Lateral Diffusion of Lipids and Diffusion of Water through Lipid Bilayers in the Presence of (1,1-Dimethyl-3-Oxobutyl)Phosphonates

The influence of some amphiphilic (diethyl, dipropyl, and dibutyl) esters of (1,1-dimethyl-3-oxobutyl)phosphonic acid with the regularly changing number of CH₂ groups in the hydrocarbon (hydrophobic) moiety on the lateral diffusion of dioleoyl phosphatidylcholine lipid and transmembrane diffusion of water in the oriented multibilayer system was studied by ¹H pulsed field gradient NMR at phosphonate concentrations up to 30 mol %. The shape of the ³¹P NMR spectra and the dependence of the shape of the ¹H NMR spectra on the bilayer orientation suggest that the presence of phosphonates does not affect the phase state of the system. The lamellar liquid crystalline phase remains unchanged, and phosphonate molecules become incorporated into the bilayer and have the same orientation as phospholipid molecules. The presence of phosphonates in the lipid bilayer increases the coefficients of lipid lateral diffusion and water diffusion through bilayers. This effect depends monotonically on the number of CH₂ groups in the phosphonate molecule. The most probable place for the incorporation of amphiphilic phosphonate molecules is the hydrophilic/hydrophobic interphase region of the bilayer. The molecules incorporated into the interphase disorder the bilayer and increase lateral diffusion of lipids and bilayer permeability compared with the ester-free bilayer. When the number of CH₂ groups in the ester molecule increases from diethyl to dibutyl phosphonate, the arrangement of lipid hydrocarbon tails becomes more ordered. This decreases the lipid lateral diffusion coefficient and bilayer permeability to water molecules.     

Phase Transition,Ordering and Lateral Diffusion in Phospholipid Bilayers in the Presence of Poly(Ethylene Oxide)

The thermal behaviour, molecular orientation and lateral diffusion in the bilayered systems of dimyristoylphosphatidylcholine (DMPC) in the presence of poly(ethylene oxide) (PEO) were studied by NMR and DSC techniques, and it was found that PEO decreases the melting temperature (of vesicles and flat multibilayers) and affects the degree of orientation of DMPC molecules relative to the bilayer normal, but it does not influence the lateral diffusion of DMPC molecules.     

Influence of perfluorinated compounds on the properties of model lipid membranes

The influence of selected perfluorinated compounds (PFCs), perfluorooctanoic acid (PFOA) or perfluorooctanesulfonic acid (PFOS), on the structure and organization of lipid membranes was investigated using model membranes-lipid monolayers and bilayers. The simplest model--a lipid monolayer--was studied at the air-water interface using the Langmuir-Blodgett technique with surface pressure and surface potential measurements. Lipid bilayers were characterized by NMR techniques and molecular dynamics simulations. Two phospholipids, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), characterized by different surface properties have been chosen as components of the model membranes. For a DPPC monolayer, a phase transition from the liquid-expanded state to the liquid-condensed state can be observed upon compression at room temperature, while a DMPC monolayer under the same conditions remains in the liquid-expanded state. For each of the two lipids, the presence of both PFOA and PFOS leads to the formation of a more fluidic layer at the air-water interface. Pulsed field gradient NMR measurements of the lateral diffusion coefficient (DL) of DMPC and PFOA in oriented bilayers reveal that, upon addition of PFOA to DMPC bilayers, DL of DMPC decreases for small amounts of PFOA, while larger additions produce an increased DL. The DL values of PFOA were found to be slightly larger than those for DMPC, probably as a consequence of the water solubility of PFOA. Furthermore, 31P and 2H NMR showed that the gel-liquid crystalline phase transition temperature decreased by the addition of PFOA for concentrations of 5 mol % and above, indicating a destabilizing effect of PFOA on the membranes. Deuterium order parameters of deuterated DMPC were found to increase slightly upon increasing the PFOA concentration. The monolayer experiments reveal that PFOS also penetrates slowly into already preformed lipid layers, leading to a change of their properties with time. These experimental observations are in qualitative agreement with the computational results obtained from the molecular dynamics simulations showing a slow migration of PFCs from the surrounding water phase into DPPC and DMPC bilayers.     

Molecular dynamics investigation of dynamical properties of phosphatidylethanolamine lipid bilayers

We describe the dynamic behavior of a 1-stearoyl-2-oleoyl-phosphatidylethanolamine (SOPE) bilayer from a 20 ns molecular dynamics simulation. The dynamics of individual molecules are characterized in terms of (2)H spin-lattice relaxation rates, nuclear overhauser enhancement spectroscopy (NOESY) cross-relaxation rates, and lateral diffusion coefficients. Additionally, we describe the dynamics of hydrogen bonding through an analysis of hydrogen bond lifetimes and the time evolution of clusters of hydrogen bonded lipids. The simulated trajectory is shown to be consistent with experimental measures of internal, intermolecular, and diffusive motion. Consistent with our analysis of SOPE structure in the companion paper, we see hydrogen bonding dominating the dynamics of the interface region. Comparison of (2)H T(1) relaxation rates for chain methylene segments in phosphatidylcholine and phosphatidylethanolamine bilayers indicates that slower motion resulting from hydrogen bonding extends at least three carbons into the hydrophobic core. NOESY cross-relaxation rates compare well with experimental values, indicating the observed hydrogen bonding dynamics are realistic. Calculated lateral diffusion rates (4 +/ -1 x 10(-8) cm(2)s) are comparable, though somewhat lower than, those determined by pulsed field gradient NMR methods.     

Combined NMR and quantum chemical studies on the interaction between trehalose and dienes relevant to the antioxidant function of trehalose

In a previous study (Oku, K.; Watanabe, H.; Kubota, M.; Fukuda, S.; Kurimoto, M.; Tujisaka, Y.; Komori, M.; Inoue, Y.; Sakurai, M. J. Am. Chem. Soc. 2003, 125, 12739), we investigated the mechanism of the antioxidant function of trehalose against unsaturated fatty acids (UFAs) and revealed that the key factor relevant to the function is the formation of OH...pi and CH...O hydrogen bonds between trehalose and the cis double bonds of the UFA. Here, we investigate whether such intriguing interactions also occur between this sugar and cis double bonds in other unsaturated compounds. For this purpose, we selected various diene compounds (1,3-butadiene, 1,3-pentadiene, 1,4-pentadiene, and 2,5-heptadiene) as interaction partners. All NMR experiments performed, including 1H-1H NOESY measurements, indicated that trehalose selectively interacts with the cis-olefin proton pair in the above diene with a 1:1 stoichiometry, and the C-3 (C-3') and C-6' (C-6) sites of the sugar are responsible for the interaction. Similar interactions were not observed for the mixtures of the diene and other saccharides (neotrehalose, kojibiose, nigerose, maltose, isomaltose, sucrose, maltitol, and sorbitol). Quantum chemical calculations revealed that the OH-3 and OH-6 groups bind to the olefin double bonds of the diene through OH...pi and CH...O types of hydrogen bonds, respectively, and the stabilization energy of the resulting complex is 5-6 kcal mol(-1). These results strongly support the above NMR results. Finally, the activation energies were calculated for the hydrogen abstraction reactions from the activated methylene group of heptadiene. In particular, when the reaction was initiated by a methyl radical, the activation energy was only 10 kcal mol(-1) for the free heptadiene, but on complexation with trehalose it drastically increased to ca. 40 kcal mol(-1). This indicates that trehalose has a significant depression effect on the oxidation of the diene compounds. These results strongly support the antioxidant mechanism deduced in the previous study and indicate that the formation of unique multiple hydrogen bonds between trehalose and cis-olefin bonds is rather a general event not confined to the case of UFA.     

Temperature driven annealing of perforations in bicellar model membranes

Bicellar model membranes composed of 1,2-dimyristoylphosphatidylcholine (DMPC) and 1,2-dihexanoylphosphatidylcholine (DHPC), with a DMPC/DHPC molar ratio of 5, and doped with the negatively charged lipid 1,2-dimyristoylphosphatidylglycerol (DMPG), at DMPG/DMPC molar ratios of 0.02 or 0.1, were examined using small angle neutron scattering (SANS), (31)P NMR, and (1)H pulsed field gradient (PFG) diffusion NMR with the goal of understanding temperature effects on the DHPC-dependent perforations in these self-assembled membrane mimetics. Over the temperature range studied via SANS (300-330 K), these bicellar lipid mixtures exhibited a well-ordered lamellar phase. The interlamellar spacing d increased with increasing temperature, in direct contrast to the decrease in d observed upon increasing temperature with otherwise identical lipid mixtures lacking DHPC. (31)P NMR measurements on magnetically aligned bicellar mixtures of identical composition indicated a progressive migration of DHPC from regions of high curvature into planar regions with increasing temperature, and in accord with the "mixed bicelle model" (Triba, M. N.; Warschawski, D. E.; Devaux, P. E. Biophys. J.2005, 88, 1887-1901). Parallel PFG diffusion NMR measurements of transbilayer water diffusion, where the observed diffusion is dependent on the fractional surface area of lamellar perforations, showed that transbilayer water diffusion decreased with increasing temperature. A model is proposed consistent with the SANS, (31)P NMR, and PFG diffusion NMR data, wherein increasing temperature drives the progressive migration of DHPC out of high-curvature regions, consequently decreasing the fractional volume of lamellar perforations, so that water occupying these perforations redistributes into the interlamellar volume, thereby increasing the interlamellar spacing.     

Inclusion complex of n-octyl beta-D-glucopyranoside and alpha-cyclodextrin in aqueous solutions: thermodynamic and structural characterization

Complex formation between octyl beta-D-glucopyranoside (OG) and alpha-cyclodextrin (alphaCD) was investigated on the basis of three highly accurate and appropriate experimental techniques. First, surface tension measurements showed that alphaCD directly acts on the surfactant monomers in the aqueous phase, leading to progressive depletion of the air-water interface with increasing cyclodextrin contents. Significant shift of OG critical micelle concentration (cmc) was consequently observed: the higher alphaCD concentration, the higher the cmc value. Experiments performed at surfactant and cyclodextrin concentrations in the Gibbs regime of surface tension versus OG content were performed on one hand to establish Job's plot that showed 1:1 stoichiometry of the OG-alphaCD complex and on the other hand to calculate the association constant found equal to (1.85 +/- 0.35) x 10(3) L mol(-1). An inclusion process of the surfactant alkyl residue within the cyclodextrin cavity was confirmed by one-dimensional (1)H NMR, and the structure of the mixed assembly was extensively characterized by two-dimensional NOESY (1)H NMR. OG penetrates alphaCD so that its hydrocarbon chain is embedded inside the cyclodextrin cavity, and its polar head as well as the alpha-methylene group emerges outside the alphaCD secondary face. Solubility behavior of the OG-alphaCD complex in a wide range of host-guest ratios and concentrations was finally examined by turbidity recording and optical microscopy. At very low free cyclodextrin levels in the solution, the complex presented high solubility behavior up to more than 70 mM. By increasing nonassociated alphaCD in the mixture, propensity of the cyclodextrin molecules to crystallize was observed at concentrations far below the 100 mM aqueous solubility of the pure cyclodextrin. The hexagonal shape of the crystals seen in the optical microscopy images suggested they were, partially at least, composed of the solid complex.     

Density functional theory-based conformational analysis of a phospholipid molecule (dimyristoyl phosphatidylcholine)

The conformational space of the dimyristoyl phosphatidylcholine (DMPC) molecule has been studied using density functional theory (DFT), augmented with a damped empirical dispersion energy term (DFT-D). Fourteen ground-state isomers have been found with total energies within less than 1 kcal/mol. Despite differences in combinations of their torsion angles, all these conformers share a common geometric profile, which includes a balance of attractive, repulsive, and constraint forces between and within specific groups of atoms. The definition of this profile fits with most of the structural characteristics deduced from measured NMR properties of DMPC solutions. The calculated vibrational spectrum of the molecule is in good agreement with experimental data obtained for DMPC bilayers. These results support the idea that DMPC molecules preserve their individual molecular structures in the various assemblies.     

Complexation behavior of alpha-, beta-, and gamma-cyclodextrin in modulating and constructing polymer networks

A systematic study of the host-guest complexation by alpha-, beta-, and gamma-cyclodextrin (CD) in either the free state or as substituents of poly(acrylic acid) (PAA) with the hydrophobic n-octadecyl groups, C18, substituted onto PAA (HMPAA) and its effect on polymer aggregation and network formation is reported. Free alpha-CD, beta-CD, and gamma-CD mask hydrophobic associations between the C18 substituent of HMPAA in aqueous solution and form host-guest complexes with a 1:1 or CD:C18 substituent stoichiometry at 0.5 wt % polymer concentration. For alpha-CD this host-guest stoichiometry changes to 2:1 or 2alpha-CD:C18 at > or =1 wt % polymer concentrations but not for beta-CD and gamma-CD. Shear-thickening occurs when gamma-CD complexes C18 HMPAA substituents. Upon addition of sodium dodecyl sulfate, SDS (SDS:CD = 1:1), the hydrophobic associations between C18 diminished by alpha-CD masking were fully restored, were only partly restored in the case of beta-CD, and not restored for gamma-CD. When alpha- and beta-CD substituted PAA (alpha-CDPAA and beta-CDPAA) were mixed with HMPAA polymer, networks formed. As for free beta-CD, the beta-CD substituents of beta-CDPAA also formed 1:1 or beta-CD:C18 stoichiometry host-guest complexes with the C18 substituents of HMPAA. The alpha-CD substituents of alpha-CDPAA also formed 1:1 or alpha-CD:C18 stoichiometry host-guest complexes with some indication of the formation of 2:1 or 2alpha-CD:C18 stoichiometry host-guest complexes at polymer concentrations > or =1 wt %. The polymer networks formed by beta-CDPAA with HMPAA are less viscous than those formed by alpha-CDPAA, for which shear-thickening occurs at polymer concentrations > or =2 wt %. It is evident that the difference in CD annular size and its match with the C18 of HMPAA control the diversity of the interactions of alpha-CD, beta-CD, gamma-CD, alpha-CDPAA, and beta-CDPAA with HMPAA.     

Influence of polyaspartic acid and phosphophoryn on octacalcium phosphate growth kinetics

Polyaspartic acid (PAA) and phosphophoryn (PPn) have been suggested to adsorb specifically on the (100) and (010) faces, respectively, of octacalcium phosphate (Ca₄H(PO₄)₃ · 2.5H₂O, OCP). In this study, the extent of adsorption and influence of these molecules on OCP crystal growth has been investigated. For kinetic studies, protein effects on crystal growth were examined in solutions sustained at a constant level of supersaturation at pH 6.00 and ionic strength of 0.08 mol l⁻¹. The maximum adsorbed mol surface concentration for PPn was 100-fold less than that for PAA. Inhibitory effects interpreted in terms of mol surface coverage showed PPn to retard OCP growth more effectively than PAA. However, when considering percentage of crystal face covered by protein, PAA and PPn showed similar maximum adsorption concentrations onto the (100) and (010) crystal faces, respectively. PAA inhibited OCP growth by 20% when only 1% of the (100) face (1% total crystal area) was covered. PPn had to reach over 200% (010) face coverage (or 28% total crystal area) before a similar level of crystal growth inhibition was obtained. This difference in inhibitory effect may be the result of a more effective β-strand conformation of the shorter PAA molecule or may indicate that growth at the (100) face is rate controlling and, therefore, less than 1% coverage of this face is needed before a significant decrease in rate is observed.     

Multi-dimensional pulsed field gradient magic angle spinning NMR experiments on membranes

The benefits of gradient techniques in the study of lipid membranes are demonstrated on a sample of 1-palmitoyl-2-oleoyl-sn-glycero-3 phosphocholine (POPC) liposomes embedded with ibuprofen. Most techniques from gradient NMR spectroscopy on solution samples are directly applicable to membrane samples subjected to magic angle spinning (MAS). Gradient-enhanced homo- and heteronuclear chemical shift correlation techniques were used to make resonance assignments. Gradient NOESY experiments provide insight into the location and dynamics of lipids, ibuprofen and water. Application of gradients not only reduces experiment time but also the t(1) noise in the multi-dimensional spectra. Diffusion measurements with pulsed field gradients characterize lateral movements of lipid and drug molecules in membranes. The theoretical framework for data analysis of MAS diffusion experiments on randomly oriented multilamellar liposomes is presented.     

Polyacrylamide-Grafted Silica as Special Type of Polymer-Colloid Complex

Structure and properties of polyacrylamide-grafted silica have been investigated with the help of thermal analysis methods, NMR spectroscopy and by measuring the ability solubilization in benzene when compared with homopolymer polyacrylamide (PAA). More homogeneous structure, low rigidity and density of packing have been revealed for the polymer shell. PAA form a dense polymer shell, which interact with silica surface through H-bonds. ¹H NMR spectroscopy suggests no influence of silica particles on PAA stereoregularity. PAA-grafted silica considered as a special type of polymer-colloid complex where polymer chains are covalently bound to silica with one end and polymer segments along the chain are hydrogen bound to the particle surface.     

Formation of an Inclusion Complex of a New Transition Metal Ligand in β-Cyclodextrin

The inclusion complex of a new transition metal ligand, 2,4,9-trithia-tricyclo[3.3.1.1^3,7]decane-7-carboxylic acid (2,4,9-trithia-adamantane-7-carboxylic acid, TPCOOH) in β-cyclodextrin was studied by ¹H NMR, 2D NOESY NMR spectroscopy, host-induced CD spectroscopy, and tandem mass spectrometry. ¹H NMR, MS–MS and NOESY data show that the TPCOOH guest forms a 1:1 inclusion complex with the host β-cyclodextrin. The NOESY experiments also show that TPCOOH is oriented in the complex with the thioketal end preferentially located at the larger opening of β-cyclodextrin. The orientation of the guest in the host molecule is also confirmed by the induced CD of the ligand, which shows a positive Cotton effect. An association constant of 660±20?M^?1 was determined by ¹H NMR titration for the complex at room temperature in D₂O.     

Lateral diffusion of molecules in two-component lipid bilayer: a Monte Carlo simulation study

Lateral diffusion of membrane components makes possible any in-plane membrane reaction and has a key role in signaling in cell membranes. In this report the equilibrium lateral diffusion of intrinsic molecules in an equimolar DMPC/DSPC mixture is simulated using a thoroughly tested two-state model of two-component phospholipid bilayers. The model has been successful in calculating the excess heat capacity function, the most frequent center-to-center distances between DSPC clusters, and the fractal dimensions of gel clusters (Sugar, I. P., Thompson, T. E., Biltonen, R. L. Biophys. J. 1999, 76, 2099-2110). In the gel/fluid mixed phase region, a diffusing intrinsic molecule may change its state from fluid to gel (or from gel to fluid) at any time. A common characterization of the diffusion of intrinsic molecules is given by the simulated average first-passage time curves. We find that these curves can be described as power functions containing two parameters, alpha and beta, except near the percolation threshold of gel/fluid or compositional clusters. We find also that the intrinsic molecules are involved in approximately normal diffusion, i.e., beta approximately 2 in the extreme gel and fluid phase regions, while in the gel/fluid and gel/gel mixed phase regions the diffusion is anomalous, i.e., beta not equal 2. In the mixed phase regions, when the initial local state of the diffusing molecule is not specified, each component is involved in sub-diffusion (beta > 2). In the gel/fluid mixed phase region molecules situated initially inside a fluid cluster are involved in sub-diffusion, but DMPC molecules situated initially inside a gel cluster are involved in super-diffusion (beta < 2). The possibility of anomalous diffusion in membranes apparently arises because the diffusing molecule visits a variety of different environments characterized by its relative proximity to various membrane components. The diffusion is actually anomalous when the components of the bilayer are nonrandomly distributed. The deviation from random distribution is strongly correlated with beta. Similar to the results of the NMR experiments, the calculated relative diffusion coefficient continuously decreases in the gel/fluid mixed phase region with decreasing temperature. In apparent contradiction, diffusion measured by fluorescence recovery after photobleaching (FRAP) demonstrates the existence of a threshold temperature, below which long-range diffusion of FRAP probe molecules is essentially blocked. This threshold temperature is highly correlated with the percolation temperature of gel clusters.     

Interaction of polyacrylic acid with lipid bilayers: effect of polymer mass

Polyanion‐coated lipid vesicles are proposed to have an appreciable potential for drug delivery because of their ability to control the permeability of lipid bilayers by environmental parameters such as pH and temperature. However, details of the interaction of this class of polymers with lipids and their mechanisms of induced permeability are still being debated. In this work, we applied ¹H NOESY to study details of the interaction of polyacrylic acid (PAA) fractions of molecular weights 5 and 240 kDa with dimyristoylphosphatidylcholine vesicles. We showed that PAA of two different molecular masses modifies lipid bilayers increasing disorder and probability of close contact between polar and hydrophobic groups. PAA molecules adsorb near the interface of lipid bilayers but do not penetrate into the hydrophobic core of the bilayer and, thus, cannot participate in formation of transbilayer channels, proposed in earlier works. Increasing the molecular mass of PAA from 5 kDa to 240 kDa does not change the effect of PAA on the bilayer, although PAA240 forms a more compact structure (either intra‐molecular or inter‐molecular) and interacts more strongly with interface lipid protons. Copyright © 2013 John Wiley & Sons, Ltd.     

Interaction between pentaethylene glycol n-octyl ether and poly(acrylic acid): effect of the polymer molecular weight

The effect of the polymer molecular weight on the interaction between pentaethylene glycol n-octyl ether (C(8)E(5)) and poly(acrylic acid) (PAA) has been investigated by a combined experimental strategy including tensiometry, potentiometry, calorimetry, fluorescence quenching and intradiffusion (pulsed gradient spin echo-NMR) measurements. PAA samples with an average molecular weight varying in a wide range (M (w)=2000, 100,000, 250,000, and 450,000) have been considered. The measurements have been performed at constant polymer concentration (0.1% w/w) with varying surfactant molality. In all the considered systems, at low surfactant concentration, adsorption of surfactant monomers onto the polymer chain has been detected. At a C(8)E(5) molality (T(1)) independent of the PAA M (w), surfactant molecules start to aggregate, forming clusters to which the polymer co-participates. Above this concentration, the behavior of the system depends on M (w). In fact, if polymer samples with high molecular weight (M (w)100,000) are employed, all the added surfactant aggregates onto the polymer leading to the polymer saturation and, subsequently, to free micelles formation. Both saturation and free micellization occur at surfactant concentrations which are independent of the polymer molecular weight. C(8)E(5) aqueous mixtures containing PAA with low molecular weight (M (w)=2000) behaves differently, in that, above T(1), only a fraction ( approximately 20%) of the added surfactant molecules interact with the polymer, forming aggregates to which more than one PAA chain participate. In this case, C(8)E(5) free micellization occurs before polymer saturation. The experimental evidences have been interpreted in terms of the subtle balance between the various molecular interactions driving the surfactant-polymer aggregation.     

Self-aggregation and supramolecular structure investigations of Triton X-100 and SDP2S by NOESY and diffusion ordered NMR spectroscopy

The self-aggregation and supramolecular micellar structure of two surfactants in aqueous solution, the anionic surfactant SDP2S (sodium dodecyl dioxyethylene-2 sulfate) and the nonionic surfactant Triton X-100 (octylphenol-polyoxyethylene ether with 9.5 ethoxy groups), were investigated by NMR spectroscopy. The critical micellar concentration (CMC), the size, and shape of the aggregates were determined by diffusion ordered NMR spectroscopy (DOSY), while 2D NOESY NMR spectra were used to study the mutual spatial arrangement of surfactant molecules in the aggregated state. A nonlinear increase of the micellar hydrodynamic radius, indicating possible sphere-to-rod shape transition, was found for SDP2S at higher surfactant concentrations. Triton X-100 micelles were found to be almost spherical at low surfactant concentrations, but formation of ellipsoid shaped particles and/or micellar aggregation was observed at higher concentrations. The NOESY data show that at low concentration Triton X-100 forms a two-layer spherical structure in the micelles, with partially overlapping internal and external layers of Triton X-100 molecules and no distinct hydrophilic-hydrophobic boundary.     

Vertical diffusion of water molecules near the surface of ice

We studied diffusion of water molecules in the direction perpendicular to the surface of an ice film. Amorphous ice films of H(2)O were deposited on Ru(0001) at temperature of 100-140 K for thickness of 1-5 bilayer (BL) in vacuum, and a fractional coverage of D(2)O was added onto the surface. Vertical migration of surface D(2)O molecules to the underlying H(2)O multilayer and the reverse migration of H(2)O resulted in change of their surface concentrations. Temporal variation of the H(2)O and D(2)O surface concentrations was monitored by the technique of Cs(+) reactive ion scattering to reveal kinetics of the vertical diffusion in depth resolution of 1 BL. The first-order rate coefficient for the migration of surface water molecules ranged from k(1)=5.7(+/-0.6) x 10(-4) s(-1) at T=100 K to k(1)=6.7(+/-2.0) x 10(-2) s(-1) at 140 K, with an activation energy of 13.7+/-1.7 kJ mol(-1). The equivalent surface diffusion coefficients were D(s)=7 x 10(-19) cm(2) s(-1) at 100 K and D(s)=8 x 10(-17) cm(2) s(-1) at 140 K. The measured activation energy was close to interstitial migration energy (15 kJ mol(-1)) and was much lower than diffusion activation energy in bulk ice (52-70 kJ mol(-1)). The result suggested that water molecules diffused via the interstitial mechanism near the surface where defect concentrations were very high.     

Morphological evolution of inorganic crystal into zigzag and helical architectures with an exquisite association of polymer: a novel approach for morphological complexity

The morphology of potassium sulfate (K(2)SO(4)) crystals grown in a viscous polymer solution of poly(acrylic acid) (PAA) was remarkably changed from the tilted columnar assembly into zigzag and helical architectures with increasing PAA concentration. The habit modification of orthorhombic K(2)SO(4) with adsorption of PAA molecules on a specified crystal face fundamentally led to the formation of tilted unit crystals. Concurrently with the habit modification, a diffusion-limited condition controlling the assembly of tilted units was achieved in the presence of PAA molecules in the matrix. Various complex morphologies, including zigzag and helical assembly, emerged through the formation of twinned crystals with the variation of the diffusion condition. Understanding the morphogenesis observed in this report would provide a novel approach for sophisticated crystal design by using an exquisite association of organic and inorganic materials.