Alison Paul

Tuneable mechanical properties in low molecular weight gels

The mechanical properties of gels are critical to the final targeted applications. Depending on the application, different properties may be required. Here, we show that the mechanical strength and ability to recover of gels formed using a low molecular weight gelator can be controlled by two independent factors (i) the volume fraction of co-solvent (in this case DMSO) in the system and (ii) the temperature cycle used. These differences correlate with the large scale structure of the network that is formed from the self-assembled fibres. This opens up the potential to prepare gels with very different properties at the same final conditions, allowing the effect of microstructure to be probed.

Fluoro-surfactants at air/water and water/CO2 interfaces

Aqueous phase behaviour and water-in-carbon-dioxide (w/c) microemulsion formation were studied with various fluorinated sulfosuccinate surfactants. For aqueous solutions of two different compounds surface coverages measured by neutron reflection and surface tension were consistent, giving reliable values for mean areas per molecule at the air/water (a/w) interface. At 20°C and pressures above about 250 bar, seven different surfactants were found to be effective at stabilising w/c microemulsions. With reference to recent work (J. Eastoe, A. Downer, A. Paul, D. C. Steytler and E. Rumsey, Prog. ColloidPolym. Sci., 2000, 115, 214) it is possible to identify a structure–performance relationship for these surfactants in water–CO2 systems. Comparison of the phase behaviour shows that sodium bis(1H,1H-nonafluoropentyl)-2-sulfosuccinate (di-CF4) forms microemulsions at the lowest pressure, e.g. for w = 20 ([water]added/[surf]) at 30°C the w/c phase was stable down to 120 bar. High-pressure FTIR spectroscopy indicates a fraction of the added water partitions out of microemulsion droplets, thereby saturating the bulk CO2 . Furthermore, high-pressure small-angle neutron scattering (SANS) is characteristic of a simple spherical droplet structure in the microemulsions. SANS data also indicate temperature-induced changes in radius, and this is consistent with partitioning of water to maintain saturation of the bulk CO2. For three different surfactants it has been possible to compare adsorption at a/w and w/c surfaces, highlighting differences in packing requirements to stabilise these two different interfaces.

Low Fluorine Content CO2-philic Surfactants

The article addresses an important, and still unresolved question in the field of CO(2) science and technology: what is the minimum fluorine content necessary to obtain a CO(2)-philic surfactant? A previous publication (Langmuir 2002, 18, 3014) suggested there should be an ideal fluorination level: for optimization of possible process applications in CO(2), it is important to establish just how little F is needed to render a surfactant CO(2)-philic. Here, optimum chemical structures for water-in-CO(2) (w/c) microemulsion stabilization are identified through a systematic study of CO(2)-philic surfactant design based on dichain sulfosuccinates. High pressure small-angle neutron scattering (HP-SANS) measurements of reversed micelle formation in CO(2) show a clear relationship between F content and CO(2) compatibility of any given surfactant. Interestingly, high F content surfactants, having lower limiting aqueous surface tensions, γ(cmc), also have better performance in CO(2), as indicated by lower cloud point pressures, P(trans). The results have important implications for the rational design of CO(2)-philic surfactants helping to identify the most economic and efficient compounds for emerging CO(2) based fluid technologies.

Demonstrating the importance of polymer-conjugate conformation in solution on its therapeutic output: Diethylstilbestrol (DES)-polyacetals as prostate cancer treatment

The design of improved polymeric carriers to be used in the next generation of polymer therapeutics is an ongoing challenge. Biodegradable systems present potential advantages regarding safety benefit apart from the possibility to use higher molecular weight (Mw) carriers allowing PK optimization, by exploiting the enhanced permeability and retention (EPR)-mediated tumor targeting. Within this context, we previously designed pH-responsive polyacetalic systems, tert-polymers, where a drug with the adequate diol-functionality was incorporated within the polymer mainchain. The synthetic, non-steroidal estrogen, diethylstilboestrol (DES) clinically used for the treatment of advanced prostate cancer was chosen as drug. In order to improve the properties of this tert-polymer, novel polyacetalic systems as block-co-polymers, with more defined structure have been obtained. This second generation polyacetals allowed higher drug capacity than the tert-polymer, a biphasic DES release profile at acidic pH and due to its controlled amphiphilic character readily formed micelle-like structures in solution. These features result in an enhancement of conjugate therapeutic value in selected prostate cancer cell models. Exhaustive physico-chemical characterization focusing on nanoconjugate solution behavior and using advanced techniques, such as, pulsed-gradient spin-echo NMR (PGSE-NMR) and small-angle neutron scattering (SANS), has been carried out in order to demonstrate this hypothesis. Clear evidence of significantly different conformation in solution has been obtained for both polyacetals. These results demonstrate that an adequate control on molecular or supramolecular conformation in solution with polymer therapeutics is crucial in order to achieve the desired therapeutic output.

Adsorption of fluoro surfactants at air-water and water-carbon dioxide interfaces

Aqueous phase behaviour, and water-in-carbon dioxide microemulsion formation, were studied with two model fluorinated anionic surfactants, sodium bis(1H/,lH/-per-fluoropentyl)-2-sulfosuccinate (di-CF4) and sodium bis(1H,1H,5H-octafluoropentyl)-2-sulfosuccinate (di-HCF4). The properties are compared to the common, branched-chain, hydrocarbon surfactant Aerosol-OT (AOT). For the fluoro surfactants, surface excesses at the air-solution interface, measured by tensiometry and neutron reflection, agreed well, and the γ-ln(activity) curves were consistent with a pre-factor of 2 in the Gibbs equation. Both di-CF4 and di-HCF4 were effective at stabilising water-in-carbon dioxide (w/c) microemulsions. Pressure-temperature studies indicated that di-CF4 is the most effective, since it gave microemulsions at lower pressures, e.g. for w = 20 ([water]/[surf]) at 30 °C the w/c phase was stable down to 120 bar. High-pressure small-angle neutron scattering showed the microemulsions have a simple spherical droplet structure. In water-CC2 mixtures these surfactants behave in a similar fashion to AOT in conventional hydrocarbon systems. Therefore, the fluoro-succinates represent a well-characterised model series of surfactants, which may be useful for understanding the effects of surfactant chemistry on w/c microemulsion phase properties.

Polymer-doxycycline conjugates as fibril disrupters: An approach towards the treatment of a rare amyloidotic disease

The term amyloidosis describes neurological diseases where an abnormal protein is misfolded and accumulated as deposits in organs and tissues, known as amyloid, disrupting their normal function. In the most common familial amyloid polyneuropathy (FAP), transthyretin (TTR) displays this role primarily affecting the peripheral nervous system (PNS). Advanced stages of this inherited rare amyloidosis, present as fibril deposits that are responsible for disease progression. In order to stop disease progression, herein we designed an efficient family of nanoconjugates as fibril disrupters. These polymer conjugates are based on doxycycline (doxy), already in phase II trials for Alzheimers disease, covalently linked to poly-l-glutamic acid (PGA). The conjugates were rationally designed, looking at drug loading and drug release rate by adequate linker design, always considering the physiological conditions at the molecular target site. Conjugation of doxycycline exhibited greater potential towards TTR fibril disaggregation in vitro compared to the parent drug. Exhaustive physico-chemical evaluation of these polymer-drug conjugates concluded that drug release was unnecessary for activity, highlighting the importance of an appropriate linker. Furthermore, biodistribution studies through optical imaging (OI) and the use of radiolabelled polymer-drug conjugates demonstrated conjugate safety profile and renal clearance route of the selected PGA-doxy candidate, settling the adequacy of our conjugate for future in vivo evaluation. Furthermore, preliminary studies in an FAP in vivo model at early stages of disease development showed non-organ toxicity evidences. This nanosized-system raises a promising treatment for advanced stages of this rare amyloidotic disease, and also presents a starting point for possible application within other amyloidosis-related diseases, such as Alzheimers disease.

Conformational consequences of cooperative binding of a coiled-coil peptide motif to poly(N-(2-hydroxypropyl) methacrylamide) HPMA copolymers

Small-angle neutron scattering and pulsed-gradient spin-echo NMR have been used to examine the solution conformation of a series of water soluble poly(N-(2-hydroxypropyl) methacrylamide) P(HPMA) co-polymer drug delivery vehicles incorporating a coiled-coil peptide motif as a novel pH sensitive non-covalent linker. The conformation of the HPMA homopolymer is well-described by a Gaussian coil model and changing pH from pH 7 to pH 5 has little effect on the solution conformation, as quantified via the radius of gyration. Copolymerisation with 5-10mol% of the K3 peptide bearing methacrylate monomer (K3-MA), gave a series of copolymers that exhibited an increase in radius of gyration at both pHs, despite being typically 30% lower in molecular weight, indicating that the K3-MA causes a perturbation (expansion) of the copolymer conformation. Subsequent addition of an equimolar amount of the complementary peptide E3 makes little further difference to the conformation, indicative of the intimate binding (coiled-coil motif) between the two peptides. Again, the effects of pH are small. Only the addition of a large aromatic structure such as methotrexate causes a further perturbation of the structure - the hydrophobic interaction between the MTX units causes a significant collapse of the polymer coil. These findings further elaborate the understanding of those factors that determine the solution conformation of novel polymer therapeutics.

Explaining the phase behaviour of the pharmaceutically relevant polymers poly(ethylene glycol) and poly(vinyl pyrrolidone) in semi-fluorinated liquids.

Phase behaviour studies of low molecular weight poly(ethylene glycol) (denoted PEG 600 and PEG 1000, corresponding to molecular weights of 600 and 1000 g mol−1, respectively) have been carried out in 2H, 3H‐perfluoropentane (HPFP) with and without added poly(vinyl pyrrolidone). The concentration and temperature dependencies of their phase behaviour and the effect of moisture on these systems have been established. Furthermore, the solubility of PEG 600 in binary mixtures of HPFP and per‐fluoropentane (PFP), as well as HPFP and perfluorodecalin (PFD) have been considered at high HPFP contents. A phase separation phenomenon in fluorinated non‐aqueous media is reported for the first time: PEG 600 and PEG 1000 both show a lower critical solution temperature type phase separation boundary. The size of the PEGs was obtained from small‐angle neutron scattering (radius of gyration) and pulsed‐gradient spin‐echo NMR (hydrodynamic radius) measurements. It is shown that polymer conformation follows a regular trend with solution concentration; the size increases from <10 Å at 3wt% in HPFP to 45 ± 2 Å at 20wt% PEG. On changing the solvent composition by substitution of HPFP by PFP or PFD, the size decreases, consistent with a decrease in the hydrogen‐bonding capacity of the solvent mixture. Computer modelling indicates an interaction between the PEG oxygen and the hydrogen of HPFP, an interaction that is absent for the fully fluorinated solvents. This indicates that hydrogen bonding is the driving force for polymer solubility in these solvents.

Structure–property relationships in metallosurfactants

The morphology of micelles formed by three sub-classes of metallosurfactants—those with macrocyclic, linear and gemini head groups—has been studied by small-angle neutron scattering (SANS) for a series of metal- and counter-ions. All the data may be described by a model that invokes a globular micelle morphology in which the dimensions of the micelle are consistent with the known chemical structure of the constituent groups within the metallosurfactant. For two macrocyclic head group metallosurfactants, viz. 1-(2-hydroxy-tetradecyl)-1,4,7-triazacyclonane that forms predominantly spherical micelles and 1-(2-hydroxy-tetradecyl)-1,4,7,10-tetraazacyclononane that forms disc-like micelles, the metal ion and its counter-ion have a negligible effect on the morphology of the micelle. Binary mixtures of surfactants with these two macrocyclic head groups (with homo- or hetero-metal ions/counter-ions) form mixed micelles whose morphology is an average of the two single component micelles. Further, as found for the single surfactant solutions, the metal and counter-ion had no effect on the morphology of the mixed surfactant micelle. Lastly, the micelle morphology of two gemini surfactants was also shown to be insensitive to the number and nature of the metal and counter-ions present, but sensitive to the structure of the head group. These observations considerably extend our understanding of the relationship between chemical structure and micelle morphology for these interesting molecules.

Drug Mimic Induced Conformational Changes in Model Polymer-Drug Conjugates Characterized by Small-Angle Neutron Scattering

Copolymers based on poly(N-(2-hydroxypropylmethacrylamide)) with conjugated Doxrubicin are established as candidate anticancer therapeutics. Two HPMA-co-polymers (ca. 35000 g mol(-1)) with 2.5 and 8 mol % gly-phe-leu-gly peptidyl side-chain content have been modified using linear hydrocarbon and small aromatic molecules as simple drug mimics. This first contrast-variation SANS study on these systems demonstrates, combined with detailed modeling, a controlled switch from random coil to a more defined morphology induced by inclusion of a series of model drug mimics. Relatively small changes in drug-mimic type and loading can significantly alter the solution conformation, and we tentatively propose a helical type structure that is more or less tightly wound, depending on both hydrophobe loading and type. The results presented have important implications for understanding the influence of conjugate structure on solution properties, which is an important factor influencing biological and clinical activity.