Cryptophane-A, comprised of two cyclotriguaiacylenes joined by three ethylene linkers, is a prototypal organic host molecule that binds reversibly to neutral small molecules via London forces. Of note are trifunctionalized, water-soluble cryptophane-A derivatives, which exhibit exceptional affinity for xenon in aqueous solution. In this paper, we report high-resolution X-ray structures of cryptophane-A and trifunctionalized derivatives in crown–crown and crown–saddle conformations, as well as in complexes with water, methanol, xenon or chloroform. Cryptophane internal volume varied by more than 20% across this series, which exemplifies 'induced fit' in a model host–guest system.

Peptide-modified cryptophane enables sensitive detection of protein analytes using hyperpolarized 129Xe NMR spectroscopy. Here we report improved targeting and delivery of cryptophane to cells expressing αvβ3 integrin receptor, which is overexpressed in many human cancers. Cryptophane was functionalized with cyclic RGDyK peptide and Alexa Fluor 488 dye, and cellular internalization was monitored by confocal laser scanning microscopy. Competitive blocking assays confirmed cryptophane endocytosis through an αvβ3 integrin receptor-mediated pathway. The peptide–cryptophane conjugate was determined to be nontoxic in normal human lung fibroblasts by MTT assay at the micromolar cryptophane concentrations typically used for hyperpolarized 129Xe NMR biosensing experiments. Flow cytometry revealed 4-fold higher cellular internalization in cancer cells overexpressing the integrin receptor compared to normal cells. Nanomolar inhibitory concentrations (IC50 = 20–30 nM) were measured for cryptophane biosensors against vitronectin binding to αvβ3 integrin and fibrinogen binding to αIIbβ3 integrin. Functionalization of the conjugate with two propionic acid groups improved water solubility for hyperpolarized 129Xe NMR spectroscopic studies, which revealed a single resonance at 67 ppm for the 129Xe-cryptophane–cyclic RGDyK biosensor. Introduction of αIIbβ3 integrin receptor in detergent solution generated a new “bound” 129Xe biosensor peak that was shifted 4 ppm downfield from the “free” 129Xe biosensor.

Xenon and radon have many similar properties, a difference being that all 35 isotopes of radon ( 195 Rn– 229 Rn) are radioactive. Radon is a pervasive indoor air pollutant believed to cause significant incidence of lung cancer in many geographic regions, yet radon affinity for a discrete molecular species has never been determined. By comparison, the chemistry of xenon has been widely studied and applied in science and technology. Here, both noble gases were found to bind with exceptional affinity to tris-(triazole ethylamine) cryptophane, a previously unsynthesized water-soluble organic host molecule. The cryptophane–xenon association constant, K a  = 42,000 ± 2,000 M -1 at 293 K, was determined by isothermal titration calorimetry. This value represents the highest measured xenon affinity for a host molecule. The partitioning of radon between air and aqueous cryptophane solutions of varying concentration was determined radiometrically to give the cryptophane–radon association constant K a  = 49,000 ± 12,000 M -1 at 293 K.

Folate-conjugated cryptophane was developed for targeting cryptophane to membrane-bound folate receptors that are overexpressed in many human cancers. The cryptophane biosensor was synthesized in 20 nonlinear steps, which included functionalization with folate recognition moiety, solubilizing peptide, and Cy3 fluorophore. Hyperpolarized 129Xe NMR studies confirmed xenon binding to the folate-conjugated cryptophane. Cellular internalization of biosensor was monitored by confocal laser scanning microscopy and quantified by flow cytometry. Competitive blocking studies confirmed cryptophane endocytosis through a folate receptor-mediated pathway. Flow cytometry revealed 10-fold higher cellular internalization in KB cancer cells overexpressing folate receptors compared to HT-1080 cells with normal folate receptor expression. The biosensor was determined to be nontoxic in HT-1080 and KB cells by MTT assay at low micromolar concentrations typically used for hyperpolarized 129Xe NMR experiments.

Efficient syntheses of trisubstituted cryptophane-A derivatives that are versatile host molecules for many applications are reported. Trihydroxy cryptophane was synthesized in six or seven steps with yields as high as 9.5%. By a different route, trihydroxy cryptophane modified with three propargyl, allyl, or benzyl protecting groups was synthesized with yields of 4.1-5.8% in just six steps. Hyperpolarized 129 Xe NMR chemical shifts of 57-65 ppm were measured for these trisubstituted cryptophanes.Cryptophane organic host molecules, constructed from two cyclotriguaiacylene (CTG) units connected by three alkane linkers, possess a hydrophobic cavity that can encapsulate a wide variety of guests. One important application involves xenon binding to cryptophane, which can be delivered to specific cellular targets for detection and resolution by 129 Xe magnetic resonance spectroscopy or imaging. 1 Currently, water-soluble cryptophane-A derivatives show the highest known xenon affinity with K A ≈ 30,000 M −1 in buffer at rt. 2 129 Xe can be hyperpolarized to generate ~10 5 NMR signal enhancements and provides a greater than 200 ppm 129 Xe NMR chemical shift window, with resonance frequencies that depend sensitively on the molecular environment. 3 Thus, cryptophane hosts functionalized with different recognition moieties allow the simultaneous detection of multiple targets (i.e., multiplexing), as is desirable for biomolecular imaging. 4 The importance of in vivo studies has motivated the development of synthetic routes capable of producing large quantities of functionalized cryptophane. 5 A previously described multi-step template strategy allowed the synthesis of diverse mono6 and tri-functionalized cryptophane-A derivatives2 ,7 as well as enantiopure (−)-cryptophane-A.8 However, even improved synthetic routes typically involve nine or more steps with low yields.5b The preparation of separate connecting linkers and CTG units is time-consuming, and the hydroxyl functionalities must be protected to avoid side-products during the cryptophane synthesis. Moreover, the two cyclization reactions to produce first CTG and finally cryptophane typically involve strong acid such as perchloric acid in methanol or formic acid.9 These conditions are incompatible with the synthesis of new CTG derivatives bearing acid-sensitive moieties, and very often dilute conditions are required to avoid polymerization, as in the case with propargyl groups.2 Recently, Brotin and coworkers * Corresponding author: Dmochowski, Ivan J., ivandmo@sas.upenn.edu. Supporting Information Available:Experimental procedures and characterization data for all synthesized compounds and 129 Xe NMR data. This material is available free of charge via the Internet at http://pubs.acs.org. reported cyclization reaction conditions using a milder reagent as Lewis acid, Sc(OTf) 3 . 10 Notably, in some cases even better yields were obtained, and the purification steps were made easier. NIH Public AccessBased on these observations, we developed a shorter, 6-step synt...