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Andrew R Hirst

Publications and source records attributed to Andrew R Hirst.

10 recordsLinked to original sources
PubMed2005

Unique nanoscale morphologies underpinning organic gel-phase materials.

This study investigates the self-assembly of simple aliphatic diamines with a dendritic peptide. By controlling the molar ratio of this two-component system, new nanoscale morphologies were generated. In the presence of relatively long aliphatic chains (C10, C12) a transition from nanoscale fibres to platelets was observed on changing the molar ratio, whereas, for shorter spacer chains (e.g., C9 and C8), interesting and unique morphological changes were observed by low voltage field emission gun scanning electron microscopy (SEM), with "nanosquares" or nanoscale "rosette" structures being formed. Remarkably, these discrete nanoscale structures were able to form sample-spanning networks capable of supporting a gel-phase material; whereas, most gels are usually based on fibrillar assemblies. In addition to SEM, the gels were characterised by using thermal measurements and circular dichroism spectroscopy. The length of the diamine spacer and the molar ratio of components controlled the self-assembly process by modifying the spatial organisation of the dendritic head groups at the molecular level, which is transcribed into the aspect ratio of the self-assembled state at the microscopic level. Ultimately, this led to diamine-induced control of the macroscopic material's behaviour. When present in excess, the diamine controlled the observed nanoscale morphology as a consequence of undergoing a dendritically controlled nanocrystallisation process to form a network, an unusual and significant result.

Anthracenes↗
PubMed2005

Two-component gel-phase materials--highly tunable self-assembling systems.

In the past 10 years, the molecular self-assembly and network formation of small molecule gelators has become one of the most active frontiers of the emergent area of nanochemistry. Increasingly, research efforts have begun to focus on multicomponent gelators, which rely on the initial interaction of distinct individual components to form a complex that subsequently self-assembles into a fibrous supramolecular polymer. In true two-component systems, an individual component can be present in isotropic solution, and only on addition of the second component will a gel actually form. In some cases, however, two-component gels are reported in which the second component significantly modifies the behaviour of a known gelator. Both systems are discussed in this article. The additional level of supramolecular control in the hierarchical self-assembly of two-component gels confers exquisite tunability and controllability. Functionality can be readily built into the material by simple variation of one of the individual components. This article discusses the key approaches used to control self-assembly by manipulating single molecular-recognition events and illustrates how controlling the transcription of information from the molecular to the macroscopic level by the simple addition of a second component allows complex functional materials to be selectively assembled from simple building blocks.

Journal Article↗
PubMed2005

A direct comparison of one- and two-component dendritic self-assembled materials: elucidating molecular recognition pathways.

This paper compares and contrasts, for the first time, one- and two-component gelation systems that are direct structural analogues and draws conclusions about the molecular recognition pathways that underpin fibrillar self-assembly. The new one-component systems comprise l-lysine-based dendritic headgroups covalently connected to an aliphatic diamine spacer chain via an amide bond. One-component gelators with different generations of headgroup (from first to third generation) and different length spacer chains are reported. The self-assembly of these dendrimers in toluene was elucidated using thermal measurements, circular dichroism (CD) and NMR spectroscopies, scanning electron microscopy (SEM), and small-angle X-ray scattering (SAXS). The observations are compared with previous results for the analogous two-component gelation system in which the dendritic headgroups are bound to the aliphatic spacer chain noncovalently via acid-amine interactions. The one-component system is inherently a more effective gelator, partly as a consequence of the additional covalent amide groups that provide a new hydrogen bonding molecular recognition pathway, whereas the two-component analogue relies solely on intermolecular hydrogen bond interactions between the chiral dendritic headgroups. Furthermore, because these amide groups are important in the assembly process for the one-component system, the chiral information preset in the dendritic headgroups is not always transcribed into the nanoscale assembly, whereas for the two-component system, fiber formation is always accompanied by chiral ordering because the molecular recognition pathway is completely dependent on hydrogen bond interactions between well-organized chiral dendritic headgroups.

Journal Article↗
PubMed2004

Solvent effects on supramolecular gel-phase materials: two-component dendritic gel.

The self-assembly of diaminododecane with dendritic l-lysine-based peptides to form gel-phase materials was investigated in a range of different solvents. The degree of structuring was modulated by the solvent employed, an effect which induced subtle changes in the mesoscale aggregate morphology and macroscopic behavior of the self-assembled state. In this paper a range of different solvent parameters are investigated, and it is clearly shown that macroscopic gelation can be related to a solvent polar solubility parameter for this system. The results also show a dependence on Kamlet-Taft hydrogen bonding parameters, and this clearly demonstrates the role of the solvent environment in terms of dendron--dendron intermolecular hydrogen bonding and its impact on the supramolecular chiral organization of the assembled superstructure.

Circular Dichroism↗
PubMed2004

Two-component dendritic gel: effect of stereochemistry on the supramolecular chiral assembly.

The self-assembly of diaminododecane solubilised by four different stereoisomeric dendritic peptides to form gel-phase materials in toluene was investigated. The second generation dendritic peptides were based on D- and L-lysine building blocks, and each contained three chiral centres. By designing dendritic peptides in which the configurations of the chiral centres were modified, and applying them as gelator units, the assembly of stereoisomers could be investigated. In all cases, the self-assembly of gelator units resulted in macroscopic gelation. However, the degree of structuring was modulated by the stereoisomers employed, an effect which changed the morphology and macroscopic behavior of the self-assembled state. Enantiomeric (L,L,L or D,D,D) gelator units formed fibrous molecular assemblies, whilst the racemic gel (50 % L,L,L : 50 % D,D,D) formed a flat structure with a "woven" appearance. Gelator units based on L,D,D or D,L,L dendritic peptides also formed fibrous assemblies, but small-angle X-ray scattering indicated significant morphological differences were caused by the switch in chirality. Furthermore, the macroscopic stability of the gel was diminished when these peptides were compared with their L,L,L or D,D,D analogues. In this paper it is clearly shown that individual stereocentres, on the molecular level, are directly related to the helicity within the fibre. It is argued that the chirality controls the pattern of hydrogen bonding within the assembly, and hence determines the extent of fibre formation and the macroscopic gel strength.

Circular Dichroism↗
PubMed2004

Self-assembly of two-component peptidic dendrimers: dendritic effects on gel-phase materials.

The self-assembly of diaminododecane solubilised by different dendritic peptides, possessing increasing levels of dendritic branching, was investigated. The dendritic peptides were based on l-lysine building blocks and were of first, second and third generation, containing one, three and seven amino acid repeat units respectively. By applying these structures as potential gelator units, the dendritic effect on gelation was investigated. The degree of structuring was modulated, with the dendritic peptide controlling the aggregate morphology and the ability of the self-assembled state to manifest itself macroscopically as gelation. First generation gelator units (G1) did not induce macroscopic gelation with diaminododecane under any conditions, whilst those self-assemblies based on second (G2) and third (G3) generation branches did form gel-phase materials. Furthermore, gel-phase materials based on G2 exhibited optimum gelation behaviour compared to those based on G3(in terms of the thermal strength of the materials). Circular dichroism showed that the dendritic effect, programmed in at the molecular level, is directly related to the degree of chiral organisation within the self-assembled state. The dendritic generation of the peptide controls the pattern of amide-amide hydrogen bonding in terms of binding strength and alignment as determined using NMR methods. The mode of self-assembly can be qualitatively rationalised in terms of an attractive enthalpic interaction (i.e., amide-amide hydrogen bonding), a repulsive interaction (i.e., steric interactions between dendritic peptides) and an entropic term related to the hierarchical organisation of the gelator building blocks. It is argued that the balance between these factors determines the nature of the dendritic effect.

Biocompatible Materials↗
PubMed2004

Two-component dendritic gel: effect of spacer chain length on the supramolecular chiral assembly.

The present study investigates in detail the physical gelation of toluene induced by the addition of simple aliphatic diamines to a dendritic L-lysine-based peptide. The gel-phase material obtained was characterized using differential scanning calorimetry, scanning electron microscopy, small-angle X-ray scattering, circular dichroism, 1H NMR, and X-ray diffraction. When the length of the aliphatic diamine is incrementally increased (C6-C12), the thermally reversible gel-sol transition temperature is dramatically increased (4-105 degrees C). This paper shows that the molecular information preset in the diamine is transcribed into the supramolecular assembly on the microscale and that this, in turn, controls the highly tunable macroscopic materials properties. The results also demonstrate the importance of chirality in the assembly process and highlight the role played by the aliphatic diamine in modulating the transcription of chirality from the molecular to the microscopic level.

Journal Article↗
PubMed2004

One-component gels based on peptidic dendrimers: dendritic effects on materials properties.

This paper describes the gelation of symmetric dendrimers based on building blocks constructed from L-lysine. These dendrimers form gel-phase materials in nonpolar organic solvents. The thermal properties and concentration dependence of the gelation were investigated, and it was found that there was a clear dendritic effect on the behavior of the soft materials formed, with higher generation dendrimers giving rise to more thermally stable gels. Variable temperature 1H NMR studies indicated that this behavior was probably a consequence of more extensive interdendrimer hydrogen bonding occurring between the peptidic groups in the higher generation dendrimers. The supramolecular aggregates were found to have a fibrillar structure, with the dimensions and alignment of the fibers being dependent on dendritic generation. Circular dichroism measurements confirmed that these fibers possessed chiral organization of the peptidic groups on the supramolecular (nano) scale, assigned as helicity. This paper indicates that dendritic functionalization provides a useful way of tuning gel-phase materials properties, with clear dendritic effects on gel formation being quantified for the first time, hence illustrating the way dendritic functionalization can play a positive role in the formation of highly functional organic materials with desirable properties.

Journal Article↗
PubMed2003

Two-component dendritic gels: easily tunable materials.

This paper reports the tunability of a two-component gel system based on dendritic l-lysine with a focal point carboxylic acid group and aliphatic diamines. The microscopic structure and macroscopic properties of the gel can be modulated by changing the concentration of the components, altering their molecular structures, or tuning their relative molar ratio. In the latter case, there is a complete change in the morphology of the gel that has a direct impact on its macroscopic properties, specifically its gel-sol transition temperature.

Journal Article↗