Molecular clip and tweezer introduce new mechanisms of enzyme inhibition

Artificial molecular clips and tweezers, designed for cofactor and amino acid recognition, are able to inhibit the enzymatic activity of alcohol dehydrogenase (ADH). IC50 values and kinetic investigations point to two different new mechanisms of interference with the NAD(+)-dependent oxidoreductase: While the clip seems to pull the cofactor out of its cleft, the tweezer docks onto lysine residues around the active site. Both modes of action can be reverted to some extent, by appropriate additives. However, while cofactor depletion by clip 1 was in part restored by subsequent NAD(+) addition, the tweezer (2) inhibition requires the competitive action of lysine derivatives. Lineweaver-Burk plots indicate a competitive mechanism for the clip, with respect to both substrate and cofactor, while the tweezer clearly follows a noncompetitive mechanism. Conformational analysis by CD spectroscopy demonstrates significant ADH denaturation in both cases. However, only the latter case (tweezer-lysine) is reversible, in full agreement with the above-detailed enzyme switch experiments. The complexes of ADH with clips or tweezer can be visualized in a nondenaturing gel electrophoresis, where the complexes migrate toward the anode, in contrast to the pure enzyme which approaches the cathode. Supramolecular chemistry has thus been employed as a means to control protein function with the specificity of artificial hosts opening new avenues for this endeavor.     

Characterization of non-polar aromatic hydrocarbons in crude oil using atmospheric pressure laser ionization and Fourier transform ion cyclotron resonance mass spectrometry (APLI FT-ICR MS)

We report on the successful application of the recently introduced atmospheric pressure laser ionization (APLI) method as a novel tool for the analysis of crude oil and its components. Using Fourier transform ion cyclotron resonance mass spectrometry, unambiguous determination of key compounds in this complex matrix with unprecedented sensitivity is presented.     

Mass-spectrometric analysis of complex volatile and nonvolatile crude oil components: a challenge

Crude oil analysis has long been an inspiration for the development of analytical techniques. Especially mass spectrometry has flourished as a result of the challenge these extremely complex problems offer. Here an overview of different analytical methods is presented that shows different ways to analyze volatile and nonvolatile crude oil components. Focus has been placed on the use of mass spectrometry and especially the new developments that have been introduced using the emerging technique of Fourier transform ion cyclotron resonance mass spectrometry. These studies are examples of how far the development of analytical methods has come for the task of studying such complex problems.     

Zweidimensionale Polymere mit hydrophober Umhüllung: Kristallstrukturen der isostrukturellen Metallkomplexe [M{C6H4(SO2)2N}(H2O)] (M = K,Rb) und des strukturverwandten Ammoniumsalzes [(NH4){C6H4(SO2)2N}(H2O)]

Metal Salts of Benzene‐1,2‐di(sulfonyl)amine. 4. Hydrophobically Wrapped Two‐Dimensional Polymers: Crystal Structures of the Isostructural Metal Complexes [M{C₆H₄(SO₂)₂N}(H₂O)] (M = K, Rb) and of the Structurally Related Ammonium Salt [(NH₄){C₆H₄(SO₂)₂N}(H₂O)] The previously unreported compounds KZ · H₂O ( 1 ), RbZ · H₂O ( 2 ) and NH₄Z · H₂O ( 3 ), where Z^– is Ndeprotonated ortho‐benzenedisulfonimide, are examples of layered inorgano‐organic solids, in which the inorganic component is comprised of metal or ammonium cations, N(SO₂)₂ groups and water molecules and the outer regions are formed by the planar benzo rings of the anions. The metal complexes 1 and 2 were found to be strictly isostructural, whereas 3 is structurally related to them by a non‐crystallographic mirror plane ( 1 – 3 : monoclinic, space group P2₁/c, Z = 4; single crystal X‐ray diffraction at low temperatures). In each structure, the five‐membered 1,3,2‐dithiazolide heterocycle possesses an envelope conformation, the N atom lying about 40 pm outside the mean plane of the S–C–C–S moiety. The metal complexes feature two‐dimensional coordination networks interwoven with O–H…O hydrogen bonds originating from the water molecules. The metal centres adopt an irregular nonacoordination formed by five sulfonyl O atoms, two N atoms and two μ₂‐bridging water molecules; each M⁺ is connected to four different anions. When NH₄⁺ is substituted for M⁺, the metal–ligand bonds are replaced by N⁺–H…O hydrogen bonds, but the general topology of the lamella is not affected. In the three structures, the lipophilic benzo groups protrude obliquely from the surfaces of the polar lamellae and display marked interlocking between adjacent layers.     

Unusual Optical Properties of Mn‐doped ZnO: The Search for a New Red Pigment—A Combined Experimental and Theoretical Study

A pigment of your imagination : A range of polycrystalline solid solutions of a zinc‐rich Zn_x?1Mn_xO system (see figure) have been prepared and studied in terms of their colour, diffuse reflectance spectra, Mn valence state and electronic structure. The intense optical absorption arises from Mn²⁺ doping and is thought to be due to forbidden or partially forbidden transitions between the valence and the conduction band.

     

Photocytotoxicity of the Fluorescent Nonsteroidal Androgen Receptor Ligand TDPQ†

1,2,3,4‐tetrahydro‐2,2‐dimethyl‐6‐(trifluoromethyl)‐8‐pyridono[5,6‐g]quinoline (TDPQ), a selective nonsteroidal androgen receptor (AR) ligand, is a fluorescent compound. We characterized its spectral properties in comparison with the structural precursor carbostyril 151 (C151) and with its racemic structural isomer 4‐ethyl‐1,2,3,4‐tetrahydro‐6‐(trifluoromethyl)‐8‐pyridino[5,6‐g]quinoline (ETPQ). The absorption maximum in CH₃CN of either TDPQ or ETPQ is 400 nm whereas that of C151 is 350 nm. The fluorescence lifetimes (τ) and quantum yields (?_f) in CH₃CN are typical of fluorescent dyes: TDPQ (4.2 ns, 0.8) and ETPQ (4.6 ns, 0.76). C151 showed lower τ and ?_f of 0.2 ns and 0.02, respectively. TDPQ can function as a fluorescent label at (sub)micromolar concentrations. We detected TDPQ fluorescence in human breast tumor cells using confocal microscopy. While the fluorescence maxima of the compounds were solvent insensitive, the ?_f for ETPQ decreased in aqueous solutions regardless of the presence of albumin or DNA. The ?_f of TDPQ was less affected. The quantum yield of singlet oxygen (¹O₂) photosensitization (?_so) by TDPQ and ETPQ was about 7% in CH₃CN, sufficient to induce photocytotoxicity. TDPQ was photocytotoxic in AR‐positive MDA‐MB‐453 breast cancer cells but not in AR‐negative MDA‐MB‐231 cells. The combination of AR selectivity with photocytotoxicity makes TDPQ a promising candidate for selective targeting of AR‐positive cells during photodynamic therapy.     

A photocontrolled beta-hairpin peptide

Beta-hairpins constitute the smallest beta-type structures in peptides and proteins. The development of highly stable, yet monomeric beta-hairpins based on the tryptophan zipper motif was therefore a remarkable success [A. G. Cochran, N. J. Skelton, M. A. Starovasnik, Proc. Natl. Acad. Sci USA 2001, 98, 5578-5583]. We have been able to design, synthesize and characterize a hairpin based on this motif which incorporates an azobenzene-based photoswitch, that allows for time-resolved folding studies of beta-structures with unprecedented time resolution. At room temperature the trans-azo isomer exhibits a mostly disordered structure; however, light-induced isomerization to the cis-azo form leads to a predominantly extended and parallel conformation of the two peptide parts, which are linked by the novel photoswitch, [3-(3-aminomethyl)phenylazo]phenylacetic acid (AMPP). While in the original sequence the dipeptide Asn-Gly forms a type I' beta-turn which connects the two strands of the hairpin, this role is adopted by the AMPP chromophore in our photoresponsive beta-hairpin that can apparently act as a beta I'-turn mimetic. The beta-hairpin structure was determined and confirmed by NMR spectroscopy, but the folding process can be monitored by pronounced changes in the CD, IR and fluorescence spectra. Finally, incorporation of the structurally and functionally important beta-hairpin motif into proteins by chemical ligation might allow for the photocontrol of protein structures and/or functions.