Aurel Radulescu

Eumelanin buildup on the nanoscale: aggregate growth/assembly and visible absorption development in biomimetic 5,6-dihydroxyindole polymerization.

Establishing structure-property relationships in the black insoluble eumelanins, the key determinants of human pigmentation and skin photoprotective system, is a considerable conceptual and experimental challenge in the current drive for elucidation of the biological roles of these biopolymers and their application as advanced materials for organoelectronics. Herein, we report a new breakthrough toward this goal by the first detailed investigation on the nanoscale level of the oxidative polymerization of 5,6-dihydroxyindole (DHI), a model process of eumelanin synthesis. On the basis of a combined use of spectrophotometry, dynamic light scattering (DLS), and small-angle neutron scattering (SANS) investigations, it was possible to unveil the dynamics of the aggregation process before precipitation, the key relationships with visible light absorption and the shape of fundamental aggregates. The results indicated a polymerization mechanism of the type: Polymer(n) + DHI(x) = Polymer(n+x), where DHI(x) indicates monomer, dimer, or low oligomers (x ≤ 5). During polymerization, visible absorption increases rapidly, reaching a plateau. Particle growth proceeds slowly, with formation of 2-D structures ~55 nm thick, until precipitation occurs, that is, when large aggregates with a maximum hydrodynamic radius (R(h)) of ~1200 nm are formed. Notably, markedly smaller R(h) values, up to ~110 nm, were determined in the presence of poly(vinyl alcohol) (PVA) that was shown to be an efficient aggregation-preventing agent for polymerizing DHI ensuring water solubilization. Finally, it is shown that DHI monomer can be efficiently and partially irreversibly depleted from aqueous solutions by the addition of eumelanin suspensions. This behavior is suggested to reflect oxidant-independent competing pathways of polymer synthesis and buildup via monomer conversion on the active aggregate surface contributing to particle growth. Besides filling crucial gaps in DHI polymerization, these results support the attractive hypothesis that eumelanins may behave as a peculiar example of living biopolymers. The potential of PVA as a powerful tool for solution chemistry-based investigations of eumelanin supramolecular organization and for technological manipulation purposes is underscored.

Interactions between block copolymers and single-walled carbon nanotubes in aqueous solutions: a small-angle neutron scattering study.

The amphiphilic copolymers of the Pluronic family are known to be excellent dispersants for single-walled carbon nanotubes (SWCNT) in water, especially F108 and F127, which have rather long end-blocks of poly(ethylene oxide) (PEO). In this study, the structure of the CNT/polymer hybrid formed in water is evaluated by measurements of small-angle neutron scattering (SANS) with contrast variation, as supported by cryo-transmission electron microscopy (cryo-TEM) imaging. The homogeneous, stable, inklike dispersions exhibited very small isolated bundles of carbon nanotubes in cryo-TEM images. SANS experiments were conducted at different D(2)O/H(2)O content of the dispersing solvent. The data for both systems showed surprisingly minimal intensity values at 70% D(2)O solvent composition, which is much higher than the expected value of 17% D(2)O that is based on the scattering length density (SLD) of PEO. At this near match point, the data exhibited a q(-1) power law relation of intensity to the scattering vector (q), indicating rodlike entities. Two models are evaluated, as extensions to Pedersons block copolymer micelles models. One is loosely adsorbed polymer chains on a rodlike CNT bundle. In the other, the hydrophobic block is considered to form a continuous hydrated shell on the CNT surface, whereas the hydrophilic blocks emanate into the solvent. Both models were found to fit the experimental data reasonably well. The model fit required special considerations of the tight association of water molecules around PEO chains and slight isotopic selectivity.

Large Domain Fluctuations on 50-ns Timescale Enable Catalytic Activity in Phosphoglycerate Kinase

Large-scale domain motions of enzymes are often essential for their biological function. Phosphoglycerate kinase has a wide open domain structure with a hinge near the active center between the two domains. Applying neutron spin echo spectroscopy and small-angle neutron scattering we have investigated the internal domain dynamics. Structural analysis reveals that the holoprotein in solution seems to be more compact compared to the crystal structure but would not allow the functionally important phosphoryl transfer between the substrates if the protein were static. Brownian large-scale domain fluctuation dynamics on a timescale of 50 ns was revealed by neutron spin echo spectroscopy. The dynamics observed was compared to the displacement patterns of low-frequency normal modes. The displacements along the normal-mode coordinates describe our experimental results reasonably well. In particular, the domain movements facilitate a close encounter of the key residues in the active center to build the active configuration. The observed dynamics shows that the protein has the flexibility to allow fluctuations and displacements that seem to enable the function of the protein. Moreover, the presence of the substrates increases the rigidity, which is deduced from a faster dynamics with smaller amplitude.

Internal nanosecond dynamics in the intrinsically disordered myelin basic protein.

Intrinsically disordered proteins lack a well-defined folded structure and contain a high degree of structural freedom and conformational flexibility, which is expected to enhance binding to their physiological targets. In solution and in the lipid-free state, myelin basic protein belongs to that class of proteins. Using small-angle scattering, the protein was found to be structurally disordered similar to Gaussian chains. The combination of structural and hydrodynamic information revealed an intermediary compactness of the protein between globular proteins and random coil polymers. Modeling by a coarse-grained structural ensemble gave indications for a compact core with flexible ends. Neutron spin-echo spectroscopy measurements revealed a large contribution of internal dynamics to the overall diffusion. The experimental results showed a high flexibility of the structural ensemble. Displacement patterns along the first two normal modes demonstrated that collective stretching and bending motions dominate the internal modes. The observed dynamics represent nanosecond conformational fluctuations within the reconstructed coarse-grained structural ensemble, allowing the exploration of a large configurational space. In an alternative approach, we investigated if models from polymer theory, recently used for the interpretation of fluorescence spectroscopy experiments on disordered proteins, are suitable for the interpretation of the observed motions. Within the framework of the Zimm model with internal friction (ZIF), a large offset of 81.6 ns is needed as an addition to all relaxation times due to intrachain friction sources. The ZIF model, however, shows small but systematic deviations from the measured data. The large value of the internal friction leads to the breakdown of the Zimm model.

Anticancer cationic ruthenium nanovectors: From rational molecular design to cellular uptake and bioactivity

An efficient drug delivery strategy is presented for novel anticancer amphiphilic ruthenium anionic complexes, based on the formation of stable nanoparticles with the cationic lipid 1,2-dioleyl-3-trimethylammoniumpropane chloride (DOTAP). This strategy is aimed at ensuring high ruthenium content within the formulation, long half-life in physiological media, and enhanced cell uptake. An in-depth microstructural characterization of the aggregates obtained mixing the ruthenium complex and the phospholipid carrier at 50/50 molar ratio is realized by combining a variety of techniques, including dynamic light scattering (DLS), small angle neutron scattering (SANS), neutron reflectivity (NR), electron paramagnetic resonance (EPR), and zeta potential measurements. The in vitro bioactivity profile of the Ru-loaded nanoparticles is investigated on human and non-human cancer cell lines, showing IC(50) values in the low μM range against MCF-7 and WiDr cells, that is, proving to be 10-20-fold more active than AziRu, a previously synthesized NAMI-A analog, used for control. Fluorescence microscopy studies demonstrate that the amphiphilic Ru-complex/DOTAP formulations, added with rhodamine-B, are efficiently and rapidly incorporated in human MCF-7 breast adenocarcinoma cells. The intracellular fate of the amphiphilic Ru-complexes was investigated in the same in vitro model by means of an ad hoc designed fluorescently tagged analog, which exhibited a marked tendency to accumulate within or in proximity of the nuclei.

Structure and dynamics of polymer rings by neutron scattering: breakdown of the Rouse model

We present a static and quasi-elastic neutron scattering study on both the structure and dynamics of a ring polymer in a ring and linear polymer melt, respectively. In the first case, the ring structure proved to be significantly more compact compared to the linear chain with the same molecular weight. In the mixture, the ring molecules swell as was confirmed by small angle neutron scattering (SANS) in accordance with both theory and simulation work. The dynamical behavior of both systems, which for the first time has been explored by neutron spin echo spectroscopy (NSE), shows a surprisingly fast center of mass diffusion as compared to the linear polymer. These results agree qualitatively with the presented atomistic MD simulations. The fast diffusion turned out to be an explicit violation of the Rouse model.

Peptide modified nanocarriers for selective targeting of bombesin receptors

The present work describes new supramolecular aggregates obtained by co-assembling two different amphiphilic molecules, one containing the bioactive bombesin peptide (BN), or a scramble sequence, and the other, the DOTA chelating agent, (C18)(2)DOTA, capable of forming stable complexes with the radioactive (111)In(III) isotope. The peptide in the amphiphilic monomer is spaced by the lipophilic moiety through ethoxylic spacers of different length: a shorter spacer with five units of dioxoethylene moieties in (C18)(2)L5-peptide, or a longer spacer consisting of a Peg3000 residue in (C18)(2)Peg3000-peptide. Structural characterization by SANS and DLS techniques indicates that, independently from the presence of the peptide containing monomer in the final composition, the predominant aggregates are liposomes of similar shape and size with a hydrodynamic radius R(h) around 200 nm and bilayer thickness, d, of 4 nm. In vitro data show specific binding of the (111)In-(C18)(2)DOTA/(C18)(2)L5-[7-14]BN 90:10 liposomes in receptor expressing cells. However, the presence of the Peg3000 unit on the external liposomal surface, could hide the peptide and prevent the receptor binding. In vivo experiments using (111)In-(C18)(2)DOTA/(C18)(2)L5-[7-14]BN show the expected biological behavior of aggregates of such size and molecular composition, moreover there is an increase in concentration of the GRPR targeting aggregate in the tumors compared to control at the 48 h time point evaluated (2.4% ID/g versus 1.6% ID/g).

Inner structure of adsorbed ionic microgel particles.

Microgel particles of cross-linked poly(NIPAM-co-acrylic acid) with different acrylic acid contents are investigated in solution and in the adsorbed state. As a substrate, silicon with a poly(allylamine hydrochloride) (PAH) coating is used. The temperature dependence of the deswelling of the microgel particles was probed with atomic force microscopy (AFM). The inner structure of the adsorbed microgel particles was detected with grazing incidence small angle neutron scattering (GISANS). Small angle neutron scattering (SANS) on corresponding microgel suspensions was performed for comparison. Whereas the correlation length of the polymer network shows a divergence in the bulk samples, in the adsorbed microgel particles it remains unchanged over the entire temperature range. In addition, GISANS indicates changes in the particles along the surface normal. This suggests that the presence of a solid surface suppresses the divergence of internal fluctuations in the adsorbed microgels close to the volume phase transition.

Characterization of liposomes formed by lipopolysaccharides from Burkholderia cenocepacia, Burkholderia multivorans and Agrobacterium tumefaciens: from the molecular structure to the aggregate architecture

The microstructure of liposomes formed by the lipopolysaccharides (LPS) derived from Burkholderia cenocepacia ET-12 type strain LMG 16656, Burkholderia multivorans strain C1576 and Agrobacterium tumefaciens strain TT111 has been investigated by a combined experimental strategy, including dynamic light scattering (DLS), small-angle neutron scattering (SANS) and electron paramagnetic resonance (EPR). The results highlight that the LPS molecular structure determines, through a complex interplay of hydrophobic, steric and electrostatic interactions, the morphology of the aggregates formed in aqueous medium. All the considered LPS form liposomes that in most cases present a multilamellar arrangement. The thickness of the hydrophobic domain of each bilayer and the local ordering of the acyl chains are determined not only by the molecular structure of the LPS glycolipid portion (lipid A), but also, indirectly, by the bulkiness of the saccharidic portion. In the case of a long polysaccharidic chain, such as that of the LPS derived from Burkholderia multivorans, liposomes coexist with elongated micellar aggregates, whose population decreases if a typical phospholipid, such as dioleoyl phosphatidylethanolamine (DOPE) is introduced in the liposome formulation. The effect of temperature has also been considered: for all the considered LPS an extremely smooth transition of the acyl chain self-organization from a gel to a liquid crystalline phase is detected around 30-35 °C. In the biological context, our results suggest that the rich biodiversity of LPS molecular structure could be fundamental to finely tune the structure and functional properties of the outer membrane of Gram negative bacteria.

Small-angle neutron scattering study of volcanic rocks

Small-angle neutron scattering (SANS) and scanning electron microscopy (SEM) have been used to study the microstructure of several volcanic rocks (basalt, rhyolite, phonolitic pumice, phonolite, rhyolitic obsidian). Magnitudes of the power-law scattering exponent were 3 < α < 4 or α ≈ 4 for the majority of the rocks. The influence of the thermal history on the rock microstructure has been investigated by SANS experiments on thermally treated samples. Specific surface areas were determined for pumice (3 m2/g) and a basalt (2.5 m2/g and 0.2 m2/g) depending on the thermal history. The small-angle scattering in obsidian is shown to be caused by Fe-rich partially crystalline aggregates. The obsidian has been further characterized by ultrasonic resonance and thermal expansion measurements.