Matteo Calvaresi

The devil and holy water: protein and carbon nanotube hybrids.

Integrating carbon nanotubes (CNTs) with biological systems to form hybrid functional assemblies is an innovative research area with great promise for medical, nanotechnology, and materials science applications. The specifics of molecular recognition and catalytic activity of proteins combined with the mechanical and electronic properties of CNTs provides opportunities for physicists, chemists, biologists, and materials scientists to understand and develop new nanomachines, sensors, or any of a number of other molecular assemblies. Researchers know relatively little about the structure, function, and spatial orientation of proteins noncovalently adsorbed on CNTs, yet because the interaction of CNTs with proteins depends strongly on the tridimensional structure of the proteins, many of these questions can be answered in simple terms. In this Account, we describe recent research investigating the properties of CNT/protein hybrids. Proteins act to solvate CNTs and may sort them according to diameter or chirality. In turn, CNTs can support and immobilize enzymes, creating functional materials. Additional applications include proteins that assemble ordered hierarchical objects containing CNTs, and CNTs that act as protein carriers for vaccines, for example. Protein/CNT hybrids can form bioscaffolds and can serve as therapeutic and imaging materials. Proteins can detect CNTs or coat them to make them biocompatible. One of the more challenging applications for protein/CNT hybrids is to make CNT substrates for cell growth and neural interfacing applications. The challenge arises from the structures interactions with living cells, which poses questions surrounding the (nano)toxicology of CNTs and whether and how CNTs can detect biological processes or sense them as they occur. The surface chemistry of CNTs and proteins, including interactions such as π-π stacking interactions, hydrophobic interactions, surfactant-like interactions, and charge-π interactions, governs the wealth of structures, processes, and functions that appear when such different types of molecules interact. Each residue stars in one of two main roles, and understanding which residues are best suited for which type of interaction can lead to the design of new hybrids. Nonlocally, the peptide or protein primary, secondary, and tertiary structures govern the binding of proteins by CNTs. The conjugation of proteins with CNTs presents some serious difficulties both experimentally and culturally (such as bridging the jargon barrier across disciplines). The intersection of these fields lies between communities characterized by distinctly different approaches and methodologies. However, the examples of this Account illustrate that when this barrier is overcome, the exploitation of hybrid CNT-protein systems offers great potential.

Baiting proteins with C60.

About 20 proteins are known to modify their activity upon interaction with C60. Their structures are present in a database that includes more than 1200 protein structures selected as possible targets for drugs and to represent the entire Protein Data Bank. The set was examined with an algorithm that appraises quantitatively the interaction of C60 and the surface of each protein. The redundancy of the set allows to establish the predictive power of the approach that finds explicitly the most probable site where C60 docks on each protein. About 80% of the known fullerene binding proteins fall in the top 10% of scorers. The close match between the model and experiments vouches for the accuracy of the model and validates its predictions. The sites of docking are shown and discussed in view of the existing experimental data available for protein-C60 interaction. A closer exam of the 10 top scorers is discussed in detail. New proteins that can interact with C60 are identified and discussed for possible future applications as drug targets and fullerene derivatives bioconjugate materials.

Wrapping Nanotubes with Micelles, Hemimicelles, and Cylindrical Micelles

This work uses a simple model based on hydrophobic and hydrophilic forces to investigate the molecular dynamics that lead to the supramolecular self-assembly of surfactants around carbon nanotubes (CNTs). The effects of the concentration and the structure of surfactants are explored. The bead-based mesoscopic description spontaneously develops the several micellar morphologies that are known to wrap and solvate CNTs.

Probing the Structure of Lysozyme–Carbon‐Nanotube Hybrids with Molecular Dynamics

Lysozyme has been successfully used to solvate carbon nanotubes (CNT). Extensive molecular dynamics simulations show that 1) a favorite site of adsorption exists, 2) the protein-tube interaction region is located far from the active site, 3) two protein helices act as a tweezer that grips the nanotube, 4) a localized protein re-arrangement hides the tube from the solvent, and 5) aminic and amidic moieties of lysozyme behave similarly to surfactants in the solvation of the tube.

Temperature‐Dependent Fluorescence of Cu5 Metal Clusters: A Molecular Thermometer

The accurate measurement of temperature is of increasing importance as it is required for widespread applications (electronic devices, biology, medical diagnostics). In this context, fluorescence thermometry has already shown great potential, and a variety of molecules have been proposed as luminescent molecular thermometers. Herein, we describe Cu5 metal cluster 1 (Figure 1) that presents remarkable photophysical properties, both in solution and as the solid, characterized by temperature-dependent emission intensity and lifetime that change significantly in the range between 45 and + 80 8C. These properties allow for an unprecedented accuracy in temperature determination by fluorescence measurements, with the high sensitivity and the high temporal (sub-millisecond) and spatial (sub-micrometer) resolution typical of photoluminescence spectroscopy. Complex 1 can be seen as a metal nanoparticle composed of five copper atoms bound to three highly conjugated dianionic cationic ligands (EtNC(S)PPh2NPPh2C(S)NEt) ; Figure 1A). 14] Its absorption spectrum presents a broad and unstructured band below 450 nm (Figure 2A). The system is luminescent in all phases, both at room temperature and at 77 K (Figure 2B) and no dependence on the solvent was observed. A summary of the photophysical properties is shown in Table 1.

A Simple Road for the Transformation of Few-Layer Graphene into MWNTs

We report the direct formation of multiwalled carbon nanotubes (MWNT) by ultrasonication of graphite in dimethylformamide (DMF) upon addition of ferrocene aldehyde (Fc-CHO). The tubular structures appear exclusively at the edges of graphene layers and contain Fe clusters. Fc in conjunction with benzyl aldehyde, or other Fc derivatives, does not induce formation of NT. Higher amounts of Fc-CHO added to the dispersion do not increase significantly MWNT formation. Increasing the temperature reduces the amount of formation of MWNTs and shows the key role of ultrasound-induced cavitation energy. It is concluded that Fc-CHO first reduces the concentration of radical reactive species that slice graphene into small moieties, localizes itself at the edges of graphene, templates the rolling up of a sheet to form a nanoscroll, where it remains trapped, and finally accepts and donates unpaired electron to the graphene edges and converts the less stable scroll into a MWNT. This new methodology matches the long held notion that CNTs are rolled up graphene layers. The proposed mechanism is general and will lead to control the production of carbon nanostructures by simple ultrasonication treatments.

Graphene Can Wreak Havoc with Cell Membranes

Molecular dynamics--coarse grained to the level of hydrophobic and hydrophilic interactions--shows that small hydrophobic graphene sheets pierce through the phospholipid membrane and navigate the double layer, intermediate size sheets pierce the membrane only if a suitable geometric orientation is met, and larger sheets lie mainly flat on the top of the bilayer where they wreak havoc with the membrane and create a patch of upturned phospholipids. The effect arises in order to maximize the interaction between hydrophobic moieties and is quantitatively explained in terms of flip-flops by the analysis of the simulations. Possible severe biological consequences are discussed.

A molecular dynamics investigation of structure and dynamics of SDS and SDBS micelles

Sodium dodecylbenzene sulfonate (SDSB) is a major constituent of some synthetic detergents with billions of kilograms produced and dispersed in the environment annually. It has now reached a prominent role in the emerging field of nanotechnology for possible uses in exfoliating graphitic materials. It differs from the highly computationally investigated sodium dodecyl sulfate (SDS) because of the presence of a phenyl group attached to the anionic head. In this paper, using MD simulations, we characterize computationally for the first time SDBS micelles. SDBS and SDS micelles in explicit water show that the SDBS micelle is more spherical with a ratio of eccentricities, eSDBS/eSDS, equal to 0.117/0.154. The mean micellar radius is similar for the two surfactants and equal to 19.4 A for SDS and 20.0 A for SDBS. Analysis of the solvent accessible surface area, chain direction, end-to-end distances, and carbon–carbon bond conformations shows that the hydrophobic core made by the dodecyl chains of SDBS is more tightly packed than SDS.

Electronic structure of carbon nanotubes with ultrahigh curvature.

The electronic and the vibrational structure of carbon nanotubes with ultrahigh curvature was systematically studied by resonance Raman scattering, high-resolution transmission electron microscopy (HRTEM), molecular dynamics, and ab initio DFT calculations. The ultrahigh curvature tubes were grown inside commercial HiPco tubes after filling the latter with the small but carbon-rich molecule ferrocene. TEM showed partial filling of the outer tubes with inner tubes and mobility of the latter in the electron beam. The smallest analyzed tube was of (5,0) chirality and had a DFT determined diameter of 0.406 nm and a radial breathing mode frequency of 570 cm(-1). For all inner tubes which had transitions in the visible spectral range, transition energies and RBM frequencies were determined with a resonance width of only 45 meV. Experimentally determined transition energies revealed dramatic deviations up to several electronvolts compared to tight-binding calculations and a significant family spread of more than 2 eV but were in agreement with many electron contribution corrected extended tight-binding results and with results from DFT calculations.

A computational analysis of the insertion of carbon nanotubes into cellular membranes

Carbon nanotubes have been proposed to serve as nano-vehicles to deliver genetic or therapeutic material into the interior of cells because of their capacity to cross the cell membrane. A detailed picture of the molecular mode of action of such a delivery is, however, difficult to obtain because of the concealing effects of the cell membrane. Here we report a systematic computational study of membrane insertion of individual carbon nanotubes and carbon nanotube bundles using two entirely different and unrelated techniques. First a static scan of the environmental free energy is carried out based on a membrane mimicry approach and different insertion geometries are assessed. Then the dynamics is investigated with a coarse-grained approach that was previously used in the study of the integration dynamics of nanoparticles into the bilayer. The results of both models point, for unfunctionalized carbon nanotubes, at a preference for the horizontal orientation inside the internal hydrophobic layer of the cell membrane. Finally, the energetics of the formation of bundles of carbon nanotubes is studied. The cellular membrane promotes aggregation of carbon nanotubes in its hydrophobic core and modifies the structural stability of the bundles.