Frederik Tielens

Investigation of the interface in silica-encapsulated liposomes by combining solid state NMR and first principles calculations.

In the context of nanomedicine, liposils (liposomes and silica) have a strong potential for drug storage and release schemes: such materials combine the intrinsic properties of liposome (encapsulation) and silica (increased rigidity, protective coating, pH degradability). In this work, an original approach combining solid state NMR, molecular dynamics, first principles geometry optimization, and NMR parameters calculation allows the building of a precise representation of the organic/inorganic interface in liposils. {(1)H-(29)Si}(1)H and {(1)H-(31)P}(1)H Double Cross-Polarization (CP) MAS NMR experiments were implemented in order to explore the proton chemical environments around the silica and the phospholipids, respectively. Using VASP (Vienna Ab Initio Simulation Package), DFT calculations including molecular dynamics, and geometry optimization lead to the determination of energetically favorable configurations of a DPPC (dipalmitoylphosphatidylcholine) headgroup adsorbed onto a hydroxylated silica surface that corresponds to a realistic model of an amorphous silica slab. These data combined with first principles NMR parameters calculations by GIPAW (Gauge Included Projected Augmented Wave) show that the phosphate moieties are not directly interacting with silanols. The stabilization of the interface is achieved through the presence of water molecules located in-between the head groups of the phospholipids and the silica surface forming an interfacial H-bonded water layer. A detailed study of the (31)P chemical shift anisotropy (CSA) parameters allows us to interpret the local dynamics of DPPC in liposils. Finally, the VASP/solid state NMR/GIPAW combined approach can be extended to a large variety of organic-inorganic hybrid interfaces.

Reactivity of gold nanoparticles towards N-heterocyclic carbenes

The reaction of gold nanoparticles with benzimididazol-2-ylidene ligands leads to the formation of well-defined bis-carbene gold(i) complexes, as shown by characterization techniques such as powder XRD and solid state NMR.

The amorphous silica-liquid water interface studied by ab initio molecular dynamics (AIMD): local organization in global disorder

The structural organization of water at a model of amorphous silica-liquid water interface is investigated by ab initio molecular dynamics (AIMD) simulations at room temperature. The amorphous surface is constructed with isolated, H-bonded vicinal and geminal silanols. In the absence of water, the silanols have orientations that depend on the local surface topology (i.e. presence of concave and convex zones). However, in the presence of liquid water, only the strong inter-silanol H-bonds are maintained, whereas the weaker ones are replaced by H-bonds formed with interfacial water molecules. All silanols are found to act as H-bond donors to water. The vicinal silanols are simultaneously found to be H-bond acceptors from water. The geminal pairs are also characterized by the formation of water H-bonded rings, which could provide special pathways for proton transfer(s) at the interface. The first water layer above the surface is overall rather disordered, with three main domains of orientations of the water molecules. We discuss the similarities and differences in the structural organization of the interfacial water layer at the surface of the amorphous silica and at the surface of the crystalline (0 0 0 1) quartz surface.

Stability of binary SAMs formed by omega-acid and alcohol functionalized thiol mixtures.

A simple model is presented to describe the mixing process of acid- and alcohol-terminated thiol self-assembled monolayers (SAMs) on Au(111). It was found that the low-concentration acid-terminated SAMs are enriched in acid thiol compared with the original solution from which the SAM is made. Due to the relatively strong interaction between acid and alcohol head groups, homogeneously mixed SAMs showing alcohol/acid pairs are preferred when the acid-terminated thiol fraction in solution is below or equal to 50%. This particular behavior affecting the mixing process is explained using atomistic first-principle thermodynamics. The calculated phase diagram has been discussed and interpreted with the help of XPS.

Effect of coverage and defects on the adsorption of propanethiol on Au(111) surface: a theoretical study.

Periodic density functional calculations have been carried out to investigate both the thiol adsorption on Au(111) surface and the reaction mechanism for the formation of the self-assembled monolayers, taking propanethiol as a representative example. The effect of coverage and surface defects (adatoms and vacancies) has been analyzed. It is found that the most stable physisorption (undissociated) site is an adatom site, whereas the chemisorption site for the thiol is a vacancy site or protrusion consisting of a pair of adatoms, followed by one adatom site. The results point out that the thiolate self-assembled monolayer adsorption process occurs preferentially on step edges.

Characterization of two-dimensional chiral self-assemblies L- and D-methionine on Au(111).

A combination of XPS, in situ RAIRS, LEED, and STM experiments together with ab initio DFT calculations were used to elucidate the self-assembly properties at the atomic level, and enabled the interpretation of the expression of surface chirality upon adsorption of both enantiomers of methionine on a clean Au(111) surface under UHV conditions. The combination of experimental results, in particular, LEED and STM data with quantum chemical calculations is shown to be a successful setup strategy for addressing this challenge. It was found that the methionine molecular self-assembly consists of the first molecule lying parallel to the gold surface and the second interacting with the first methionine through a 2D H-bond network. The interaction with the gold surface is weak. The stability of the assembly is mainly due to the presence of intermolecular H bonds, resulting in the formation of ziplike dimer rows on the Au(111) surface. The methionine molecules interact with each other via their amino acid functional groups. The assembly shows an asymmetric pattern due to a slightly different orientation of the methionine molecules with respect to the surface. Simulations of the STM image of methionine assemblies were consistent with the experimental STM image. The present study shows another example of Au(111) stabilizing a self-assembled biological layer, which is not chemically perturbed by the surface.

Self-assembly of (S)-glutamic acid on Ag(100): a combined LT-STM and ab initio investigation.

Self-assembly of organic molecules at metal surfaces is of greatest importance in nanoscience; in fact, it opens new perspectives in the field of molecular electronics and in the study of biocompatible materials. Combining an experimental low-temperature scanning tunneling microscopy investigation with ab initio calculations, we succeeded to describe in detail (S)-glutamic acid adsorption on Ag(100) at T = 350 K. We find that (S)-glutamic acid organizes in a squared structure and, at variance with the majority of cases reported in literature, it adsorbs in the neutral form, 4.6 A above the surface plane. The interaction with the poorly reactive Ag substrate is only due to weak van der Waals forces, while H-bonds between carboxyl groups and the formation of a OCOH-OCOH-OCOH-OCOH cycle at the vertex of the squares are the main responsible for the self-assembly.

Ab initio study of the bridging hydroxyl acidity and stability in the 12-membered ring of zeolites

Abstract The influence of Al/Si substitutions on the stability and acidic characteristics of a zeolite system is investigated. The 12-membered ring present in a number of zeolites was chosen as a realistic model system for this purpose. The stability of this system upon different double Al/Si substitutions was investigated in order to find evidence for both Lowensteins and Dempseys rules. It was found that both rules hold for the chosen model system. The acidity of the bridging hydroxyls was studied using the calculated deprotonation energy and the Mulliken and ChelpG electronic populations on the H-atom and on the O atom in the conjugated base.

Performance and basis set dependence of density functional theory dipole and quadrupole moments

Abstract A study is presented concerning the performance and basis set dependence of density functional methods in the calculation of dipole and quadrupole moments. A set of some small molecules was thereby used, together with seven commonly used exchange correlation functionals and Dunnings correlation consistent aug-cc-pVDZ, aug-cc-pVTZ, aug-cc-pVQZ and aug-cc-pV5Z basis sets. The results were compared with experiment and previously published values obtained with traditional ab initio molecular orbital methods. The best performance in the calculation of dipole and quadrupole moments is due to the hybrid functionals B3LYP and B3PW91. Moreover, the results seem to be essentially converged at the aug-cc-pVTZ level. Comparison of the LDA/aug-cc-pV5Z results with the numerical LDA results of Dickson and Becke, reveals that both sets of values are identical for most cases.

Drastic Au(111) surface reconstruction upon insulin growth factor tripeptide adsorption.

Adsorption of biomolecules at metal surfaces often creates two-dimensional ordering of the adlayers. However, metal substrate reconstruction is less commonly observed, unless upon annealing of the molecule-surface system. Here, we report on the drastic room-temperature reconstruction of the Au(111) surface, driven by the adsorption of insulin growth factor tripeptide molecules. Scanning tunneling microscopy images show that the surface reconstruction, which takes place without annealing the system, is dynamic and evolves over time. It is initiated at kinks and steps edges, but the reconstruction also takes place within defect-free terraces. Theoretical calculations are performed to explain the reconstruction at the molecular level.