Molecular receptors. Synthesis, x-ray crystal structures, and chemical properties of crown ethers bearing an intraannular phenolic group

Proton Transfer Mediated Recognition of Amines by Ionizable Macrocyclic Receptors

Ammonium ion/carboxylate ion pairing is a key interaction ubiquitous in biological systems, but amine recognition by ionizable molecular receptors, mediated by host-to-guest proton transfer, has too often been overlooked as...

Rotamerism-driven large magnitude host–guest binding change in a crown ether derivatized pyridinium-phenolate series

A large magnitude host–guest binding change driven by transducting rotamerism effects is demonstrated within a crown ether modified pyridinium-phenolate series.

Selective amine recognition driven by host-guest proton transfer and salt bridge formation

The stepwise synthesis of ionizable p-tert-butylcalix[5]arenes 1a·H and 1b·H, featuring a fixed cone cavity endowed with a carboxyl moiety at the narrow rim, is described. Single-crystal X-ray analyses have shown that in the solid state 1a·H and 1b·H adopt a cone-out conformation with the carboxylic OH group pointing in, toward the bottom of the aromatic cavity, as a result of a three- or two-center hydrogen-bonding pattern between the carboxyl group and the phenolic oxygen atom(s). The affinity of amines for calix[5]arene derivatives 1a·H and 1b·H was probed by (1)H NMR spectroscopy and single-crystal X-ray diffraction studies. These carboxylcalix[5]arenes are shown to selectively recognize linear primary amines--over branched, secondary, and tertiary amines--by a two-step process involving a proton transfer from the carboxyl to the amino group to provide the corresponding alkylammonium ion, followed by binding of the latter inside the cavity of the ionized calixarene. Proton transfer occurs only with linear primary amines, that is, when the best size and shape fit between host and substrate is achieved, while the other amines remain in their noncompeting unprotonated form. The role of the solvent in the ionization/complexation process is discussed. Structural studies on the n-BuNH(2) complexes with 1a·H and 1b·H provide evidence that binding of the in situ formed n-BuNH(3)(+) substrate to the cavity of the ionized macrocycle is ultimately secured, in the case of 1a·H, by the formation of an unprecedented salt-bridge interaction.

Bidirectional and Colorimetric Recognition of Sodium and Potassium Ions

Host 1 based on the phenolphthalein skeleton and two crown ether moieties demonstrated opposite behaviors toward sodium and potassium cations caused by bidirectional complexation. [reaction: see text]

Synthesis of Enantiomerically Pure Thiocrown Ethers Derived from 1,1'-Binaphthalene-2,2'-diol

Synthetic methodology is given for the preparation of two different types of thiocrown ethers from optically pure 1,1'-binaphthalene-2,2'-diol (10). The conceptually simplest approach starts from optically pure 10 itself, which is alkylated (4 equiv of K(2)CO(3) in DMF at 110 degrees C) with 2-chloroethanol followed by mesylation to provide 2,2'-bis(2-(mesyloxy)ethoxy)-1,1'-binaphthyl (14). When allowed to react with ethane-1,2-dithiol, propane-1,3-dithiol, 1,4,7-trithiaheptane, 1,4,8,11-tetrathiaundecane, 2,2-dimethylpropane-1,3-dithiol, 2-(mercaptomethyl)-1-propene-3-thiol, and 1,2-benzenedithiol in the presence of Cs(2)CO(3) in DMF at 60 degrees C the corresponding thiocrown ethers 22-25, 28, 30, and 32 are formed in 30-54% yields. Test reactions were carried out to establish that no racemization occurs during alkylation under these conditions. Reaction of optically pure 10 with tetrahydropyranyl (THP)-protected 3-chloropropanol under similar conditions for the preparation of 14 proceeded more sluggishly but cleanly. Removal of the THP protecting groups afforded 2,2'-bis(3-bromopropoxy)-1,1'-binaphthyl (20), which on reaction with propane-1,3-dithiol, 1,5,9-trithianonane, 2,2-dimethylpropane-1,3-dithiol, 2-(mercaptomethyl)-1-propene-3-thiol, and 1,2-bis(mercaptomethyl)benzene provided the respective thiocrown ethers 26, 27, 29, 31, and 33 in 24-68% yields. Another class of thiocrown ethers was prepared from optically active 10, which was converted via ortho-lithiation to 3,3'-bis(bromomethyl)-2,2'-dimethoxy-1,1'-binaphthyl (39) by means of methylation (K(2)CO(3)/CH(3)I), ortho-lithiation followed by formylation (n-C(4)H(9)Li/N,N,N',N'-tetramethylethylenediamine (TMEDA)/ether followed by DMF and H(2)O workup) followed by reduction (NaBH(4)) followed by bromination (PBr(3) in C(5)H(5)N). Reaction (Cs(2)CO(3) in DMF at 60 degrees C) with 1,4,7-trithiaheptane, 1,4,8-trithiaoctane, 1,4,7,10-tetrathiadecane, 1,4,8,11-tetrathiaundecane, and 1,5,10,14-tetrathiatetradecane afforded the corresponding thiocrown ethers 40-44 in 40-75% yields. Despite repeated attempts using a wide range of reagents, demethylation of the methoxy ether functionalities failed. Attempts to prepare the free phenol derivatives of the latter type of crown ethers by oxidative coupling of two naphthol units failed.

Optical chemical sensors for environmental control and system management

Several fiber optic based chemical sensors have been developed for use in plant growth systems and enclosed life support systems. Optical chemical sensors offer several distinct advantages in terms of sensitivity, calibration stability, immunity to biofouling and electrical interference, and ease of multiplexing sensors for multipoint/multiparameter analysis. Also, the ability to locate fiber optic sensors in close proximity to plant roots or leaves should improve the measurement reliability by obviating the need for handling and transport which can compromise sample integrity. Polestar Technologies and GEO-CENTERS have developed and tested many types of optical chemical sensors which utilize novel glass and polymeric materials as the sensor substrate. Analytes are detected using immobilized colorimetric indication systems or molecular recognition elements. Typically transduction is via wavelength specific absorption changes with multiwavelength detection for drift compensation. Sensors have been developed for solution pH, NH3, ethylene, CO2, and dissolved metal ions. In addition, unique PC-compatible optoelectronic interfaces, as well as distributed measurement systems, so that integrated detection systems are now available. In this paper recent efforts to develop sensors for critical nutrient ions are presented.

Refined crystal structure of beta-lactamase from Citrobacter freundii indicates a mechanism for beta-lactam hydrolysis

Beta-Lactamases (EC 3.5.2.6, 'penicillinases') are a family of enzymes that protect bacteria against the lethal effects of cell-wall synthesis of penicillins, cephalosporins and related antibiotic agents, by hydrolysing the beta-lactam antibiotics to biologically inactive compounds. Their production can, therefore, greatly contribute to the clinical problem of antibiotic resistance. Three classes of beta-lactamases--A, B and C--have been identified on the basis of their amino-acid sequence; class B beta-lactamases are metalloenzymes, and are clearly distinct from members of class A and C beta-lactamases, which both contain an active-site serine residue involved in the formation of an acyl enzyme with beta-lactam substrates during catalysis. It has been predicted that class C beta-lactamases share common structural features with D,D-carboxypeptidases and class A beta-lactamases, and further, suggested that class A and class C beta-lactamases have the same evolutionary origin as other beta-lactam target enzymes. We report here the refined three-dimensional structure of the class C beta-lactamase from Citrobacter freundii at 2.0-A resolution and confirm the predicted structural similarity. The refined structure of the acyl-enzyme formed with the monobactam inhibitor aztreonam at 2.5-A resolution defines the enzyme's active site and, along with molecular modelling, indicates a mechanism for beta-lactam hydrolysis. This leads to the hypothesis that Tyr 150 functions as a general base during catalysis.