Characterization of long-chain carboxylic esters with CH3OBOCH 3 + in a small fourier-transform ion cyclotron resonance mass spectrometer

The usefulness of CH₃OBOCH ₃ ⁺ as a chemical ionization reagent was examined by allowing the ion to react with carboxylic esters of various chain lengths in a small Fourier-transform ion cyclotron resonance mass spectrometer equipped with a permanent magnet. CH₃OBOCH ₃ ⁺ is a strong electrophile and readily abstracts an oxygen-containing group from the carboxylic esters. Long-chain esters exclusively lose the alkoxide moiety to give the acylium ion. The same reaction was observed for saturated, unsaturated, branched and cyclic esters. In each case, the acylium ion reacts further with a neutral ester molecule by proton transfer to yield the protonated ester as a secondary product. This remarkably simple product distribution reveals the molecular weight of the ester, the chain length of its acid moiety, and the degree of unsaturation in the acid and alcohol moieties.     

New Quantization Method for Evaluation of Eigenenergies

A quantum dynamical equation is constructed as the limit of a sequence of functions (called Semiquantum momentum functions or SQMF). The quantum action variable J is defined as the limit of the sequence of contour integrals of SQMFs such that the quantization condition is J = n, where n is a nonnegative integer for eigenvalues and a noninteger for off eigenvalues. This quantization condition is exact and J is an analytic function of energy. Based on new definitions, an accurate numerical method is developed for obtaining eigenenergies. The method can be applied to both real and PT symmetric complex potentials. The validity and the accuracy of this new method is demonstrated with three illustrations.     

Synthesis of Boc-protected bicycloproline

The synthesis of a highly constrained quaternary carbocyclic α-amino acid, (+)-N-Boc-bicycloproline, has been achieved starting from sodium cyclopentadienylide. Key steps include a rhodium-catalyzed nitrenoid C-H insertion to install the tert-alkylamine and a ring-closing metathesis reaction to form the pyrrolidine ring.     

Mechanistic implications of the equality of compensation temperatures in chromatography

A common interpretation of the observation that two processes exhibit similar compensation temperatures in an enthalpy-entropy plot is that the two processes occur via the same "mechanism". We show that this interpretation is not rigorously allowed. In fact, the only thing that can be concluded from the observation of identical compensation temperatures is that the relative contributions of enthalpy and entropy to the overall free energy are the same in the two processes. Since it is possible that two processes occur via different mechanisms that, by chance, result in the same relative blends of enthalpy and entropy, the observation of identical compensation temperatures cannot be used as evidence for mechanistic identity. If two processes exhibit different compensation temperatures, however, it can logically be concluded that the two processes are mechanistically distinct.     

Identification of inhibitory components toxic toward zymomonas mobilis CP4(pZB5) xylose fermentation

Zymomonas mobilis CP4(pZB5) is a recombinant bacterium that can produce ethanol from both xylose and glucose. The ethanol-producing efficiency of this organism is substantially impeded by toxic substances present in pretreated hydrolyzates or solid biomass substrates. Acetic acid and furfural (a pentose degradation product) are highly toxic to this organism at levels envisioned for a pretreated-hardwood liquid hydrolyzate. In addition, lignin degradation products and 5-hydroxymethylfurfural (a hexose degradation product) have a moderately toxic effect on the organism. Of the compounds studied, organic acids and aldehydes were found to be more inhibitory than lignin acids or the one alkaloid studied. Acetone:water and methanol extracts of solid biomass samples from red oak, white oak, and yellow poplar are toxic toZymomonas cell growth and ethanol production, with the extracts from white oak being the most toxic.     

Rapid analysis of trace organics in aqueous solutions by enrichment in uncoated capillary columns

Summary A novel technique has been developed for the analysis of trace organics in aqueous solutions. Concentration of organics is effected by passage of the solution being analysed through uncoated plastic or metal capillaries as reported. The concentrated organics are then desorbed from the capillary using an organic solvent, and the desorbed solution is subsequently analysed by gas chromatography. Organics trapped inside a variety of columns have been recovered by solvent desorption in this manner using a number of different solvents, mixed solvents, different solvent volumes, different solvent flow rates through the capillary column and at different desorption temperatures, and the effects of these variables on the efficiency of desorption are discussed.     

An ester enolate-Claisen rearrangement route to substituted 4-alkylideneprolines. studies toward a definitive structural revision of lucentamycin A

Substituted 4-alkylideneprolines represent a rare class of naturally occurring amino acids with promising biological activities. Lucentamycin A is a cytotoxic, marine-derived tripeptide that harbors a 4-ethylidine-3-methylproline (Emp) residue unique among known peptide natural products. In this paper, we examine the synthesis of Emp and related 4-alkylideneprolines employing a versatile ester enolate-Claisen rearrangement. The scope and selectivity of the key rearrangement reaction are described with a number of diversely substituted glycine ester substrates. Treatment of the allyl esters with excess NaHMDS at ambient temperature gives rise to highly substituted α-allylglycine products with good to excellent diastereoselectivities. Resolution of dipeptide diastereomers and cyclization to form the pyrrolidine rings provide rapid access to stereopure prolyl dipeptides. We have applied this strategy to the synthesis of four Emp-containing isomers of lucentamycin A in pursuit of a definitive stereochemical revision of the natural product. Our studies indicate that the Emp stereogenic centers are not the source of structural misassignment. The current strategy should find broad utility in the synthesis of additional natural product analogues and related 3-alkyl-4-alkylidene prolines.     

Determination of equilibrium constant of alkylbenzenes binding to bovine serum albumin by solid phase microextraction

Solid phase microextraction (SPME) coupled with GC has been applied to study the binding properties between bovine serum albumin (BSA) and volatile organic compounds such as benzene, toluene, ethylbenzene, propylbenzene and butylbenzene. Their protein-ligand equilibrium constants have been determined. The measurement of free and bound ligand concentrations in the aqueous solution was based on the equilibrium among the analyte in the fiber coating (Cf), headspace (Ch) and aqueous solution (Cs). The work demonstrated that SPME is a simple and effective method in the study of protein binding to measure the freely dissolved analyte concentration as well as the equilibrium constant. The theoretical aspect of the SPME applied to the equilibrium constant measurement in two-phase (liquid sample-fiber coating) and three-phase (liquid sample-headspace-fiber coating) systems has been thoroughly discussed. The results demonstrated that the interpretation of the calibration data is crucial to the determination of freely dissolved analyte concentration and the equilibrium constant especially when the sample volume is small. The error in the experimental system is discussed. It is demonstrated in this study that for the three-phase system the amount of the analyte partitioned in the headspace could be ignored only in certain circumstances, where the Henry's law constant and the ratio between headspace volume and sample volume are sufficiently small.