Simultaneous measurement of metabolic stability and metabolite identification of 7-methoxymethylthiazolo[3,2-a]pyrimidin-5-one derivatives in human liver microsomes using liquid chromatography/ion-trap mass spectrometry

A liquid chromatography/mass spectrometry (LC/MS) method using an atmospheric pressure chemical ionisation source was used to measure the metabolic stability and metabolite identification of 7-methoxymethylthiazolo[3,2-a]pyrimidin-5-one derivative (1) in human liver microsomes. After 15 min incubation with human liver microsomes, compound 1 exhibited metabolic turnover of 44%. Data-dependent tandem mass spectrometry (MS/MS) scanning was used to generate product ion spectra from the protonated ions of the compound and its metabolites. An unusual metabolite at m/z 407 corresponding to the [M-24+H]+ ion was identified for compound 1. Interestingly, the formation of the [M-24+H]+ ion was not observed in the analogues wherein the fused thieno double bond was substituted (2) and the thieno group replaced by a fused benzo derivative (3). Compounds 2 and 3 exhibited metabolic turnovers of 24 and 30%, yielding oxidative metabolites corresponding to [M+16] and [M+32]+, respectively. Based on these facts the mechanism for [M-24]+ formation in compound 1 through an initial epoxide formation on the double bond of the fused thieno ring followed by hydrolytic ring opening and deacylation is envisaged.     

Redetermination of 1,3,5‐tri­chloro‐2,4‐di­nitro­benzene

The title compound, C₆HCl₃N₂O₄, is an intermediate in the synthesis of 1,3,5‐tri­chloro‐2,4,6‐tri­nitro­benzene. The crystal structure at 153 K shows no major deviations from the previously reported structure at 295 K other than the expected contraction of the a and c axes and, correspondingly, the β angle.     

Strain estimates for small-ring cyclic allenes and butatrienes

Isodesmic and homodesmic equations at the B3LYP/6-311+G(d,p)+ZPVE level of theory have been used to estimate strain for the homologous series of cyclic allenes and cyclic butatrienes. A simple fragment deformation approach also has been applied and appears to work better for the larger rings. For the cyclic allene series, estimates for allene functional group strain (kcal/mol) include: 1,2-cyclobutadiene, 65; 1,2-cyclopentadiene, 51; 1,2-cyclohexadiene, 32; 1,2-cycloheptadiene, 14; 1,2-cyclooctadiene, 5; 1,2-cyclononadiene, 2; 1,2,4-cyclohexatriene, 34; and bicyclo[3.2.1]octa-2,3-diene, 39. For cyclic butatrienes, functional group strain estimates include: 1,2,3-cyclobutatriene, >100; 1,2,3-cyclopentatriene, 80; 1,2,3-cyclohexatriene, 50; 1,2,3-cycloheptatriene, 26; 1,2,3-cyclooctatriene, 17; and 1,2,3-cyclononatriene, 4. Barriers to interconversion of enantiomers in cyclic allenes are reduced with increasing strain. Newly predicted values include: 1,2-cyclopentadiene <1 kcal/mol and bicyclo[3.2.1]octa-2,3-diene, 7.4 kcal/mol. Estimated levels of strain parallel the known reactivity of these substances.     

Critical point estimation of the Lennard-Jones pure fluid and binary mixtures

The apparent critical point of the pure fluid and binary mixtures interacting with the Lennard-Jones potential has been calculated using Monte Carlo histogram reweighting techniques combined with either a fourth order cumulant calculation (Binder parameter) or a mixed-field study. By extrapolating these finite system size results through a finite size scaling analysis we estimate the infinite system size critical point. Excellent agreement is found between all methodologies as well as previous works, both for the pure fluid and the binary mixture studied. The combination of the proposed cumulant method with the use of finite size scaling is found to present advantages with respect to the mixed-field analysis since no matching to the Ising universal distribution is required while maintaining the same statistical efficiency. In addition, the accurate estimation of the finite critical point becomes straightforward while the scaling of density and composition is also possible and allows for the estimation of the line of critical points for a Lennard-Jones mixture.     

Ring contraction mechanisms in substituted cyclohexylium cations

The effect of an electron donating or withdrawing group on the ring contraction mechanism of a cyclohexylium (cyclohexane-derived) cation has been studied using density functional theory. The barrier to rearrangement of the parent cyclohexane cation (1) was previously calculated to be 7.7 kcal/mol using PBE/6-311++G(2d, 2p). We show in this work that addition of an electron withdrawing group (CF₃) lowers the average barrier to ring contraction, while an electron donating group (CH₃) increases the average barrier, relative to the parent, unsubstituted, cyclohexane cation. Calculated barrier heights for going from a 6-membered to 5-membered ring range from 4.3 to 23.3 kcal/mol for methyl-cyclohexylium (2), but only from 0.6 to 14.0 kcal/mol for trifluoromethyl-cyclohexylium (3). The lower barriers for 3 can be explained by (a) the starting structures involved, and (b) the use of dative bonding as a catalyst in stabilizing intermediates and transition states. For 1 and 3 the reaction involves starting from secondary cations and going downhill in energy through secondary intermediates to a tertiary product. However, 2 does not benefit from such favorable energetics since it more likely starts from a tertiary cation and has to first rearrange to secondary intermediates to derive the tertiary methyl-cyclopentane-type product.     

Colloidal aspects of protein digestion

The increase in food related health issues has sparked an interest in research on the digestion processes of the gastrointestinal tract. Because of the difficulty and expense of undertaking human trials or even animal experiments, much of the current research uses in vitro models that simulate various aspects of digestion. The results of this research indicate that the rate and extent of protein digestion is governed by accessibility of the cleavage sites to enzymes and local flexibility of the substrate molecule. However, results have also shown that digestion of an allergenic protein to small fragments does not necessarily mean that it will no longer be immunologically active. Other factors are also important. For example, adsorption to an interface increased rates of digestion as did the presence of bile acids. In fact, interaction with a range of physiological surfactants has been shown to be extremely important in protein digestion. When protein is adsorbed to an emulsified food it can be displaced by the surfactants in either the stomach or the small intestine. Lipid interaction with the protein in solution has been demonstrated to be important in effecting rates of proteolysis and phospholipids in particular have provided a protective effect for some milk proteins. Conversely the presence of specific proteins has been shown to affect rates of lipid digestion. The number of such colloidal interactions that we now know may play a role in protein digestion highlights the importance of this area to understanding how we can produce food that optimises nutrition for the consumer.