Effect of glycine substitution on Fmoc-diphenylalanine self-assembly and gelation properties

We have investigated the self-assembly behavior of fluorenyl-9-methoxycarbonyl (Fmoc)-FG, Fmoc-GG, and Fmoc-GF and compared it to that of Fmoc-FF using potentiometry, fluorescence and infrared spectroscopy, transmission electron microscopy, wide-angle X-ray scattering, and oscillatory rheometry. Titration experiments revealed a substantially shifted apparent pK(a) transition for Fmoc-FG, Fmoc-GG, and Fmoc-GF. The apparent pK(a) values observed correlated with the hydrophobicity (log P) of the Fmoc-dipeptide molecules. Fmoc-GG and Fmoc-GF were found to self-assemble only in their protonated form (below their apparent pK(a)), while Fmoc-FG formed self-assembled structures above and below its apparent pK(a). Fmoc-GG and Fmoc-FG were found to form hydrogels below their apparent pK(a) transitions in agreement with the entangled fibers morphologies revealed by TEM. Unlike Fmoc-FF and Fmoc-GG, Fmoc-FG showed unusual gelation behavior as gels were found to form upon heating. Fmoc-GF formed precipitates instead of a hydrogel below its apparent pK(a) in agreement with the formation of micrometer scale sheetlike structures observed by TEM. The fact that all four Fmoc-dipeptides were found to self-assemble suggests that the main driving force behind the self-assembly process is a combination of the hydrophobic and π-π interactions of the fluorenyl moieties with a secondary role for hydrogen bonding of the peptidic components. The nature of the peptidic tail was found to have a pronounced effect on the type of self-assembled structure formed. This work indicates that the substitution of phenylalanine by glycine significantly impacts on the mode of assembly and illustrates the versatility of aromatic peptide amphiphiles in the formation of structurally diverse nanostructures.     

Designing peptide based nanomaterials

This tutorial review looks at the design rules that allow peptides to be exploited as building blocks for the assembly of nanomaterials. These design rules are either derived by copying nature (alpha-helix, beta-sheet) or may exploit entirely new designs based on peptide derivatives (peptide amphiphiles, pi-stacking systems). We will examine the features that can be introduced to allow self-assembly to be controlled and directed by application of an externally applied stimulus, such as pH, light or enzyme action. Lastly the applications of designed self-assembly peptide systems in biotechnology (3D cell culture, biosensing) and technology (nanoelectronics, templating) will be examined.     

Spherically symmetric inhomogeneous dust collapse in higher dimensional space-time and cosmic censorship hypothesis

We consider a collapsing spherically symmetric inhomogeneous dust cloud in higher dimensional space-time. We show that the central singularity of collapse can be a strong curvature or a weak curvature naked singularity depending on the initial density distribution.     

Organically modified xerogels as supports for solid-phase chemistry

Organically modified xerogels (OMX_NH2) can be used as an easy to handle and chemically stable support in solid-phase chemistry and are compatible with enzymatic transformations.     

Scattering by small defects in the neighbourhood of a fluid-solid interface

The scattering of incident plane elastic waves by a varietyof different defects that lie upon a fluid-solid interface isconsidered here using matched asymptotic expansions. The expansionscheme is developed in terms of a parameter , the ratio of typicaldefect length scale to a typical wavelength of the incidentfield, taken to be small. Three different canonical situations occur and these are illustratedvia three specific examples treated here: a rigid strut, anedge crack, and a rigid strip. In each case the leading-ordermatching is performed to identify the leading-order contributionof the defect to the acoustic field in the far field. In particular,each defect is identified with a source of dipole response ininterfacial stress of displacement. It is shown in the limit as s<<s1 that in the inner problemsthe fluid and solid pieces uncouple in a particularly convenientmanner allowing analytical solutions to be deduced. These arethen matched with appropriate outer solutions.     

Biocatalytic Pathway Selection in Transient Tripeptide Nanostructures

Structural adaption in living systems is achieved by competing catalytic pathways that drive assembly and disassembly of molecular components under the influence of chemical fuels. We report on a simple mimic of such a system that displays transient, sequence‐dependent formation of supramolecular nanostructures based on biocatalytic formation and hydrolysis of self‐assembling tripeptides. The systems are catalyzed by α‐chymotrypsin and driven by hydrolysis of dipeptide aspartyl‐phenylalanine‐methyl ester (the sweetener aspartame, DF‐OMe). We observed switch‐like pathway selection, with the kinetics and consequent lifetime of transient nanostructures controlled by the peptide sequence. In direct competition, kinetic (rather than thermodynamic) component selection is observed.     

Raman optical activity of an achiral element in a chiral environment

Raman optical activity (ROA) is a relatively new technique used to determine the structure of chiral molecules and is proving useful in the study of biological molecules such as proteins and DNA/RNA. Here, for the first time, we demonstrate the applicability of ROA as a technique to study achiral groups in chiral environments, detecting the induced chirality of N‐(fluorenyl‐9‐methoxycarbonyl) (Fmoc) in a chiral self‐assembled structure of Fmoc‐dipeptides. This technique is therefore of interest to those studying self‐assembled systems that adopt a chiral structure. Copyright © 2009 John Wiley & Sons, Ltd.     

On a Simple Nonisothermal Adsorption Experiment with Organic Vapors and an Inertial Microbalance To Study the Surface Properties of Hybrid (Organic/Inorganic) Porous Materials

A nonisothermal adsorption experiment using a controlled flow of cyclopentane in the 333-313 K range is used to simultaneously estimate the specific surface area and micropore volume of a hybrid (organic/inorganic) alcogel. For reference, the method is also applied to an all-inorganic material with a more rigid structure, namely, a high surface area SiO(2)-Al(2)O(3). The proposed data analysis provides guidelines to determine whether adsorption data on a certain adsorbate/adsorbent system can be modeled effectively as a convolution of BET (meso- and macropore) and Dubinin-Radushkevitch (DR, micropore) contributions. Copyright 2000 Academic Press.     

Lipase-catalyzed kinetic resolution on solid-phase via a "capture and release" strategy

The lipase-catalyzed kinetic resolution of (R/S)-3-phenylbutyric acid 2 using solid-supported cyclohexane-1,3-dione (CHD) 6 is described. In each case the predominant enantiomer observed, after cleavage from the resin, was (R)-(-)-3-phenylbutyric acid (R)-2 (ee > 99%) rather than the expected (S)-enantiomer of 2. This observation is in contrast to the fact that Chromobacterium viscosum lipase shows high enantiospecificity for the (S)-enantiomer in the corresponding solution-phase hydrolysis reactions. The (R)-acyl group was subsequently released from the resin by NaOH hydrolysis, and the yield of the reaction could be improved by triple acylation of the resin.