Ehud M. Landau

Langmuir Monolayers Designed for the Oriented Growth of Glycine and Sodium Chloride Crystals at Air/Water Interfaces

Abstract Oriented growth of crystals of α-glycine and sodium chloride under compressed Langmuir monolayers at air-water interfaces was achieved. For α-glycine, a variety of monolayers containing resolved glycyl head groups and different hydrophobic moieties were used, thus establishing the structural requirements necessary for oriented crystallization. For sodium chloride, monolayers of positive, negative and zwitterionic charge induced crystal nucleation from faces of the type {100}, {110} and {111}, the latter two faces not being naturally occurring.

A Synchrotron X-ray Study of a Solid-Solid Phase Transition in a Two-Dimensional Crystal

A measurement and interpretation on a molecular level of a phase transition in an ordered Langmuir monolayer is reported. The diagram of surface pressure (π) versus molecular area of a monolayer of chiral (S)-[CF3-(CF2)9-(CH2)2-OCO-CH2-CH (NH3+)CO2-] over water shows a change in slope at about πs= 25 millinewtons per meter. Grazing-incidence x-ray diffraction and specular reflectivity measurements indicate a solid-solid phase transition at πs. The diffraction pattren at low pressures reveals two diffraction peaks of equal intensities, with lattice spacings d of 5.11 and 5.00 angstroms; these coalesce for π ≥πs. Structural models that fit the diffraction data show that at π> πs the molecules pack in a two-dimensional crystal with the molecules aligned vertically. At π < πs there is a molecular tilt of 16 � � 7 �. Independent x-ray reflectivity data yield a tilt of 26 � � 7�. Concomitant with the tilt, the diffraction data indicate a transition from a hexagonal to a distorted-hexagonal lattice. The hexagonal arrangement is favored because the -(CF2)9CF3 moiety adopts a helical conformation. Compression to 70 millinewtons per meter yields a unit cell with increased crystallinity and a coherence length exceeding 1000 angstroms.

Synchrotron X-ray study of the structure of a Langmuir monolayer and the attached solute molecular layer☆

We have determined the packing arrangement of a floating monolayer of palmitoyl-(R)-lysine at the air-water interface by grazing incidence X-ray diffraction and reflection measurements. These techniques utilize the unique properties of synchrotron radiation: high intensity within a small natural collimation. In the grazing angle diffraction experiment two peaks were detected in the “two-dimensional powder” pattern from a monolayer of palmitoyl-(R)-lysine. Their widths indicated coherence lengths of 500 A. The positions and intensities of these peaks allowed us to choose between various models and to determine the monolayer structure. The packing arrangement of the α-amino acid headgroups in the model proved to be very similar to that found in the crystal structures of the α form of glycine and several hydrophobic α-amino acids, thus providing a simple explanation for the oriented crystallization of α-glycine at monolayer-solution interfaces. The tilt of the molecule calculated from the model is consistent with the results from reflectivity measurements and X-ray powder diffraction data of the crystalline powder material. Reflectivity measurements indicate that at surface pressures as high as 30 mN m−1 the monolayer covers only about 90% of the surface. Reflectivity measurements of the monolayer over water and over solutions of 2% (S)-glutamine showed significant differences, indicating binding of the solute molecules to the monolayer.

Biophysical Compaction Studies of Effects of Abscisic Acid on Membrane Phospholipids: Possible Role in Stomatal Physiology and Senescence

Summary In vitro biopysical compaction studies based on Langmuir tensiometry and Contact Angle measurements combined with Mass Spectral in vivo characterization of dipalmitoylphosphatidylcholine (DPPC) in guard cell microsomal membranes has indicated that abscisic acid (ABA) may exert a specifically located compacting effect. It is suggested that the rigidity imposed by the ABA-sensitive DPPC, in contrast to other membranes, as in lung surfactant tissue may fulfill a collapse-resisting function in stomatal physiology.