Stearic acid solubility and cubic phase volume

Development of an optimized sample preparation method for quantification of free fatty acids in food using liquid chromatography-mass spectrometry

Accurate and precise determination of free fatty acid (FFA) contents is essential for quality control and assurance in food production. Herein, a mass spectrometric study was performed to develop a sample preparation protocol that can minimize exogenous FFA contamination during the quantification of FFA in food. The quantities of exogenous FFAs were measured using various combinations of seven pretreatment methods for a sample tube, three extraction methods, and four types of sample tubes. Methanol washing could effectively reduce exogenous palmitic acid (PA) and stearic acid (SA) by 73 and 64%, respectively, in contrast to furnace baking resulting in a decrease in the amount of PA and SA contaminants by 50 and 37%, respectively. A lower amount of FFA contaminants was extracted from glass tubes during comparative analysis of the four different sample tubes studied. A combination of a methanol-washed glass tube and chloroform extraction solvent was determined to be the optimal method for minimizing the error in FFA quantification. The optimized sample preparation protocol used for FFA quantification can lower the amount of foreign palmitic acid and stearic acid to the sub-nanomolar level in the analysis of FFAs in skimmed milk.

Unit cell volume and liquid-phase immiscibility in oleate-stearate lipid mixtures

The structural basis of immiscibility of stearic acid (SA) in two unsaturated lipids at room temperature was examined. A 5% SA mixture in octadec-9-enoic acid (OA) is cloudy; a 5% SA solution in the methyl ester of OA (MeOA) is fully miscible. At 10% SA in MeOA, a clear and immiscible phase formed. The composition of this immiscible phase however was not 10% SA, but 25% SA. Adding additional SA altered the amount of the second phase, not its stoichiometry. Molecular mechanics explains the molecular basis for the ratio of saturated and unsaturated lipids in the different phases. Packing order of lipids within a unit volume explains the discrete lipid composition ratio forming each of the observed miscible and immiscible phases. The smallest unit cell volume that explains the observed stoichiometry is a cube (not a sphere).

Growth hormone (GH)-associated nitration of Janus kinase-2 at the 1007Y-1008Y epitope impedes phosphorylation at this site: mechanism for and impact of a GH, AKT, and nitric oxide synthase axis on GH signal transduction

A generalized increase in liver protein tyrosine nitration (3'-nitrotyrosine, 3'-NT) occurs after GH injection in a time frame consistent with observed acute GH hyporesponsiveness. Here we investigated whether the GH-associated nitration process might be targeted to the (1007)Y-(1008)Y-phosphorylation epitope of Janus kinase (JAK)-2 because of its homology to a defined peptide nitration motif. Using antibodies we developed to the 3'NT-substituted peptide analog of the (1007)Y-(1008)Y-JAK2 site (nitro-JAK2), we demonstrated a rapid increase in membrane-associated nitro-JAK2 after GH. In vivo (bovine liver) and in vitro (porcine hepatocytes), GH-induced cellular levels of nitro-(1007)Y-(1008)Y-JAK2 persisted significantly longer after a stimulatory GH pulse than did levels of phospho-JAK2. Treatment of cultured cells with inhibitors of AKT or endothelial nitric oxide synthase prior to GH challenge attenuated the increases in nitro-JAK2 predominantly in the membrane subcellular fraction. In instances in which GH effected orthophosphorylation of (694)Y-signal transducer and activator of transcription (STAT)-5b, the addition of AKT and endothelial nitric oxide synthase inhibitors prior to GH significantly increased the levels of phospho-(694)Y-STAT5b and phospho-(1007)Y-JAK2 over those arising from GH alone. Nuclear magnetic resonance molecular modeling of natural and 3'-NT- and orthophosphate-substituted peptide analogs of the (1007)Y-(1008)Y site demonstrated significant effects of 3'-nitration on the planar orientation and intramolecular stabilizing points of the affected tyrosines. When these peptides were used as substrates for in vitro tyrosine kinase phosphorylation reactions, 3'-NT in the (1007)Y and/or (1008)Y positions blocked the generation of (1007)Y-phosphotyrosine. The data suggest that the nitration of JAK2 may act as an inhibitory counterpart to phosphorylation activation, reflecting a very localized break on the progression of GH signal transduction processes spanning JAK-STAT-AKT interactions.

Phase behaviour of stearic acid–stearonitrile mixtures

The solid-liquid phase behaviour of stearic acid (SA) and stearonitrile (SN) in binary mixtures was investigated by differential scanning calorimetry (DSC), and the formation of SA-SN mixed monolayers at the air-water interface was followed by surface pressure-area (pi-A) measurements and by Brewster angle microscope (BAM) observation. The solid-liquid phase diagram is a eutectic type phase diagram, with the eutectic composition 0.90<X(SN)<0.95 and T(eut)=40.9 degrees C. The DSC results also suggest that the two components are immiscible in the solid phase but form a liquid mixture with positive deviations to the ideal behaviour. At the air-water interface, the two components form liquid condensed monolayers in the entire range of compositions, at low surface pressures, while solid mixed monolayers only form at high surface pressures for X(SN)<0.8. Thermodynamic analysis indicates that SA and SN are miscible in the liquid condensed phase, with negative deviations from the ideal behaviour. The variation of the collapse surface pressure of mixed monolayers also indicates miscibility at the air-water interface.