Martin C. Grossel

Effects of PEGylation and acetylation of PAMAM dendrimers on DNA Binding, cytotoxicity and in vitro transfection efficiency

Poly(amidoamine) (PAMAM) dendrimers are promising multipotent gene delivery vectors, providing favorable DNA condensation properties also in combination with the possibility of conjugation of different targeting ligands to their surface. They have been used for transfection both in vitro and in vivo, but their application is currently somewhat limited due to inherent cytotoxicity. In this work we investigate how two types of surface modification, acetylation and PEGylation, affect the DNA binding characteristics, the cytotoxicity and the in vitro transfection efficiency of generation 4 and 5 PAMAM dendrimers. Particularly, we address how the morphology of DNA-dendrimer complexes, formed under low salt conditions, changes upon dilution in cell growth medium, an event that inevitably occurs before the complexes reach the cell surface in any transfection experiment. We find that acetylation and PEGylation essentially eliminates the inherent dendrimer cytotoxicity. However, the transfection efficiency of the modified dendrimers is lower than that of the corresponding unmodified dendrimers, which can be rationally understood by our observations that DNA is less condensed when complexed with these modified dendrimers. Although small DNA-dendrimer particles are formed, the availability for ethidium intercalation and nuclease degradation is significantly higher in the modified DNA-dendrimer complexes than in unmodified ones. Dilution in cell growth medium has a drastic effect on these electrostatically assembled complexes, resulting in increase in size and DNA availability. Our results strongly add to the notion that it is of importance to perform a biophysical characterization under conditions as close to the transfection situation as possible, to enable conclusions regarding structure-activity relations of gene delivery vectors.

Rotor in a cage: infrared spectroscopy of an endohedral hydrogen-fullerene complex

We report the observation of quantized translational and rotational motion of molecular hydrogen inside the cages of C(60). Narrow infrared absorption lines at the temperature of 6 K correspond to vibrational excitations in combination with translational and rotational excitations and show well-resolved splittings due to the coupling between translational and rotational modes of the endohedral H(2) molecule. A theoretical model shows that H(2) inside C(60) is a three-dimensional quantum rotor moving in a nearly spherical potential. The theory provides both the frequencies and the intensities of the observed infrared transitions. Good agreement with the experimental results is obtained by fitting a small number of empirical parameters to describe the confining potential, as well as the relative concentration of ortho- and para-H(2).

On the twist-bend nematic phase formed directly from the isotropic phase

Abstract The intriguing twist-bend nematic (NTB) phase is formed, primarily, by liquid crystal dimers having odd spacers. Typically, the phase is preceded by a nematic (N) phase via a weak first-order transition. Our aim is to obtain dimers where the NTB phase is formed directly from the isotropic (I) phase via a strong first-order phase transition. The analogy between such behaviour and that of the smectic A (SmA)–N–I sequence suggests that this new dimer will require a short spacer. This expectation is consistent with the prediction of a molecular field theory, since the decrease in the spacer length results in an increase in the molecular curvature. A vector of odd dimers based on benzoyloxybenzylidene mesogenic groups with terminal ethoxy groups has been synthesised with spacers composed of odd numbers of methylene groups. Spacers having 5, 7, 9 and 11 methylene groups are found to possess the conventional phase sequence NTB–N–I; surprisingly, for the propane spacer, the NTB phase is formed directly from the I phase. The properties of these dimers have been studied with care to ensure that the identification of the NTB phase is reliable. Graphical Abstract

Optimisation of polymer scaffolds for retinal pigment epithelium (RPE) cell transplantation

Aim To evaluate a variety of copolymers as suitable scaffolds to facilitate retinal pigment epithelium (RPE) transplantation. Methods Five blends of poly(l-lactic acid) (PLLA) with poly(d,l-lactic-glycolic acid) (PLGA) were manufactured by a solid–liquid phase separation technique. The blends were 10:90, 25:75, 50:50, 75:25 and 90:10 (PLLA:PLGA). All blend ratios were validated by nuclear magnetic resonance spectroscopy. Samples of polymer blends were coated with laminin. Coated and uncoated blends were seeded with a human RPE cell line. Cell attachment, viability and retention of phenotype were assessed. Results As the lactide unit content increased pore size generally became smaller. The 25:75 PLLA:PLGA blend was the most porous (44%) and thinnest (134 μm) scaffold produced. ARPE-19 cells retained an appropriate phenotype with minimal cell death for up to 4 weeks in vitro. Cell density was maintained on only one of the fabricated ratios (25% PLLA:75% PLGA). A consistent decrease in apoptotic cell death with time was observed on coated samples of this blend. A decrease in polymer thickness concomitant with an increase in porosity characteristic of degradation was observed with all polymer blends. Conclusions This study demonstrates that a 25:75 copolymer blend of PLLA:PLGA is a potentially useful scaffold for ocular cell transplantation.

Polymorphism in pyridine-2,6-dicarboxylic acid: Competition between “robust” synthons

A new polymorph of anhydrous pyridine-2,6-dicarboxylic (dipicolinic) acid in which salt formation is favoured over carboxylic acid dimerisation is reported, together with the solid-state architectures of a novel dipicolinic acid dihydrate and a monohydrate structure of the 4-chloro analogue.

Comparison of Chiral and Racemic Forms of Zinc Cyclohexane trans‐1,2‐Dicarboxylate Frameworks: A Structural, Computational, and Calorimetric Study

An integrated study of the organic-inorganic framework material zinc cyclohexane trans-1,2-dicarboxylate involving synthesis, structure elucidation, computer simulation, and calorimetry shows that the chiral R,R form (right in picture) is less stable than its racemic R,R/S,S analogue (left) and adopts a layered structure with a fundamentally different topology. The results point to the possibility that the structural diversity of racemic frameworks and their homochiral analogues may be much greater than has hitherto been suspected.

The chemistry of retinal transplantation: the influence of polymer scaffold properties on retinal cell adhesion and control

Age-related macular degeneration is the most common cause of blindness in the UK. Cellular replacement of retinal pigment epithelium cells is a potential therapeutic option to treat the cellular loss and dysfunction which is characteristic of age-related macular degeneration and other progressive retinopathies. A supportive scaffold, natural or artificial, may be required to facilitate cell delivery to the eye. Research to improve the biomimetic properties of such scaffolds, in order to optimise cell attachment and functionality following implantation, is ongoing. This short review will focus on the potential of biomaterials for ocular tissue engineering and how surface modification and the physical properties of these scaffolds can be tailored to help realise the full clinical potential of retinal pigment epithelium cell transplantation.

Infrared emitting PbSe nanocrystals for telecommunications window applications

We demonstrate the colloidal synthesis of PbSe nanocrystal quantum dots, via an organometallic-precursor route, developed from recently reported techniques. This synthesis typically yields a particle size distribution of approximately 5–10%, as may be inferred from the sharp spectral features seen in absorption and from our effective-mass model correlating spectral features to nanocrystal size. An accurate quantitative analysis, using an infrared reference dye, shows these nanocrystals to exhibit infrared photoluminescence from intrinsic quantum-confined states, with high quantum efficiencies of up to 60% in solution. The wavelength of the photoluminescence may also be conveniently size tuned in order to access the 1.3–1.5 µm ‘telecommunications window’. We discuss the significance of this work in the context of future optoelectronic applications.

Using the photoinduced reversible refractive-index change of an azobenzene co-polymer to reconfigure an optical Bragg grating

A photoactive material has been synthesised in which azobenzene units are attached to a methyl methacrylate/2-hydroxyethyl methacrylate co-polymer and shows repeated photoresponsive switching upon irradiation at 365 nm and 254 nm with long term thermal stability. In combination with a direct UV-written silica-on-silicon Bragg grating, this has been used to fabricate a prototype optical device which undergoes reversible refractive index changes at telecom wavelengths. The 63 GHz tuning response demonstrated by this device has potential applicability for reconfigurable dispersion compensation for use in optical networks.

UV written waveguides using crosslinkable PMMA-based copolymers

Crosslinkable copolymers poly(methylmethacrylate/2-methacryloylethylmethacrylate) (P(MMA/MAOEMA)) were developed for waveguide applications. P(MMA/MAOEMA) can crosslink under either UV exposure or heating. The UV-induced refractive index change in unreacted P(MMA/MAOEMA) is found to depend on the fluence. UV exposure of thermally crosslinked P(MMA/MAOEMA) can induce further structure change and thus index change, and therefore, was found to be useful for creating the core layers in optical waveguides. The photosensitivity of the thermally crosslinked polymers is sufficient for the fabrication of low loss (<1 dB/cm) channel waveguides in the thermally crosslinked copolymer system.