The binary phase behavior of 1,3-dimyristoyl-2-stearoyl-sn-glycerol and 1,2-dimyristoyl-3-stearoyl-sn-glycerol

The binary phase behavior of pure 1,3-dimyristoyl-2-stearoyl-sn-glycerol (MSM) and 1,2-dimyristoyl-3-stearoyl-sn-glycerol (MMS) was investigated in terms of polymorphism, melting and crystallization behavior, SFC, hardness and microstructure. Samples were crystallized at cooling rates of 3.0 and 0.1 degrees C/min. The asymmetric TAG demonstrated lower melting and crystallization points at both cooling rates. All samples crystallized in the beta' polymorph when cooled at 0.1 degrees C/min and in the alpha polymorph when cooled at 3.0 degrees C/min. The experimentally determined kinetic phase diagram of MSM-MMS was monotectic for both cooling rates. This data was well described by a thermodynamic model using the Bragg-Williams approximation for non-ideality of mixing and suggested that in both the solid and liquid states, like pair interactions (MSM-MSM and MMS-MMS) were favored over MSM-MMS interaction. A strong tendency to phase separation in the solid phase was also observed. For both cooling rates, the fit of the SFC (%)-time curves to a modified form of the Avrami model indicated that crystallization occurred in two distinct kinetic steps. Depressions seen in SFC did not correspond to depressions in hardness or melting temperatures.     

The binary phase behavior of 1,3-dicaproyl-2-stearoyl-sn-glycerol and 1,2-dicaproyl-3-stearoyl-sn-glycerol

The phase behavior of a binary system constituted of purified 1,3-dicaproyl-2-stearoyl-sn-glycerol (CSC) and 1,2-dicaproyl-3-stearoyl-sn-glycerol (CCS) was investigated at a very slow (0.1 °C/min) and a relatively fast (3.0 °C/min) cooling rate using differential scanning calorimetry (DSC), low resolution NMR, X-ray diffraction (XRD), and polarized light microscopy (PLM). Related forms of the β′ polymorph were detected for all mixtures as well as a β form for CSC-rich mixtures. A double chain length (DCL) stacking of the non-mixed CCS-CCS and CSC-CSC phases and a triple chain length (TCL) stacking of mixed CCS-CSC structure were detected for the different β′ forms. The kinetic phase diagram demonstrated an apparent eutectic at the 0.5_CSC composition when cooled at 0.1 °C/min and at the 0.25_CSC composition when cooled at 3.0 °C/min. The application of a thermodynamic model based on the Hildebrand equation suggests that compounds CSC and CCS are not fully miscible. In addition, the miscibility changes according to the structure of the growing solid phase which is dependent on CSC molar ratio as well as on the kinetics. It was also shown that the miscibility is concentration dependent and that the solid phase, which is growing at conditions well away from equilibrium, is determined kinetically. The molecular interactions were found to be strong and to favor the formation of CSC-CCS pairs in the liquid state. CSC and CCS were also shown to be immiscible in the solid state. Depressions in solid fat content (SFC) were observed for both rates. Relatively complex networks made of needle-like, spherulitic and granular crystals were observed in the CSC/CCS system. A pure CSC phase was found to be instrumental in promoting a higher SFC, and more stable polymorphic forms. The microstructure was shown to be strongly dependent on the cooling rate and was linked to the different polymorphic forms observed by DSC and XRD. Correlations between SFC and the eutectic behavior have been observed for the 3.0 °C/min cooling rate, but not directly in the case of the 0.1 °C/min cooling rate, where slower kinetics which favors the metastable to stable phase conversion processes prevented the same shifts in behavior.     

Structure and thermal history dependant phase behavior of hydrated synthetic sphingomyelin analogue: 1,2-dimyristamido-1,2-deoxyphosphatidylcholine

The physical properties of hydrated multilamellar sample of 1,2-dimyristamido-1,2-deoxyphosphatidylcholine (DDPC) were investigated by means of differential scanning calorimetry (DSC), static X-ray diffraction, and simultaneous DSC and X-ray diffraction. The DDPC is a synthetic sphingomyelin analogue and has two amide bonds in its hydrophobic parts. This paper reports on metastable phase behavior of the hydrated DDPC sample. By cooling from a chain-melted state at the rates of greater than 4 °C min⁻¹, hydrated DDPC bilayers form a metastable gel phase. In the gel phase, the hydrophobic chains are tilted with respect to the bilayer normal, as like the gel phase of glycero-phosphatidylcholines. By heating, the metastable gel phase is transformed in to a stable phase associated with an exothermic heat event at 18.3 °C (ΔH = 14.6 kJ mol⁻¹) and then the stable phase is transformed into a liquid-crystalline phase at 25.6 °C (ΔH = 42 kJ mol⁻¹). The incubation at 17 °C for more than 1 h also induces the formation of the stable phase. In the stable phase, the hydrophobic chains are packed into highly ordered crystal-like structure. However, the X-ray diffraction pattern of the stable phase suggested that the entire DDPC molecules do not form a two-dimensional molecular ordered lattice, differing from normal subgel phase of glycero-phosphatidylcholines. The structure and phase behavior of DDPC revealed by the present study are discussed from the viewpoint of hydrogen bonds.     

Triglycerides obtained by dry fractionation of milk fat: 2. Thermal properties and polymorphic evolutions on heating

The crystallographic and thermal properties of milk fat and fractions were investigated on heating using the coupling of synchrotron X-ray diffraction with differential scanning calorimetry. We showed that re-crystallisations occurred during the heating of the stearin and the olein fractions with the formation of a β′ 2L (41.1-42.6 Å) structure and a β′ 3L (66 Å) structure, respectively. By creating a quantified solid-liquid phase behaviour versus temperature diagram, the amount of the solid and liquid phases and the relative proportion of each of the crystalline structures within the solid phase were determined.     

Effect of urea, dimethylurea, and tetramethylurea on the phase behavior of dioleoylphosphatidylethanolamine

The phase behavior of dioleoylphosphatidylethanolamine in aqueous solutions of urea, N,N'-dimethylurea (DMU), and N,N,N',N'-tetramethylurea (TMU) has been characterized by synchrotron X-ray diffraction and differential scanning calorimetry. All three solutes stabilize the lamellar liquid-crystalline phase at the expense of lamellar-gel phase and inverted hexagonal phase of the phospholipid when present in concentrations up to 3 M. X-ray diffraction data demonstrated that the repeat spacing of DOPE increased with increasing urea concentration, but decreased as the DMU and TMU concentrations increased. The repeat spacing of DOPE in the liquid-crystal phase dispersed in the three solutes is d(urea)>d(DMU)>d(TMU). The molecular mechanisms underlying these observations are discussed in terms of either membrane Hofmeister effect, where urea acts as a water structure breaker, or a direct insertion effect of the amphiphilic DMU and TMU molecules into the lipid head groups in the interfacial region of the phospholipid bilayer.     

Interactions of La3+ with phosphatidylserine vesicles binding,phase transition,leakage and fusion

The interaction of La³⁺ with phosphatidylserine vesicles is elucidated by binding studies, differential scanning calorimetry, X-ray diffraction, freeze fracture electron microscopy, and release of vesicle contents. La³⁺ effectively competes with Ca²⁺ for phosphatidylserine binding sites. The saturation level is close to a La/lipid ratio of 1:3. A concentration of 0.1 mM of La³⁺ is sufficient to induce fusion between sonicated vesicles.     

The thermotropic phase behaviour and phase structure of a homologous series of racemic beta-D-galactosyl dialkylglycerols studied by differential scanning calorimetry and X-ray diffraction

The thermotropic phase behaviour of aqueous dispersions of some synthetic 1,2-di-O-alkyl-3-O-(beta-D-galactosyl)-rac-glycerols (rac-beta-D-GalDAGs) with both odd and even hydrocarbon chain lengths was studied by differential scanning calorimetry (DSC), small-angle (SAXS) and wide-angle (WAXS) X-ray diffraction. DSC heating curves show a complex pattern of lamellar (L) and nonlamellar (NL) phase polymorphism dependent on the sample's thermal history. On cooling from 95 degrees C and immediate reheating, rac-beta-D-GalDAGs typically show a single, strongly energetic phase transition, corresponding to either a lamellar gel/liquid-crystalline (L(beta)/L(alpha)) phase transition (N< or =15 carbon atoms) or a lamellar gel/inverted hexagonal (L(beta)/H(II)) phase transition (N> or =16). At higher temperatures, some shorter chain compounds (N=10-13) exhibit additional endothermic phase transitions, identified as L/NL phase transitions using SAXS/WAXS. The NL morphology and the number of associated intermediate transitions vary with hydrocarbon chain length. Typically, at temperatures just above the L(alpha) phase boundary, a region of phase coexistence consisting of two inverted cubic (Q(II)) phases are observed. The space group of the cubic phase seen on initial heating has not been determined; however, on further heating, this Q(II) phase disappears, enabling the identification of the second Q(II) phase as Pn3 m (space group Q(224)). Only the Pn3 m phase is seen on cooling. Under suitable annealing conditions, rac-beta-D-GalDAGs rapidly form highly ordered lamellar-crystalline (L(c)) phases at temperatures above (N< or =15) or below (N=16-18) the L(beta)/L(alpha) phase transition temperature (T(m)). In the N< or =15 chain length lipids, DSC heating curves show two overlapping, highly energetic, endothermic peaks on heating above T(m); corresponding changes in the first-order spacings are observed by SAXS, accompanied by two different, complex patterns of reflections in the WAXS region. The WAXS data show that there is a difference in hydrocarbon chain packing, but no difference in bilayer dimensions or hydrocarbon chain tilt for these two L(c) phases (termed L(c1) and L(c2), respectively). Continued heating of suitably annealed, shorter chain rac-beta-D-GalDAGs from the L(c2) phase results in a phase transition to an L(alpha) phase and, on further heating, to the same Q(II) or H(II) phases observed on first heating. On reheating annealed samples with longer chain lengths, a subgel phase is formed. This is characterized by a single, poorly energetic endotherm visible below the T(m). SAXS/WAXS identifies this event as an L(c)/L(beta) phase transition. However, the WAXS reflections in the di-16:0 lipid do not entirely correspond to the reflections seen for either the L(c1) or L(c2) phases present in the shorter chain rac-beta-D-GalDAGs; rather these consist of a combination of L(c1), L(c2) and L(beta) reflections, consistent with DSC data where all three phase transitions occur within a span of 5 degrees C. At very long chain lengths (N> or =19), the L(beta)/L(c) conversion process is so slow that no L(c) phases are formed over the time scale of our experiments. The L(beta)/L(c) phase conversion process is significantly faster than that seen in the corresponding rac-beta-D-GlcDAGs, but is slower than in the 1,2-sn-beta-D-GalDAGs already studied. The L(alpha)/NL phase transition temperatures are also higher in the rac-beta-D-GalDAGs than in the corresponding rac-beta-D-GlcDAGs, suggesting that the orientation of the hydroxyl at position 4 and the chirality of the glycerol molecule in the lipid/water interface influence both the L(c) and NL phase properties of these lipids, probably by controlling the relative positions of hydrogen bond donors and acceptors in the polar region of the membrane.     

The effect of dolichol on the structure and phase behaviour of phospholipid model membranes

The effect of dolichol C₉₅ on the structure and thermotropic phase behaviour of dipalmitoylphosphatidylcholine, dipalmitoylphosphatidylethanolamine and stearoyloleoylphosphatidylethanolamine has been examined by synchrotron X-ray diffraction and differential scanning calorimetry. The presence of dolichol C₉₅ had no detectible effects on the temperature of either the gel to ripple or the ripple to liquid-crystal phase transition of dipalmitoylphosphatidylcholine. A proportionate increase of a few degrees in the temperature of the gel to lamellar liquid-crystal phase transition is observed in dispersions of dipalmitoylphosphatidylethanolamine and significantly there is a decrease in the temperature of the lamellar to non-lamellar phase transition of stearoyloleoylphosphatidylethanolamine. There was no significant change in the bilayer repeat spacing of all three mixed dispersions in gel phase in the presence of up to 20 mol% dolichol C₉₅. Electron density calculations showed that there was no change of bilayer thickness of dipalmitoylphosphatidylcholine with incorporation of up to 7.5 mol% dolichol C₉₅. These data suggest that effect of dolichol on the phospholipid model membranes depend on both the head group and the hydrocarbon chains of the phospholipid molecules. The presence of dolichol in phosphatidylcholine bilayers conforms to a model in which the polyisoprene compound is phase separated into a central domain sandwiched between the two monolayers in gel phase. In bilayers of phosphatidylethanolamines dolichol tends to stabilize the bilayers in gel phase at low temperatures and destabilize the bilayers in lamellar disordered structure at high temperatures. Non-lamellar structures coexist with lamellar disordered phase over a wide temperature range suggesting that dolichol is enriched in domains of non-lamellar structure and depleted from lamellar phase. These findings are useful to understand the function of dolichol in cell membranes.     

Native fat globules of different sizes selected from raw milk: thermal and structural behavior

The aim of this study was to characterize differences in the thermal and structural behavior between different sized native milk fat globules. A novel microfiltration process permits the selection of native small fat globules (SFG, 1-3 microm) and large fat globules (LFG, >5 microm) in raw milk, that were analyzed by X-ray diffraction (XRD) coupled to differential scanning calorimetry (DSC). There were no major differences in triglyceride crystalline structures between SFG and LFG, after eliminating thermal history and the influence of cooling rates. The three main 3L and 2L crystalline structures appearing under slow cooling existed regardless of globule size. The supercooling increased for the SFG, mainly due to heterogeneous nucleation in winter milk, and also to compositional variations in spring milk. Differences appeared regarding stabilized crystalline forms at 20 degrees C and subsequent cooling: the SFG contained less 2L triglyceride structures than the LFG. These results can be important in dairy manufactures using tempering periods.     

Crystallization and phase behavior of 1,3-propanediol esters II. 1,3-propanediol distearate/1,3-propanediol dipalmitate (SS/PP) and 1,3-propanediol distearate/1,3-propanediol dimyristate (SS/MM) binary systems

Polymorphic influences on the phase behavior of two types of binary mixtures of saturated monoacid 1,3-propanediol esters (PADEs), dipalmitate/distearate (PP/SS) and dimyristate/distearate (MM/SS) were examined by X-ray diffraction (XRD), differential scanning calorimetry (DSC), and by solid fat content (SFC), hardness and microscopy measurements. Three stacking modes have been found in the PP/SS binary system. Mixed SS-PP bilayers were detected in all mixtures, SS-SS bilayers in x(PP)=0.0-0.4 mixtures and PP-PP bilayers in x(PP)=0.6-0.1 mixtures. Two different but close beta polymorphs and one beta' polymorph were detected for this system. beta' was only detected in x(PP)=0.5-0.9 mixtures for the mixed bilayers. For the MM/SS binary system, only MM-MM and SS-SS bilayers were detected and both solid phases crystallized in two different beta forms. XRD data evidenced clearly that the MM and SS components were completely immiscible in the solid state. The phase diagrams constructed using DSC data, exhibited a typical eutectic-type phase boundary. The presence of eutectics, the shape of the solidus lines as well as the analysis of the individual enthalpies of melting indicated typical phase separation for both systems. A thermodynamic study based on the Hildebrand equation and using the Bragg-Williams approximation for non-ideality of mixing confirmed the phase separation in the solid phase and suggested that the PP and SS were miscible in the liquid phase and that SS formed an ideal mixing with MM. Avrami analysis of SFC vs. time curves indicated heterogeneous nucleation and spherulitic crystal development from sporadic nuclei, and suggested that the nucleation rate was higher for the mixture at the eutectic composition. The relative hardness was correlated with the enthalpies, the final SFC and the microscopy measurements.     

The crystal structure and physicochemical characteristics of 2-hydroxy-N-[3(5)-pyrazolyl]-1,4-naphthoquinone-4-imine,a new antitrypanosomal compound

This study was designed to investigate the physical characteristics and crystalline structure of 2-hydroxy-N-[3(5)-pyrazolyl]-1,4-naphthoquinone-4-imine (PNQ), a new active compound against Trypanosoma cruzi, the causative agent of American trypanosomiasis. Methods used included differential scanning calorimetry, thermogravimetry, hot stage microscopy, polarized light microscopy (PLM), Fourier-transform infrared (FTIR) spectroscopy, and high-resolution X-ray powder diffraction (HR-XRPD). According to PLM and HR-XRPD data, PNQ crystallized as red oolitic crystals (absolute methanol) or prisms (dimethyl sulfoxide [DMSO]-water) with the same internal structure. The findings obtained with HR-XRPD data (applying molecular location methods) showed a monoclinic unit cell [a = 18.4437(1) A, b = 3.9968(2) A, c = 14.5304(1) A, alpha = 90 degrees , beta = 102.71(6) degrees , gamma = 90 degrees , V = 1044.9(1) A(3), Z = 4, space group P2(1)/c], and a crystal structure (excluding H-positions) described by parallel layers in the direction of the b-axis, with molecules held by homochemical (phenyl-phenyl and pyrazole-pyrazole) van der Waals interactions. In addition, FTIR spectra displayed the NH-pyrazole stretch overlapped with the OH absorption at 3222 cm(-1), typical of -NH and -OH groups associated through H-bondings; and a carbonyl stretching absorption at 1694 cm(-1), indicating a nonextensively H-bonded quinonic C=O, which was in accordance with the solved crystal structure of PNQ. The existence of such cohesive forces shed light on the thermoanalytical data, which revealed that PNQ is a stable solid, unaffected by oxygen that decomposed without melting above 260 degrees C.     

Influence of cross-linked potato starch treated with POCl3 on DSC, rheological properties and granule size

Differential scanning calorimetry (DSC), rheological measurements and granule size analyses were performed to characterize the influence of phosphorylation substitution levels on the properties of cross-linked potato starch. Phosphorus oxychloride (POCl₃) was used to produce the cross-linked potato starch. The levels of the reagent used for the reaction ranged between 40 and 5000 ppm (dwb). Storage (G′) and loss (G″) moduli were measured for a 5% (w/w) gelatinized starch dispersion stored at 20 °C for 24 h after heating at 85 °C for 30 min. The samples from 80 to 500 ppm were recognized as ‘strong gel'systems, whereas native potato starch showed ‘weak gel'behavior. Steady shear and dynamic viscoelastic properties of gelatinized starch dispersion were compared. Furthermore, granule mean diameter was measured by laser scattering for a 1% (w/w) dispersion heated at 85 °C for 30 min. The granules in the 100 ppm sample swelled to a maximum of about 2.6 times the native starch granule mean diameter.     

Interactions of La with phosphatidylserine vesicles binding, phase transition, leakage and fusion

The interaction of La³⁺ with phosphatidylserine vesicles is elucidated by binding studies, differential scanning calorimetry, X-ray diffraction, freeze fracture electron microscopy, and release of vesicle contents. La³⁺ effectively competes with Ca²⁺ for phosphatidylserine binding sites. The saturation level is close to a La/lipid ratio of 1:3. A concentration of 0.1 mM of La³⁺ is sufficient to induce fusion between sonicated vesicles.     

Interaction of a new anticancer prodrug, gemcitabine-squalene, with a model membrane : Coupled DSC and XRD study

Gemcitabine is an anticancer nucleoside analogue active against a wide variety of solid tumors. However it is rapidly deaminated to an inactive metabolite, leading to short biological half-life and induction of resistance. A new prodrug of gemcitabine, coupling squalene to gemcitabine (GemSq), has been designed to overcome the above drawbacks. It has been previously shown that this prodrug displays significantly higher anticancer activity than gemcitabine against leukemia. In the present study the structural modifications of dipalmitoylphosphatidylcholine (DPPC) model membranes induced by increasing concentrations of GemSQ have been investigated using small and wide angle X-ray scattering (SWAXS) and differential scanning calorimetry (DSC). At room temperature an unusual inverse bicontinuous cubic phase formed over a broad composition range. The basic bilayer structure displayed an intermediate order between those of the gel and fluid phases of DPPC. A reversible transition to a fluid lamellar phase occurred upon heating. The transitions between these two phases were governed by different mechanisms depending on the GemSq concentration in the membrane. Finally, the biological relevance of these observations for the cytotoxic activity of GemSq has been discussed.     

New organoantimony complexes with the isomers of chlorophenylacetic acid: Syntheses, characterizations and crystal structures of 1D polymeric chain, 2D network structure and 3D framework

A series of novel organoantimony(V) complexes have been synthesised by the reactions of the isomers of chlorophenylacetic acids with triphenylantimony(V) dichloride or tetraphenylantimony(V) bromide in 1:2 or 1:1 stoichiometries. All the complexes have been characterized by elemental analysis, IR and NMR (¹H, ¹³C) spectra analyses; furthermore, complexes 1, 2, 3 and 4 have been determined by X-ray single crystal diffraction. The structure of complexes show that the five-coordinated and six-coordinated antimony(V) atoms adopt distorted trigonal bipyramidal geometry and octahedral geometry. And the structural analyses show that complexes 1 and 3 have 2D network structures; complex 2 possesses a 1D polymeric chain structure and complex 4 has a 3D supramolecular framework.     

Synthesis, structure and magnetic behavior of new 1D metal-organic coordination polymer with Fe3O core

New metal-organic coordination polymer, (Me₃NH)[Fe₃(μ₃-O)(μ-HCOO)₈]·DMF (1), was synthesized by heating of Fe(NO₃)₃·9H₂O and [NH(CH₃)₃]Cl in DMF/HCOOH solution. The single-crystal X-ray structure analysis reveals this compound has 1D anionic framework structure built from Fe₃O(μ-HCOO)₆ cluster units linked by formate bridges. Compound 1 was characterized by elemental analysis, IR and TGA. The magnetic susceptibility measurements indicate a strong antiferromagnetic exchange interaction between Fe^III ions in the cluster and intercluster magnetic interaction.     

Effect of fatty acid chain length,fatty acid hydroxylation,and various cations on phase behavior of synthetic cerebroside sulfate

Synthetic species of CBS containing palmitic, stearic, lignoceric, D-2-hydroxy palmitic, or D-2-hydroxy stearic acid were prepared and their phase behavior in the presence of a number of mono- and divalent cations was studied by differential scanning calorimetry and the use of fatty acid spin labels. The results showed that both the non-hydroxy fatty acid (NFA) and hydroxy fatty acid (HFA) forms of cerebroside sulfate (CBS) can occur in two different gel states, a metastable state and a lower entropy stable state. The phase behavior is more sensitive to the type and concentration of cation present than in the case with acidic phospholipids. The sensitivity of the transition temperature (T_m) to cation concentration reflects, in part, increased participation of the lipid in intermolecular hydrogen bonding interactions as the negative charge of the sulfate is shielded. The extra hydroxyl group on the HFA also contributes to the intermolecular hydrogen bonding network causing a significant increase in the T_m.The HFA has an even more significant effect in causing inhibition of formation of the stable state. Formation of the stable state is also inhibited by Li⁺ and divalent cations. A similar mechanism may be involved, i.e.; cross-linking of adjacent lipids or increased intermolecular interactions inhibit the molecular rearrangement necessary to form the stable state. This inhibition is counteracted by an increase in fatty acid chain length. The results suggest that the stable state may be interdigitated as a result of the unequal chain length between the sphingosine base and the fatty acid.     

The cyanocarbanion (C[C(CN)2]3) as monodentate ligand: Synthesis, structure and magnetic properties of [Mn2(bpym)3(tcpd)2(H2O)2] (tcpd = (C[C(CN)2]3) and bpym = 2,2′-bipyrimidine)

One-pot reaction between MnCl₂·4H₂O, K₂tcpd (tcpd²⁻ = [C₁₀N₆]²⁻ = (C[C(CN)₂]₃)²⁻ = 2-dicyanomethylene-1,1,3,3-tetracyanopropanediide anion) and 2,2′-bipyrimidine (bpym = C₈H₆N₄) in aqueous solution yields the new compound [Mn₂(bpym)₃(tcpd)₂(H₂O)₂] (1). The molecular structure of 1 consists of a centrosymmetrical binuclear complex which includes unprecedented unidentate tcpd ligands with two bidentate and a bis-chelate bpym units. Examination of the intermolecular distances reveals that the dinuclear units are held together by hydrogen bonds involving coordinated water molecules and two nitrile groups of the tcpd ligand, giving rise to a 2D structure overall. Variable-temperature magnetic susceptibility data show the occurrence of slight antiferromagnetic coupling (J = −0.58 cm⁻¹) between the Mn(II) ions through bridging bpym (the exchange Hamiltonian being defined as ).     

Acid-base behavior of a simple metal bis(dithiolate) system: Synthesis, crystal structure and spectroscopy of [Bu4N]2[M(ppdt)2] (M = Ni, Pt; ppdt = pyrido[2,3-b]pyrazine-2,3-dithiolate)

The syntheses, crystal structures and properties of compounds [Bu₄N]₂[Ni(ppdt)₂] (1) and [Bu₄N]₂[Pt(ppdt)₂] (2) (ppdt = pyrido[2,3-b]pyrazine-2,3-dithiolate) have been described. Compound 1 crystallizes in P2₁/c space group (monoclinic system), whereas compound 2 crystallizes in C2/c space group (monoclinic system). The crystal structures of both compounds 1 and 2 have been characterized by C-H?S and C-H?N hydrogen bonding interactions between cation and anions resulting in three-dimensional supramolecular networks in the crystals of 1 and 2, respectively. The acid-base behavior of the ground states of both [Bu₄N]₂[Ni(ppdt)₂] (1) and [Bu₄N]₂[Pt(ppdt)₂] (2) and also the excited state of compound [Bu₄N]₂[Pt(ppdt)₂] (2) in solutions has been studied. The pH dependent changes in the charge transfer absorption and emission spectra are attributed to the protonation on an imine nitrogen of the ppdt ligand. The ground-state basicity constants of the two complexes 1 and 2 have been determined from spectrophotometric analysis by titrating with an weak acid, yielding pK_b1 = 8.0 for complex [Bu₄N]₂[Ni(ppdt)₂] (1) and pK_b1 = 7.8 for complex [Bu₄N]₂[Pt(ppdt)₂] (2). The excited-state basicity constant pK_b1^* for complex [Bu₄N]₂[Pt(ppdt)₂] (2) has been determined by a thermodynamic equation using a Förster analysis yielding the value of 1.8. The complex 2 is electrochemically irreversible with an oxidation potential of E_1/2 = +0.41 V versus Ag/AgCl in methanol.     

Structure and phase behavior of O-stearoylethanolamine: A combined calorimetric, spectroscopic and X-ray diffraction study

Recent studies show that O-acylethanolamines (OAEs), structural isomers of the putative stress-fighting lipids, namely N-acylethanolamines (NAEs), can be derived from NAEs and are present in biological membranes under physiological conditions. In view of this, we have synthesized O-stearoylethanolamine (OSEA) as a representative OAE and investigated its phase behavior and crystal structure. The thermotropic phase transitions of OSEA dispersed in water and in 150 mM NaCl were characterized using calorimetric, spectroscopic, turbidimetric and X-ray diffraction studies. These studies have revealed that when dispersed in water OSEA undergoes a cooperative phase transition centered at 53.8 °C from an ordered gel phase to a micellar structure whereas in presence of 150 mM NaCl the transition temperature increases to 55.8 °C and most likely the bilayer structure is retained above the phase transition. O-Stearoylethanolamine crystallized in the orthorhombic space group P2₁2₁2₁ with four symmetry-related molecules in the unit cell. Single-crystal X-ray diffraction studies show that OSEA molecules adopt a linear structure with all-trans conformation in the acyl chain region. The molecules are organized in a tail-to-tail fashion, similar to the arrangement in a bilayer membrane. These studies are relevant to understanding the role of salt on the phase properties of this new class of lipids.