Effect of double bond position on lipid bilayer properties: insight through atomistic simulations

Phosphatidylcholines (PCs) are among the most common phospholipids in plasma membranes. Their structural and dynamic properties are known to be strongly affected by unsaturation of lipid hydrocarbon chains, yet the role of the exact positions of the double bonds is poorly understood. In this work, we shed light on this matter through atomic-scale molecular dynamics simulations of eight different one-component lipid bilayers comprised of PCs with 18 carbons in their acyl chains. By introducing a single double bond in each acyl chain and varying its position in a systematic manner, we elucidate the effects of a double bond on various membrane properties. Studies in the fluid phase show that a number of membrane properties depend on the double bond position. In particular, when the double bond in an acyl chain is located close to the membrane-water interface, the area per lipid is considerably larger than that found for a saturated lipid. Further, when the double bond is shifted from the interfacial region toward membrane center, the area per lipid is observed to increase and have a maximum when the double bond is in the middle of the chain. Beyond this point, the surface area decreases systematically as the double bond approaches membrane center. These changes in area per lipid are accompanied by corresponding changes in membrane thickness and ordering of the chains. Further changes are observed in the tilt angles of the chains, membrane hydration together with changes in the number of gauche conformations, and direct head group interactions. All of these effects can be associated with changes in acyl chain conformations and local effects of the double bond on the packing of the surrounding atoms.     

Synthesis,Crystal Structure and Spectroscopy of <Emphasis Type="Italic">catena-</Emphasis>poly-bis(azido-<Emphasis Type="Italic">N1,N1</Emphasis>)(2-Aminopyrimidine)Copper(II)

Abstract The compound [Cu(ampym)(μ_1,1-N₃)₂]_n (ampym = 2-aminopyrimidine) has been synthesized and characterized by X-ray crystallography and infrared spectroscopy. In addition, Ligand Field and powder EPR measurements have been performed. The structure is solved in space group P21/c with a = 7.303(2), b = 19.716(4), c = 5.949(1) ?, β = 98.17(3), V = 847.9(3) ?³, Z = 2 with final R = 0.0382. The coordination geometry around the Cu(II) ion is distorted square pyramidal, with four nitrogen atoms of four bridging azido anions in the basal plane with Cu–N distances that range from 1.998(3) to 2.069(3) ?. The apical position is occupied by a nitrogen atom of the ampym molecule at a Cu–N distance of 2.169(3) ?. The trans-basal angles are 165.7(1) and 143.9(1)°. Weak hydrogen bonding is observed between the two amine hydrogen atoms and nitrogen of an azide anion and the pyrimidine-ring nitrogen atom of a neighbouring molecule (N···N distances 3.174(5), 3.106(4) ?). These last hydrogen bonds (N7···N3) are forming so-called “Watson-Crick type” hydrogen bonds. In the infrared the vibrations of the coordinated azide anion are observed at 2,062, 1,273 and 655 cm⁻¹, while the Cu–N vibrations are observed at 370 and 224 cm⁻¹. Ligand-field and EPR spectra are uneventful and give spectral parameters expected in the range for such Cu(II) compounds. Magnetic susceptibility measurements reveal a weak antiferromagnetic interaction between the Cu(II) ions. Index Abstract A new Cu(II) compound, [Cu(2-aminopyrimidine)(μ_1,1-N₃)₂]_n has been synthesized and characterized by X-ray crystallography and by IR, Ligand Field, EPR spectroscopy and magnetic susceptibility. The coordination geometry around the Cu(II) ion is distorted square pyramidal, with four nitrogen atoms of four bridging azido ligands in the basal plane with Cu–N distances that range from 1.998(3) to 2.069(3) ?. The apical position is occupied by a nitrogen atom of the ampym molecule at a Cu–N distance of 2.169(3) ?. Also a very interesting hydrogen bond system occurs between pairs of ligands in the lattice. .     

Mass spectra of halogenated esters 6—Methyl esters of some trihalogenated propanoic and butanoic acids

The mass spectral fragmentation of trihalogenated methyl esters, formed in the reactions of monochlorinated methyl propenoates and 2-butenoates with Cl₂, BrCl and Br₂, have been investigated. In most cases α-cleavage gives the base peak, [COOCH₃]⁺, the peaks originating from the subsequent losses of one or two halogen atoms also being abundant. The primary loss of a halogen atom is more prominent in the C₄ derivatives, Br˙ and Cl˙ being preferentially lost from the 2- and 3-positions, respectively. The McLafferty rearrangement yields in one case the base peak; the 2-halo compounds could in general be distinguished by that fragmentation. Typical for all 2-bromo-substituted methyl butanoates studied is the base peak, [C₃H₃]⁺, at m/z 39, and for some 3-halo compounds the peaks at m/z 95, [C₂H₄ClO₂]⁺ and 139, [C₂H₄BrO₂]⁺.     

Mass spectra of chlorinated aromatics formed in pulp bleaching 2—Chlorinated guaiacols

The fragmentation of chlorinated guaiacols (2-methoxyphenols) on electron impact has been studied. The most common fragmentation processes are interpreted and in some cases the small differences between spectra of positional isomers are explained. In addition to the well-known alkyl-oxygen fission (loss of methyl radical), metastable ion studies and deuterium labelling have indicated several new fragmentation pathways. The most characteristic are the formation of [M? CH₃? HCl]⁺ and [M? CH₃? Cl]^+· ions. In general, however, the spectra of positional isomers are shown to be very similar.     

Addition of chlorine,bromine and bromine chloride to some α,β-unsaturated methyl esters

The additions of chlorine, bromine and bromine chloride to trans methyl 2-butenoate 1, trans methyl 2-methyl-2-butenoate 2 and methyl 3-methyl-2-butenoate 3 under ionic conditions were studied. Bromine chloride addition always gave as a major regioisomer the 2-bromo-3--chloro compound,almost quantitatively in the case of 3. The mechanism of bromonium ion ring-opening (S_N1 or S_N2) is discussed with respect to the double bond substitution and regioisomer proportions. The dihalo products were identified by MS, ¹H and ¹³C NMR.     

Effect of NaCl and KCl on phosphatidylcholine and phosphatidylethanolamine lipid membranes: insight from atomic-scale simulations for understanding salt-induced effects in the plasma membrane

To gain a better understanding of how monovalent salt under physiological conditions affects plasma membranes, we have performed 200 ns atomic-scale molecular dynamics simulations of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) lipid bilayers. These two systems provide representative models for the outer and inner leaflets of the plasma membrane, respectively. The implications of cation-lipid interactions in these lipid systems have been considered in two different aqueous salt solutions, namely NaCl and KCl, and the sensitivity of the results on the details of interactions used for ions is determined by repeating the simulations with two distinctly different force fields. We demonstrate that the main effect of monovalent salt on a phospholipid membrane is determined by cations binding to the carbonyl region of a membrane, while chloride anions mostly stay in the water phase. It turns out that the strength and character of the cation-lipid interactions are quite different for different types of lipids and cations. PC membranes and Na+ ions demonstrate strongest interactions, leading to notable membrane compression. This finding was confirmed by both force fields (Gromacs and Charmm) employed for the ions. The binding of potassium ions to PC membranes (and the overall effect of KCl), in turn, was found to be much weaker mainly due to the larger size of a K+ ion compared to Na+. Furthermore, the effect of KCl on PC membranes was found to be force-field sensitive: The binding of a potassium ion was not observed at all in simulations performed with the Gromacs force-field, which seems to exaggerate the size of a K+ ion. As far as PE lipid bilayers are concerned, they are found to be influenced by monovalent salt to a significantly lesser extent compared to PC bilayers, which is a direct consequence of the ability of PE lipids to form both intra- and intermolecular hydrogen bonds and hence to adopt a more densely packed bilayer structure. Whereas for NaCl we observed weak binding of Na+ cations to the PE lipid-water interface, in the case of KCl we witnessed almost complete lack of cation binding. Overall, our findings indicate that monovalent salt ions affect lipids in the inner and outer leaflets of plasma cell membranes in substantially different ways.     

Mass spectra of halogenated esters 5—Chloromethyl esters of aliphatic C2?C12 n-carboxylic acids and their monochlorinated derivatives

A study has been made of the mass spectral fragmentation upon electron impact of aliphatic C₂? C₁₂ chloromethyl esters and all their 66 monochlorinated derivatives. The fragmentation pathways of the parent chloromethyl esters were elucidated with the aid of the 1st FFR metastable ions. A McLafferty rearrangement gives the base peak in the C₆? C₁₁ parent esters and in almost all the 4-chloro and ω-chloro isomers. The subsequent loss of HCl gives a very characteristic peak of the chloromethyl esters and their (3-ω)-chloro derivatives at m/z 72, [C₃H₄O₂]⁺. The 2-chloro isomers have the corresponding chlorine-containing fragment ion at m/z 106/108. The mass spectra of 2-, 3-, 4-, 5- and ω-chloro isomers give the characteristic fragment ions, the mass spectra of the other isomers being very similar.     

Synthesis and characterisation of cyclopentadienyl complexes of barium: precursors for atomic layer deposition of BaTiO3

Cyclopentadienyl complexes Ba(C5Me5)2(THF)2 (1), Ba(C5Me5)2(A) (A = THF, dien, trien, diglyme, triglyme) (2-5), Ba(Pr(i)3C5H2)2(THF)2 (6), Ba(Bu(t)3C5H2)2(THF) (7), Ba(Me2NC2H4C5Me4)2 (8) and Ba(EtOC2H4C5Me4)2 (9) were prepared and characterised with TGA/SDTA, NMR and MS. Crystal structures of 2, 4, 5, 7, 8 and 9 are presented. All complexes prepared sublime under reduced pressure and complexes 1, 6 and 7 showed volatility also under atmospheric pressure. Complexes 1, 6 and 7 lose the coordinated THF when evaporated while complexes 2-5 are sublimable as complete molecules under reduced pressure. Complexes with bulky cyclopentadienyl ligands (6 and 7) are the most thermally stable and volatile among the prepared barocenes. X-ray structure determinations reveal that all the complexes studied are monomeric. Complexes 1, 7 and 8 were successfully tested in BaTiO3 thin film depositions by atomic layer deposition (ALD).