Accurate measurements of 13C-13C J-couplings in the rhodopsin chromophore by double-quantum solid-state NMR spectroscopy

A new double-quantum solid-state NMR pulse sequence is presented and used to measure one-bond 13C-13C J-couplings in a set of 13C2-labeled rhodopsin isotopomers. The measured J-couplings reveal a perturbation of the electronic structure at the terminus of the conjugated chain but show no evidence for protein-induced electronic perturbation near the C11-C12 isomerization site. This work establishes NMR methodology for measuring accurate 1JCC values in noncrystalline macromolecules and shows that the measured J-couplings may reveal local electronic perturbations of mechanistic significance.     

Precursors of biological cofactors from ultraviolet irradiation of circumstellar/interstellar ice analogues

Biological cofactors include functionalized derivatives of cyclic tetrapyrrole structures that incorporate different metal ions. They build up structural partnerships with proteins, which play a crucial role in biochemical reactions. Porphyrin, chlorin, bacteriochlorin, and corrin are the basic structures of cofactors (heme, chlorophyll, bacteriochlorophyll, siroheme, F 430, and vitamin B12). Laboratory and theoretical work suggest that the molecular building blocks of proteins (alpha-amino acids) and nucleic acids (carbohydrates, purines, and pyrimidines) were generated under prebiotic conditions. On the other hand, experimental data on the prebiotic chemistry of cofactors are rare. We propose to search directly for the pathways of the formation of cofactors in the laboratory. Herein we report on the detection of N-heterocycles and amines in the room-temperature residue obtained after photo- and thermal processing of an interstellar ice analogue under high vacuum at 12 K. Among them, hexahydro-1,3,5-triazine and its derivatives, together with monopyrrolic molecules, are precursors of porphinoid cofactors. Hexahydropyrimidine was also detected. This is the first detection of these compounds in experiments simulating circumstellar/interstellar conditions. Except for 2-aminopyrrole and 2,4-diaminofuran, which were only found in 13C-labeled experiments, all the reported species were detected in both 12C- and 13C-labeled experiments, excluding contamination. The molecules reported here might be present in circumstellar/interstellar grains and cometary dust and could be detected by the Stardust and Rosetta missions.     

The synthesis of 3-methyl- and 3,4-dimethyltetrahydroisoquinolines by intramolecular hydroamination with n-butyllithium

The synthesis of 3-methyltetrahydroisoquinolines and 3,4-dimethyltetrahydroisoquinolines from aromatic aminoalkenes using intramolecular hydroamination reactions, mediated by sub-stoichiometric amounts of n-butyllithium, is described.     

The perception of randomness

Psychologists have studied people's intuitive notions of randomness by two kinds of tasks: judgment tasks (e.g., “is this series like a coin?” or “which of these series is most like a coin?”), and production tasks (e.g., “produce a series like a coin”). People's notion of randomness is biased in that they see clumps or streaks in truly random series and expect more alternation, or shorter runs, than are there. Similarly, they produce series with higher than expected alternation rates. Production tasks are subject to other biases as well, resulting from various functional limitations. The subjectively ideal random sequence obeys “local representativeness”; namely, in short segments of it, it represents both the relative frequencies (e.g., for a coin, 50%–50%) and the irregularity (avoidance of runs and other patterns). The extent to which this bias is a handicap in the real world is addressed.     

Exceptionally high in vitro antitumor activity of substituted triphenyltin benzoates including salicylates against a human mammary tumor,MCF-7, and a colon carcinoma,WiDr

Nine new substituted triphenyltin benzoates of the type Ar₃Sn? OOC? C₆H₂XYZ [X = Y = H, Z = 2-OCH₃ and 4-F; X = H, Y = 3-F, Z = 5-F; X = H, Y = 2-OH, Z = 5-Cl, 5-NH₂, 5-OCH₃ and 5-SO₃H; X = 2-OH, Y = 3-CH(CH₃)₂ and Z = 5-CH(CH₃)₂] were prepared and possess considerable in vitro antitumor activity against two human tumor cell lines (MCF-7, a mammary tumor, and WiDr, a colon carcinoma) comparable with that of mitomycin C.     

Synthesis,spectroscopic characterization and in vitro antitumour activity of di-n-butyltin and diethyltin trimethoxybenzoates: X-ray structure analysis of bis[di-n-butyl(3,4,5-trimethoxybenzoato)tin] oxide

Di-n-butyltin(IV) and diethyltin(IV) 2,3,4-, 2,4,5- and 3,4,5-trimethoxybenzoates [(CH₃O)₃C₆H₂COO]₂SnR [Type a] and {[(CH₃O)₃C₆H₂COO]R₂Sn}₂O [Type b] have been synthesized and characterized spectroscopically. The crystal structure of bis[di-n-butyl(3,4,5- trimethoxybenzoato)tin] oxide has been determined. The triclinic unit cell contains one centrosymmetric dimer; a = 15.919(2) Å, b = 11.711(3) Å, c = 13.475(1) Å, and α = 63.63(2)°, β = 67.49(1)°, γ = 76.61(2)°. The geometry of the dimer is very similar to that of bis[di-n-butyl(5-methoxysalicylatotin] oxide), with two different types of five-coordinate tin atoms and one central planar Sn₂O₂ ring.     

Gravity-induced aging in glasses of colloidal hard spheres

The influence of gravity on the long-time behavior of the mean squared displacement in glasses of polydisperse colloidal hard spheres was studied by means of real-space fluorescent recovery after photobleaching. We present, for the first time, a significant influence of gravity on the mean squared displacements of the particles. In particular, we observe that systems which are glasses under gravity (with a gravitational length on the order of tens of micrometers) show anomalous diffusion over several decades in time if the gravitational length is increased by an order of magnitude. No influence of gravity was observed in systems below the glass transition density. We show that this behavior is caused by gravity dramatically accelerating aging in colloidal hard sphere glasses. This behavior explains the observation that colloidal hard sphere systems which are a glass on Earth rapidly crystallize in space.     

Palladium(II)-catalyzed intramolecular 1,4-oxyacyloxylation of conjugated dienes. A stereocontrolled route to fused six-membered lactones and pyrans

Stereocontrolled palladium(II)-catalyzed intramolecular 1,4-oxidations of dienyl acids and alcohols proceed under mild conditions to give fused delta-lactones and pyrans, respectively, in good yields. The stereo- and regioselectivity was affected by the presence of LiCl and solvent composition (HOAc/acetone). The products obtained were used for further functionalizations using copper(I)-mediated reactions. Stoichiometric reactions of preformed dibutylcyanocuprates with pyrans containing an allylic acetate gave cis- and trans-fused ring systems with high gamma-selectivity and in high yields.