Design and synthesis of new cryptophanes with intermediate cavity sizes

Porous Organic Cages

Porous organic cages (POCs) are a relatively new class of low-density crystalline materials that have emerged as a versatile platform for investigating molecular recognition, gas storage and separation, and proton conduction, with potential applications in the fields of porous liquids, highly permeable membranes, heterogeneous catalysis, and microreactors. In common with highly extended porous structures, such as metal-organic frameworks (MOFs), covalent organic frameworks (COFs), and porous organic polymers (POPs), POCs possess all of the advantages of highly specific surface areas, porosities, open pore channels, and tunable structures. In addition, they have discrete molecular structures and exhibit good to excellent solubilities in common solvents, enabling their solution dispersibility and processability─properties that are not readily available in the case of the well-established, insoluble, extended porous frameworks. Here, we present a critical review summarizing in detail recent progress and breakthroughs─especially during the past five years─of all the POCs while taking a close look at their strategic design, precise synthesis, including both irreversible bond-forming chemistry and dynamic covalent chemistry, advanced characterization, and diverse applications. We highlight representative POC examples in an attempt to gain some understanding of their structure-function relationships. We also discuss future challenges and opportunities in the design, synthesis, characterization, and application of POCs. We anticipate that this review will be useful to researchers working in this field when it comes to designing and developing new POCs with desired functions.

Genetically encoded reporters for hyperpolarized xenon magnetic resonance imaging

Magnetic resonance imaging (MRI) enables high-resolution non-invasive observation of the anatomy and function of intact organisms. However, previous MRI reporters of key biological processes tied to gene expression have been limited by the inherently low molecular sensitivity of conventional (1)H MRI. This limitation could be overcome through the use of hyperpolarized nuclei, such as in the noble gas xenon, but previous reporters acting on such nuclei have been synthetic. Here, we introduce the first genetically encoded reporters for hyperpolarized (129)Xe MRI. These expressible reporters are based on gas vesicles (GVs), gas-binding protein nanostructures expressed by certain buoyant microorganisms. We show that GVs are capable of chemical exchange saturation transfer interactions with xenon, which enables chemically amplified GV detection at picomolar concentrations (a 100- to 10,000-fold improvement over comparable constructs for (1)H MRI). We demonstrate the use of GVs as heterologously expressed indicators of gene expression and chemically targeted exogenous labels in MRI experiments performed on living cells.

Synthesis and biological evaluation of ¹⁸F labeled fluoro-oligo-ethoxylated 4-benzylpiperazine derivatives for sigma-1 receptor imaging

We report the synthesis and evaluation of a series of fluoro-oligo-ethoxylated 4-benzylpiperazine derivatives as potential σ(1) receptor ligands. In vitro competition binding assays showed that 1-(1,3-benzodioxol-5-ylmethyl)-4-(4-(2-fluoroethoxy)benzyl)piperazine (6) exhibits low nanomolar affinity for σ(1) receptors (K(i)=1.85 ± 1.59 nM) and high subtype selectivity (σ(2) receptor: K(i)=291 ± 111 nM; K(i)σ(2)/K(i)σ(1)=157). [(18)F]6 was prepared in 30-50% isolated radiochemical yield, with radiochemical purity of >99% by HPLC analysis after purification, via nucleophilic (18)F(-) substitution of the corresponding tosylate precursor. The logD(pH 7.4) value of [(18)F]6 was found to be 2.57 ± 0.10, which is within the range expected to give high brain uptake. Biodistribution studies in mice demonstrated relatively high concentration of radiotracers in organs known to contain σ(1) receptors, including the brain, lungs, kidneys, heart, and spleen. Administration of haloperidol 5 min prior to injection of [(18)F]6 significantly reduced the concentration of radiotracers in the above-mentioned organs. The accumulation of radiotracers in the bone was quite low suggesting that [(18)F]6 is relatively stable to in vivo defluorination. The ex vivo autoradiography in rat brain showed high accumulation of radiotracers in the brain areas known to possess high expression of σ(1) receptors. These findings suggest that [(18)F]6 is a suitable radiotracer for imaging σ(1) receptors with PET in vivo.