Thick Molecules Research Articles

Structure formation in monolayers composed of hard bent-core molecules

ABSTRACTWe investigate the role of excluded volume interactions instabilising different structures in monolayers filled with bent-shaped molecules using the Onsager type of density functional theory supplemented by constant-pressure Monte-Carlo simulations. We study influence of molecular features, like the apex angle, thickness of the arm and the type of the arm edges on the stability of layered structures. For simple molecular shapes taken the observed phases are dominated by the lamellar antiferroelectric type as observed experimentally, but a considerable sensitivity of the ordering to details of the molecular shape is found for order parameters and wave vectors of the structures. Interestingly, for large opening angles and not too thick molecules, a window of stable nematic splay-bend phase is shown to exist.

Structure and dynamics of self-assembling beta-sheet peptide tapes by dynamic light scattering.

Oligomeric peptides can be designed which undergo one-dimensional self-assembly in solution to form beta-sheet tapes a single molecule in thickness and micrometers in length.(1) In this paper, we present the first systematic investigation of the size, shape, dynamics, and interactions of beta-sheet tapes formed by a self-assembling 24-residue peptide, K24, in 2-chloroethanol, over a wide range of peptide concentrations c (c: 10(-7)-1 mM), using photon correlation spectroscopy. The tapes behave like semiflexible chains with persistence lengths of several hundred nanometers and much longer contour lengths, even at c approximately 0.1 nM. The polarized q-dependent light-scattering intensity I fits a model of a prolate object with major and minor axes alpha approximately 630 nm and beta approximately 40 nm. This is an unexpected result in view of the previous theoretical predictions that tapelike polymers could form oblate coinlike structures in solution.(2) This experimentally observed behavior is attributed to the pronounced twist and bend of the beta-tapes, which do not allow them to form the coinlike structures, but instead they favor the formation of elongated polymers. At c approximately 10(-2) mM, the tapes are seen to start overlapping and forming networks with unusually large mesh sizes (e.g., ca. 400 nm at 15 mciroM), much larger than those of conventional polymers. With increasing peptide concentration the mesh size decreases and the network becomes a physical gel at c approximately 0.4 mM. These semidilute solutions are characterized by one main relaxation mode associated with the cooperative diffusion of the entangled tape network, and a weaker slower mode, associated with gel cluster formation. The concentration dependence of xi (xi(c) approximately c(-0.34)) is much weaker compared to the expected scaling for Gaussian or swollen chains (xi(c) approximately c(-1) for Gaussian chains, or xi(c) approximately c(-3/4) for swollen ones), but is not inconsistent with the expected scaling for rigid rods. On the basis of the concentration dependencies of the light-scattering intensity I and of the cooperative diffusion coefficient D, the cooperative friction coefficient f(c) is found to display a stronger concentration dependence (f(c) approximately c(1.34)) than in the case of semidilute flexible and semiflexible polymer solutions (f(c) approximately c(0.5)).(3) Thus, we may conclude that the network of entangled tapes approximates in its behavior that of semirigid polymers.

The use of flow micro-calorimetry and FTIR techniques for characterising filler/organic acid interactions

Reaction of filler grade magnesium hydroxide (Martinswerke Magnifin H 10) with oleic acid (cis 9-octadecenoic acid) and stearic acid (octadecanoic acid) has been investigated using flow micro-calorimetry (FMC) and solution adsorption isotherm methods (both from heptane). Diffuse reflectance Fourier transform IR spectroscopy (DRIFTS), sedimentation volume measurements and X-ray diffraction were also carried out on the isolated and dried adsorption isotherm treated samples. Stearic acid adsorbed to a greater extent (24 mg g-1) than oleic acid (12 mg g-1 ) and this is likely to correspond to vertical adsorption of both acids based on measured filler surface area and adsorbate molecular area. It is considered that stearic acid forms a layer more than one molecule in thickness and was shown by X-ray diffraction, in the dry state at least, to exhibit structural order. Oleic acid adsorbs as a single molecular layer with any additional (i.e. excess) acid present probably reacting fully with the magnesium ion, thus forming the double salt which then dissolves in the heptane. Magnesium stearate is not soluble in heptane. FTIR analysis of the supernatant liquors for the oleic acid treated filler showed no free acid, only magnesium oleate.

Engineering of peptide β-sheet nanotapes

A. Aggeli, M. Bell, N. Boden, J. N. Keen, T. C. B. McLeish, I. Nyrkova, Sheena E. Radford and A. Semenov, J. Mater. Chem., 1997, 7, 1135 DOI: 10.1039/A701088E

Ranges of low-energy electrons in protein.

A KNOWLEDGE of the effective range of low-energy electrons in protein is of considerable biological interest. The ranges as calculated by Lea1 from the Bethe theory of energy-loss cannot be accepted without experimental verification, since the validity of the assumptions made in the computations are questionable at the low energies. The sensitivity of small biologically active molecules to radiation can be used to measure the ranges by employing a layer of enzyme of thickness greater than the range of the electrons as an absorber. If a thin, uniform layer of dry enzyme is irradiated with a sufficiently large number of electrons, all the enzyme within the range of electrons is inactivated. After irradiation, the enzyme is assayed for remaining activity; knowing the amount inactivated, the surface area irradiated and the density of the material (taken as 1.3 gm./c.c.)2 the range of the electrons in the enzyme can be found. Irradiations using the enzyme invertase3 have already been carried out, and the ranges found were significantly less than those calculated by Lea. The size of the enzyme molecule itself prevented any measurements being made at energies less than 600 eV., for at that energy a layer one enzyme molecule in thickness was inactivated. The inaccuracies of the assay made the determinations at energies greater than 1,500 eV. uncertain.

The structure of unimolecular films

Surface films one molecule in thickness present the simplest type of molecular mosaic. In this paper, data obtained by means of surface pressure and surface potential methods on films of various proteins are given and applied in an attempt to indicate the probable structure of the protein micelle as it exists in the film state. Gorter and Grendel 1 were the first, with the aid of a Langmuir trough, to examine thin films of proteins and spread the material dispersed in acid or alkaline solutions on the surface of various substrates. This method, ingenious as it was, could not be utilized to obtain homogeneous films in every instance as revealed by the ultramicroscopic examination of protein films spread in this manner by Zocher and Stiebe12 The apparent mean limiting thickness of the films studied by Gorter and Grendel varied from 7.5 to 30.6 A. and appeared to be complicated by the action of the dispersing agent on the proteins and the solubility of a portion of the material. Preliminary results on some protein films have been presented by Hughes, Schulman and Rideal. Recently Hughes and Rideal 4 have examined in detail the behavior of monolayers of gliadin and glutenin. Films formed by spreading the solid material directly from the arm of a quartz fiber microbalance were found to be homogeneous as revealed by surface potential and surface pressure measurements. In the case of gliadin, in which two-thirds of the side chains may be regarded as consisting of the glutamyl residue, they were

The properties and molecular structure of thin films. Part II.—Condensed films

In this and a following paper, an account will be given of work done in continuation of the experiments on thin films of palmitic acid, described in Part I of this investigation (1), and, in particular, of the results obtained by extending the range of substances composing the films, and the range of temperature over which the experiments were made. As was there pointed out, the study of these films is a peculiarly attractive one, owing to the extremely simple manner in which the molecules are arranged; the films being of one molecule in thickness and the molecules in them being arranged not indiscriminately, but perpendicular to the surface and parallel to each other, as a rule. The experimental method (due in principle to Langmuir) has also a great advantage over those more commonly employed in capillary studies, that the forces tangential to the surface are measured directly. Exceptionally direct information is therefore obtainable as to some of the forces between the molecules composing the films, and by a study of the influence of chemical constitution on the properties of the films, much light should be shed upon the important problem of the relation between chemical and capillary forces. It will not be necessary to occupy space in proving that, for any of the substances here studied, the films are one molecule in thickness. Reasons for taking this view have been given by other workers, as well as on p. 344 of my preceding paper, and it is sufficient to say that the experiments described here afford evidence of the same character, which is equally conclusive for each one of the substances investigated.