OND linkers to allow the attachment of other molecules of interest.

Synthesis and properties of the red chromophore of the green-to-red photoconvertible fluorescent protein Kaede and its analogs.

Green fluorescent protein (GFP) and homologous proteins possess a unique pathway of chromophore formation based on autocatalytic modification of their own amino acid residues. Green-to-red photoconvertible fluorescent protein Kaede carries His-Tyr-Gly chromophore-forming triad. Here, we describe synthesis of Kaede red chromophore (2-[(1E)-2-(5-imidazolyl)ethenyl]-4-(p-hydroxybenzylidene)-5-imidazolone) and its analogs that can be potentially formed by natural amino acid residues. Chromophores corresponding to the following tripeptides were obtained: His-Tyr-Gly, Trp-Tyr-Gly, Phe-Trp-Gly, Tyr-Trp-Gly, Asn-Tyr-Gly, Phe-Tyr-Gly, and Tyr-Tyr-Gly. In basic conditions they fluoresced red with relatively high quantum yield (up to 0.017 for Trp-derived compounds). The most red-shifted absorption peak at 595nm was found for the chromophore Trp-Tyr-Gly in basic DMSO. Surprisingly, in basic DMF non-aromatic Asn-derived chromophore Asn-Tyr-Gly demonstrated the most red-shifted emission maximum at 642 nm. Thus, Asn residue may be a promising substituent, which can potentially diversify posttranslational chemistry in GFP-like proteins.