Circular dichroism studies of low molecular weight hydrogelators: The use of SRCD and addressing practical issues.

Circular dichroism (CD) spectroscopy has been used extensively for the investigation of the conformation and configuration of chiral molecules, but its use for evaluating the mode of self-assembly in soft materials has been limited. Herein, we report a protocol for the study of such materials by electronic CD spectroscopy using commercial/benchtop instruments and synchrotron radiation (SR) using the B23 beamline available at Diamond Light Source. The use of the B23 beamtime for SRCD was advantageous because of the unique enhanced spatial resolution achieved because of its highly collimated and small beamlight cross section (ca. 250 µm) and higher photon flux in the far UV region (175-250 nm) enhancing the signal-to-noise ratio relative to benchtop CD instruments. A set of low molecular weight (LMW) hydrogelators, comprising two Fmoc-protected enantiomeric monosaccharides and one Fmoc dipeptide (Fmoc-FF), were studied. The research focused on the optimization of sample preparation and handling, which then enabled the characterization of sample conformational homogeneity and thermal stability. CD spectroscopy, in combination with other spectroscopic techniques and microscopy, will allow a better insight into the self-assembly of chiral building blocks into higher order structural architectures.

Facet-Dependent Interactions of Islet Amyloid Polypeptide with Gold Nanoparticles: Implications for Fibril Formation and Peptide-Induced Lipid Membrane Disruption.

A comprehensive understanding of the mechanisms of interaction between proteins or peptides and nanomaterials is crucial for the development of nanomaterial-based diagnostics and therapeutics. In this work, we systematically explored the interactions between citrate-capped gold nanoparticles (AuNPs) and islet amyloid polypeptide (IAPP), a 37-amino acid peptide hormone co-secreted with insulin from the pancreatic islet. We utilized diffusion-ordered spectroscopy, isothermal titration calorimetry, localized surface plasmon resonance spectroscopy, gel electrophoresis, atomic force microscopy, transmission electron microscopy (TEM), and molecular dynamics (MD) simulations to systematically elucidate the underlying mechanism of the IAPP-AuNP interactions. Because of the presence of a metal-binding sequence motif in the hydrophilic peptide domain, IAPP strongly interacts with the Au surface in both the monomeric and fibrillar states. Circular dichroism showed that AuNPs triggered the IAPP conformational transition from random coil to ordered structures (α-helix and ß-sheet), and TEM imaging suggested the acceleration of IAPP fibrillation in the presence of AuNPs. MD simulations revealed that the IAPP-AuNP interactions were initiated by the N-terminal domain (IAPP residues 1-19), which subsequently induced a facet-dependent conformational change in IAPP. On a Au(111) surface, IAPP was unfolded and adsorbed directly onto the Au surface, while for the Au(100) surface, it interacted predominantly with the citrate adlayer and retained some helical conformation. The observed affinity of AuNPs for IAPP was further applied to reduce the level of peptide-induced lipid membrane disruption.

Effect of glucosamine HCl on dissolution and solid state behaviours of piroxicam upon milling.

Piroxicam is a non-steroidal anti-inflammatory drug that is characterised by low solubility and high permeability. In order to improve the drug dissolution rate, the co-grinding method was used as an approach to prepare piroxicam co-ground in the carriers such as glucosamine hydrochloride. As, this amino sugar (glucosamine HCl) has been shown to decrease pain and improve mobility in osteoarthritis in joints, therefore, the incorporation of glucosamine in piroxicam formulations would be expected to offer additional benefits to patients. The effect of the order of grinding on the dissolution of piroxicam was also investigated. Co-ground drug and glucosamine were prepared in different ratios using a ball mill. The samples were then subjected to different grinding times. In order to investigate the effect of the grinding process on the dissolution behaviour of piroxicam, the drug was ground separately in the absence of glucosamine. Mixtures of ground piroxicam and unground D-glucosamine HCl were prepared. Physical mixtures of piroxicam and glucosamine were also prepared for comparison. The properties of prepared co-ground systems and physical mixtures were studied using a dissolution tester, FTIR, SEM, XRPD and DSC. These results showed that the presence of glucosamine HCl can increase dissolution rate of piroxicam compared to pure piroxicam. Generally, all dissolution profiles showed the fastest dissolution rate when ground piroxicam was mixed with unground glucosamine. This was closely followed by the co-grinding of piroxicam with glucosamine where lower grinding times showed the fastest dissolution. The solid state studies showed that the grinding of piroxicam for longer times had no effect on polymorphic form of piroxicam, whereas mixtures of piroxicam-glucosamine ground for longer times (60 min) converted piroxicam polymorph II to polymorph I.

Cyclic α,ß-peptoid octamers with differing side chain patterns: synthesis and conformational investigation.

The solution-phase synthesis and cyclisation of three α,ß-peptoid octamers with differing side chain patterns is reported. One of these, compound C, showed a significantly greater resolution by NMR relative to the other two structurally related octamers. This observation was studied in detail by circular dichroism at a synchrotron light source to facilitate the correlation between the side chain patterns and conformational preference of these three peptoids. The X-ray crystal structure of cyclic octamer C, the first high-resolution structure for the α,ß-peptoid backbone, was also obtained from methanol. Combined solid- and solution-phase studies allowed the identification of the N-2-(benzyloxy)ethyl side chain on the ß-residue of the heterogeneous backbone as a key structural feature driving the increased conformational stability for octamer C.

C-3 branched δ-3,5-cis- and trans-THF sugar amino acids: synthesis of the first generation of branched homooligomers.

This article describes the efficient synthesis of the first generation of branched sugar amino acid (SAA) oligomers in solution phase via two main routes: by the use of a standard coupling reagent and via the use of active ester intermediates. Benzyl-protected dimeric carbopeptoid and methyl-protected dimeric and tetrameric, hexameric and octameric carbopeptoids were obtained from a branched δ-3,5-trans-tetrahydrofuran (THF) SAA and methyl-protected dimeric and tetrameric carbopeptoids were synthesised from a branched δ-3,5-cis-THF SAA. These systems are of interest because of their potential to display foldameric properties reminiscent of those observed in α-peptides and proteins. Amongst their many uses, foldamers provide simpler models in the study of the factors which induce the folding and unfolding of proteins and, ultimately, potential insights into their functioning.

Glucosamine HCl as a new carrier for improved dissolution behaviour: effect of grinding.

The co-grinding technique is one of the most effective methods for improving the dissolution rate of poorly water-soluble drugs and it is superior to other approaches from an economical as well as an environmental stand point, as the technique does not require any toxic organic solvents. The present work is an attempt to use d-glucosamine HCl (G-HCl) as a potential excipient to improve dissolution rate of carbamazepine (CBZ) from physical mixtures and co-grinding formulations. The effect of order of grinding on dissolution of CBZ was also investigated. Co-ground of drug and G-HCL were prepared using different ratios using ball mill. The samples were subjected to different grinding times. In order to investigate the effect of grinding process on dissolution behaviour of CBZ, the drug was ground separately in the absence of glucosamine. Then the mixture of ground CBZ and un-ground d-glucosamine HCl were prepared. Physical mixtures of CBZ and G-HCl were also prepared for comparison. The properties of prepared co-ground systems and physical mixtures were studied using a dissolution tester, FT-IR, SEM, XRPD, and DSC. These results showed that the presence of glucosamine can increase dissolution rate of CBZ compared to pure CBZ. The results showed the order of grinding had a big impact on the dissolution performance of CBZ formulations containing glucosamine. All dissolution profiles generally showed that the fastest dissolution rate was obtained when ground CBZ was mixed with un-ground glucosamine. This was closely followed by the co-grinding of CBZ with glucosamine where lower grinding times showed the fastest dissolution. XRPD showed that the grinding of CBZ can reduce the percentage crystallinity of drug crystals. DSC study of ground CBZ showed that the grinding induced polymorphism transformations in the CBZ crystals and the limit and type of these transformations were related to the grinding time.

To enhance dissolution rate of poorly water-soluble drugs: glucosamine hydrochloride as a potential carrier in solid dispersion formulations.

The solid dispersion technique is the most effective method for improving the dissolution rate of poorly water-soluble drugs, however this is reliant on a suitable carrier and solvent being selected. The work presented explores D-glucosamine HCl (G-HCl) as a potential hydrophilic carrier to improve dissolution rate of a poorly water-soluble drug, carbamazepine (CBZ), from physical mixtures and solid dispersion formulations. The effect of different solvents in the preparation of solid dispersion formulations was also investigated. Solid dispersions of the drug and G-HCl were prepared using different ratios by the conventional solvent evaporation method. Different solvents (ethanol, acetone and water) were used as second variable in the preparation of solid dispersions. Physical mixtures of CBZ and G-HCl were also prepared for comparison. The properties of all solid dispersions and physical mixtures were studied using a dissolution tester, FT-IR, SEM and DSC. These results showed that the presence of glucosamine can increase dissolution rate of CBZ compared to pure CBZ. All solid dispersions of CBZ-G-HCl showed considerably a higher dissolution rate than the corresponding physical mixtures. The presence of water during preparation of the solid dispersions reduced the dissolution rate of CBZ due to formation of carbamazepine dihydrate during the preparation of solid dispersion, as proved by DSC and FT-IR studies. To facilitate comparison, the dissolution efficiency was calculated for solid dispersions prepared with different solvents and the dissolution efficiency can generally be ranked as follows: ethanol>acetone>ethanol-water>acetone-water when the ratios of drug to carrier were 4:1 and 2:1. It has thus been shown that the use of G-HCl in solid dispersion formulations can significantly enhance the dissolution rate of poorly water-soluble drugs such as carbamazepine. This amino sugar could be used as a new carrier in solid dispersion formulations and would have significant commercial potential.

Penta-fluoro-phenyl (3R,4R,5S)-5-{[(3R,4R,5S)-5-azido-methyl-3,4-dimeth-oxy-2,3,4,5-tetra-hydro-furan-3-carboxamido]-meth-yl}-3,4-dimeth-oxy-2,3,4,5-tetra-hydro-furan-3-carboxyl-ate.

The title compound, C(22)H(25)F(5)N(4)O(9), is a stable penta-fluoro-phenyl ester inter-mediate in the synthesis of novel homo-oligomeric structures containing branched carbon chains. The structure is epimeric to the previously characterized dimeric penta-fluoro-phenyl ester with stereochemistry (3R,4R,5R), which was synthesized using d-ribose as starting material. The crystal structure of the title mol-ecule removes any ambiguities arising from the relative stereochemistries of the six chiral centres. Two hydrogen bonds, bifurcating from the NH group, stabilize the crystal: one intra-molecular and one inter-molecular, both involving O atoms of the meth-oxy groups. The asymmetric unit contains two independent mol-ecules not related by any pseudo-symmetry operators. The major conformational differences are localized, leading to one mol-ecule being extended compared to the other. The collected crystal was twinned (twin ratio is 0.939:0.061), and the azide group is positionally disordered over two positions in one mol-ecule [occupancy ratio 0.511 (18):0.489 (18)].

(3R,4R,5R)-5-(Acetamido-meth-yl)-N-benzyl-3,4-dihy-droxy-tetra-hydro-furan-3-carboxamide.

X-ray crystallographic analysis with Cu Kα radiation established the relative configurations of the stereogenic centers in the title compound, C(15)H(20)N(2)O(5), and clarified mechanistic ambiguities in the synthesis. The conformation of the five-membered ring approximates twisted, about a C-O bond. The absolute configuration of this carbon-branched dipeptide isostere was known based on the use of d-ribose as the starting material. Refinement of the Flack parameter gave an ambiguous result but the refined Hooft parameter is in agreement with the assumed (d-ribose) absolute structure. The crystal structure consists of N-H⋯O and O-H⋯O hydrogen-bonded bi-layers, with the terminal methyl and phenyl groups forming a hydro-phobic inter-layer inter-face. Some weak C-H⋯O inter-actions are also present.

Convenient solution-phase synthesis and conformational studies of novel linear and cyclic alpha,beta-alternating peptoids.

The synthesis of a novel family of peptidomimetics composed of linear and cyclic alpha,beta-alternating peptoids is described. Oligomers consisting of up to six peptoid residues (n = 1-3) were synthesized on large scale with use of an efficient iterative solution-phase method and longer oligomers (n = 4, 5) were obtained by the coupling of appropriately protected shorter oligomers. Preliminary conformational studies of these hybrid peptoids are reported.

Spectroscopic studies of oligomers containing 2,5-trans furanoid sugar amino acids.

Sugar amino acids and their oligomers, known as carbopeptoids, are commonly studied as foldamers. However, study of their conformational preference is often challenging when the adopted conformations are extended and/or disordered. This study is the first to explore the disordered nature of such carbopeptoids by utilizing a family of 2,5-trans carbopeptoids. An array of spectroscopic techniques has been used to investigate the conformational preference of these carbopeptoids. However, using this data alone it has not been possible to assign conformational preference as an ordered extended conformation or as a disordered family of closely related conformations. Computational methods need to be employed to achieve reliable interpretation of the spectroscopic data.

Classification of conformation for sugar amino acid systems using chiroptical spectroscopy.

A number of structurally related sugar amino acid systems have been examined by chiroptical methods to aid interpretation of their conformational preference. The use of circular dichroism, in addition to NMR and solution IR, has enabled classification of the conformations adopted by sugar amino acid systems as hydrogen-bonded regular, non-hydrogen-bonded regular, and non-hydrogen-bonded irregular. A set of tetrameric SAAs are examined and the effect of change in primary structure related to conformation.

Conformational studies of oligomeric oxetane-based dipeptide isosteres derived from L-rhamnose or D-xylose.

Conformational investigations have been undertaken on oligomers (dimers, tetramers, hexamers) of five closely related oxetane-based dipeptide isosteres. All the oligomers were subjected to a range of studies by NMR, FT-IR and CD spectroscopy. The oligomers derived from methyl 2,4-anhydro-5-azido-3-O-tert-butyldimethylsilyl-5-deoxy-L-rhamnonate 'monomer' all exhibited evidence of ordered conformations in chloroform and 2,2,2-trifluoroethanol (TFE) solution. 5-Acetamido and N-methylamide derivatives of the L-rhamnonate 'monomer', along with a 'dimer' lacking silyl protection at C-3, were synthesized to ascertain the role of intramolecular interactions. This led to the conclusion that, for the L-rhamnonate oligomers, steric interactions govern the conformational preference observed. The equivalent silyl-protected D-lyxonate oligomers gave ordered CD spectra in TFE solution, but NMR and FT-IR spectroscopy in chloroform solution suggested an irregular, non-hydrogen bonded system. The remaining silyl-protected 6-deoxy-L-altronate, 6-deoxy-D-gulonate and D-fuconate oligomers appear to be characterized by their lack of ordered conformation in TFE and chloroform solution.

Circular dichroism studies of carbopeptoid-cyclodextrins.

A series of sugar amino acids, based on open chain sugars, have been oligomerised and cyclised. The resulting cyclic carbopeptoids have been examined for desirable properties such as host-guest chemistry (as in cyclodextrins) or self-assembling properties (e.g., peptide nanotubes). Initial studies of these systems, by circular dichroism and X-ray crystallography, have given valuable insight into their stability and properties. One of the four cyclic species studied was found to interact with ion/molecular probes.

Helix-forming carbohydrate amino acids.

[reaction: see text] The solution-phase conformational properties of tetrameric and octameric chains of C-glycosyl alpha-d-lyxofuranose configured tetrahydrofuran amino acids (where the C-2 and C-5 substituents on the tetrahydrofuran ring are trans to each other) were examined using NMR and IR and CD in organic solvents. Studies by NMR and IR demonstrated that in chloroform solution, the tetramer 7 does not adopt a hydrogen-bonded conformation whereas the octamer 10 populates a well-defined helical secondary structure stabilized by 16-membered (i, i - 3) interresidue hydrogen bonds, similar to a pi-helix. Circular dichroism studies in trifluoroethanol are consistent with this conformation for the octamer 10, and also indicate that the tetramer 7 adopts a rigid conformation not stabilized by hydrogen bonds.

Tetrahydrofuran-based amino acids as library scaffolds.

A furanose sugar amino acid (SAA) has been utilized as a library scaffold for the first time. Two furanose SAA scaffolds were examined to illustrate their potential for derivatization. The resulting 99-member library contained three orthogonal points of diversification that allowed easy access to ethers and carbamates from a hydroxyl moiety, a range of ureas from an azide (via an amine), and a range of amides from a methyl ester. The novel amide formation (by displacement of the methoxide from the methyl ester moiety) was achieved in good yield and purity with high structural confidence. Full characterization of several library intermediates (including a crystal structure) was obtained. The library was submitted for antibacterial screening.