Pascal Jonkheijm

Probing the solvent-assisted nucleation pathway in chemical self-assembly

Hierarchical self-assembly offers a powerful strategy for producing molecular nanostructures. Although widely used, the mechanistic details of self-assembly processes are poorly understood. We spectroscopically monitored a nucleation process in the self-assembly of p-conjugated molecules into helical supramolecular fibrillar structures. The data support a nucleation-growth pathway that gives rise to a remarkably high degree of cooperativity. Furthermore, we characterize a helical transition in the nucleating species before growth. The self-assembly process depends strongly on solvent structure, suggesting that an organized shell of solvent molecules plays an explicit role in rigidifying the aggregates and guiding them toward further assembly into bundles and/or gels.

Chemical Strategies for Generating Protein Biochips

Protein biochips are at the heart of many medical and bioanalytical applications. Increasing interest has been focused on surface activation and subsequent functionalization strategies for immobilizing these biomolecules. Different approaches using covalent and noncovalent chemistry are reviewed; particular emphasis is placed on the chemical specificity of protein attachment and on retention of protein function. Strategies for creating protein patterns (as opposed to protein arrays) are also outlined. An outlook on promising and challenging future directions for protein biochip research and applications is also offered.

Photochemical Surface Patterning by the Thiol-Ene Reaction†

The immobilization of proteins on solid substrates while controlling the size and dimensions of the generated patterns is increasingly relevant in biotechnology. Site-specific immobilization and thus control over the orientation of proteins is particularly important because, as opposed to nonspecific adsorption, it generates homogeneous surface coverage and accessibility to the active site of the protein. Consequently, different types of bioorthogonal reactions have been developed to attach proteins site-specifically to surfaces and to control protein patterning. Herein, we report the photochemical coupling of olefins to thiols to generate a stable thioether bond for the covalent surface patterning of proteins and small molecules. This reaction has been applied previously in solution for carbohydrate and peptide coupling. The thiol-ene photoreaction proceeds at close to visible wavelengths (l = 365–405 nm) and in buffered aqueous solutions. As a result of its specificity for olefins, this photoreaction can be considered to be bioorthogonal, unlike other photochemical methods used previously for protein immobilization. To adopt the thiol-ene reaction for the immobilization of biomolecules, surfaces functionalized with thiols and biomolecules derivatized with olefins were prepared (Figure 1). Polyamidoamine (PAMAM) dendrimers were attached covalently to silicon oxide surfaces. An aminocaproic acid spacer was attached to the dendrimers to create distance from the surface. Cystamine was coupled to the spacer, and subsequent reduction of the disulfide yielded the desired thiolterminated surfaces. A liquid layer of terminal-olefinfunctionalized molecules dissolved in ethylene glycol was spread onto these wafers, which were then covered immediately with a photomask. Subsequent irradiation of the surfaces through the photomask led to patterning with adducts of covalently attached thioethers. To establish the method, we photochemically attached the biotin derivative 1 to a thiol-functionalized surface as described above (Figure 1). After the removal of unreacted biotin molecules, the surface was incubated with Cy5-labeled streptavidin (SAv) to produce a SAv-patterned surface. Fluorescence images of the resulting surface (Figure 1) demonstrated that lateral gradients and patterns with micrometer-sized features (5–100 mm) over areas of centimeters in width (Figure 1A) were readily accessible. Figure 1B,C and the fluorescence-intensity profile in Figure 1D show that the patterns have a well-defined shape and are homogeneous over large distances. When prolonged sonication (4 h) and stringent washing were carried our after irradiation, SAv patterns with similar fluorescence intensities were observed, whereas control experiments with biotin that lacked the olefin linker showed no distinctive SAv patterns. These results indicate that the covalent attachment of biotin to the surface occurs specifically through the proposed thiol-ene reaction and that the nonspecific adsorption of biotin is insignificant. Figure 1E shows that the amount of material immobilized can be modified by changing the irradiation time. The procedure reproducibly requires a short irradiation time of 60 s to yield sufficient surface coverage for fabricating dense SAv patterns. To obtain homogeneous fluorescence signals of the patterns, the starting concentration of the solution that is drop cast onto the surface is also important. When the solution of 1 was diluted (to 1 mm), the Cy5-fluorescence intensity decreased considerably. Further dilution (below 500 mm) resulted eventually in disrupted SAv patterns. The application of more concentrated solutions of 1 (> 20 mm) resulted in the saturation of the fluorescence intensity of the SAv patterns. This behavior corresponds well with the effects observed upon varying the irradiation time. Longer irradi[*] Dr. D. N sse, Dr. H. Schroeder, Dr. R. Wacker, Prof. Dr. C. M. Niemeyer Faculty of Chemistry Biological-Chemical Microstructuring Technical University of Dortmund Otto-Hahn-Strasse 6, 44227 Dortmund (Germany) Fax: (+49)231-755-7082 E-mail: christof.niemeyer@tu-dortmund.de

Surface-controlled self-assembly of chiral sexithiophenes

We report on the self-assembly of two enantiomeric sexithiophenes in solution and on surfaces. Circular dichromism of aggregated sexithiophenes and drop-cast films reveals, as expected, mirror image spectra for both enantiomers. The aggregation in thin deposits from sexithiophenes molecularly dispersed in a solution on different types of substrates was investigated by atomic force microscopy (AFM). On graphite, one-dimensional objects (nanowires) are formed while on mica platelets are generated. Remarkably, we found that both enantiomers form left-handed helices on silicon. This observation depends on the hydrophilicity of the silicon. Furthermore, the achiral sexithiophene did not form helical aggregates suggesting that the stereocenter is required to obtain chirality in the fibers.

Surface immobilization of biomolecules by click sulfonamide reaction.

Alkyne-modified biomolecules can be immobilized site- and chemoselectively on sulfonylazide slides under very mild conditions by means of the click sulfonamide reaction.

The influence of hydrogen bonding and pi-pi stacking interactions on the self-assembly properties of C3-symmetrical oligo(p-phenylenevinylene) discs.

Three C3-symmetrical discotics containing a 1,3,5-benzenetricarboxamide unit functionalized with pi-conjugated oligo(p-phenylenevinylene)s (OPV)s have been synthesized and fully characterized. For the two amide OPV discs a two-step transition from helical stacks to molecularly dissolved species was observed and surprisingly, the topology of the amide determines the stability and helicity of the fibers in solution and the length of the fibrils at a surface. In case of the bipyridine disc, aggregates were formed that show little chiral ordering while the stacks remain present over a large temperature range. At a surface, completely disordered structures exist probably as a result of competing types of pi-pi stacking interactions that differ in strength and orientation. The results show that the design of functional self-assembled architectures based on hydrogen bonding and pi-pi stacking interactions is an extremely delicate matter and reveal that special demands have to be taken into account to balance the topology, directionality and strength of multiple secondary interactions.

Exciton bimolecular annihilation dynamics in supramolecular nanostructures of conjugated oligomers

We present femtosecond transient absorption measurements on p-conjugated supramolecular assemblies in a high-pump-fluence regime. Oligo( p-phenylenevinylene! monofunctionalized with ureido-s-triazine ~MOPV! self-assembles into chiral stacks in dodecane solution below 75 °C at a concentration of 4310 24 M. We observe exciton bimolecular annihilation in MOPV stacks at high excitation fluence, indicated by the fluencedependent decay of 1 1 Bu-exciton spectral signatures and by the sublinear fluence dependence of time- and wavelength-integrated photoluminescence ~PL! intensity. These two characteristics are much less pronounced in MOPV solution where the phase equilibrium is shifted significantly away from supramolecular assembly, slightly below the transition temperature. A mesoscopic rate-equation model is applied to extract the bimolecular annihilation rate constant from the excitation fluence dependence of transient absorption and PL signals. The results demonstrate that the bimolecular annihilation rate is very high with a square-root dependence in time. The exciton annihilation results from a combination of fast exciton diffusion and resonance energy transfer. The supramolecular nanostructures studied here have electronic properties that are intermediate between molecular aggregates and polymeric semiconductors.

Anharmonic Magnetic Deformation of Self-Assembled Molecular Nanocapsules

High magnetic fields were used to deform spherical nanocapsules, self-assembled from bolaamphiphilic sexithiophene molecules. At low fields the deformation--measured through linear birefringence-scales quadratically with the capsule radius and with the magnetic field strength. These data confirm a long standing theoretical prediction [W. Helfrich, Phys. Lett. A 43, 409 (1973)10.1016/0375-9601(73)90396-4], and permit the determination of the bending rigidity of the capsules as (2.6+/-0.8) x 10(-21) J. At high fields, an enhanced rigidity is found which cannot be explained within the Helfrich model. We propose a complete form of the free energy functional that accounts for this behavior, and allows discussion of the formation and stability of nanocapsules in solution.

Electrical transport measurements on self-assembled organic molecular wires

The electrical properties of supermolecular assemblies of oligo(p-phenylene vinylene) were studied. These materials self-assemble into well-defined cylindrical structures in solution with lengths in the range of 100 nm-10 microm and diameters between 5 and 200 nm. Atomic force microscopy showed that by adjusting the concentration, either individual molecular wires or a dense film could be deposited. The molecular wires showed poor electrical conduction. Several tests were performed that show that it was the molecular wires themselves, not the contacts, that limit the conductivity.

Direct observation of chiral oligo(p-phenylenevinylene)s with scanning tunneling microscopy

Oligo(p-phenylenevinylene) (OPV) dimers, tetramers and hexamers, have been organized in highly organized 2D crystals on graphite by spontaneous self-assembly. The unit cell parameters of these monolayers could be determined and a profound effect of the length of the π-conjugated backbone on the adlayer structure was discovered. For all oligomers, the OPV-units were aligned in rows, with the alkyl chains residing in between the rows. Only trans-vinylene bonds were observed. In the hexamer, the organization was strongly determined by the π-system. Shortening the π-conjugated moiety led to monolayer structures controlled by the alkyl side chains. The chiral oligomers (hexamer and tetramer) formed chiral monolayers. In the case of the tetramer, the π-conjugated backbone was oriented clockwise with respect to the normal of the rows while in the case of the hexamer this orientation was counterclockwise.