Benoît Muls

Poly(amidoamine) dendrimer peripherally modified with 4-N,N-dimethylaminoethyleneamino-1,8-naphthalimide as a sensor of metal cations and protons.

A new poly(amidoamine) dendrimer from second generation whose periphery comprises sixteen fluorescent 4-N,N-dimethylaminoethylamino-1,8-naphthalimide units has been synthesized and characterized. In DMF, the dendrimer shows sensitivity to the presence of Cu(2+), Fe(3+) and protons. The changes in the fluorescence intensity of the material are in opposite directions if acids or metals are present. Fluorescence enhancements (FE from 5 to 9 depending on solvent) are recorded when the photoinduced electron transfer (PET) originating from the donating amine to the electron accepting naphthalimide is inhibited by the protonation of the N,N-dimethylamino groups. In the case of Cu(2+) cations, a fluorescence quenching (FQ of 6) is first observed, followed by fluorescence partial restoration. In the Fe(3+) case, the same behaviour is observed with a final FE of 2. The successive complexations of these cations by the dendrimer core and by the external rim of the dendrimer may explain the results.

Orientational effects in the excitation and de-excitation of single molecules interacting with donut-mode laser beams

The interactions between single molecules and three-dimensional donut modes in fluorescence microscopy are discussed based on the vector diffraction theory of light.We find that the use of donut modes generated from a linearly polarized laser beam can yield information about the orientation of immobilized single molecules, allowing for their use in orientational imaging. While fairly insensitive over a range of orientations, this technique is seen to be very sensitive for the subset of orientations where the transition dipole of the molecule is oriented close to the optical axis of the microscope and perpendicular to the input polarization. In a second part of the paper we discuss the impact of the molecular orientation on the resolution improvement in STED microscopy. We find that, even for circularly polarized excitation light, the expected resolution improvement depends on the orientation of the molecule relative to the optical axis of the microscope.

Monitoring the Interaction of a Single G-Protein Key Binding Site with Rhodopsin Disk Membranes upon Light Activation

Heterotrimeric G-proteins interact with their G-protein-coupled receptors (GPCRs) via key binding elements comprising the receptor-specific C-terminal segment of the alpha-subunit and the lipid anchors at the alpha-subunit N-terminus and the gamma-subunit C-terminus. Direct information about diffusion and interaction of GPCRs and their G-proteins is mandatory for an understanding of the signal transduction mechanism. By using single-particle tracking, we show that the encounters of the alpha-subunit C-terminus with the GPCR rhodopsin change after receptor activation. Slow as well as less restricted diffusion compared to the inactive state within domains 60-280 nm in length was found for the receptor-bound C-terminus, indicating short-range order in rhodopsin packing.

Nano-patterned layers of a grafted coumarinic chromophore

We report on the grafting of coumarin chromophores on flat silicon surfaces and in regions of nanometric dimensions drawn on silicon surfaces. The coumarin derivative was grafted by using the quaternization of a tertiary amine group of the chromophore with a ((chloromethyl)phenylethyl)-dimethylchlorosilane (CMPDCS) grafted on silicon. Complete characterization of the grafted layer was performed as a function of reaction time by X-ray photoelectron spectroscopy, X-ray reflectometry, atomic force microscopy, fluorescence spectroscopy and laser-scanning confocal microscopy. The results indicate that about one chromophore molecule is grafted every second CMPDCS molecule, resulting in a surface density of coumarin of slightly more than one coumarin per nm2. A broadening of the distribution of the fluorescence lifetimes was observed, suggesting that the grafted molecules experience a larger distribution of environments in the grafted layer than in solution. Since this reaction is fully compatible with silicon processing technology, the grafting could also be performed in nano-regions of size as small as 250 nm defined by combining electron-beam lithography with silanization. In such nano-sized regions the distribution of fluorescence lifetimes was narrower, suggesting a possible influence of the confinement on the organization of the molecules.

Defocused imaging in wide-field fluorescence microscopy

Defocused imaging in wide-field fluorescence microscopy provides information on the 3D orientationof the transition dipole moment of single molecules with nanometer spatial resolution. In this chapter,the theoretical background of defocused imaging will be presented, followed by two experimental applications.As a first example, defocused imaging was used along with fluorescence lifetime measurements to provethe fact that the molecular orientation of dye molecules in a thin film has a significant affecton their fluorescence lifetime. This is attributed to the electromagnetic boundary condition effect. Asa second example, the power of the technique for following 3D molecular rotational reorientation (molecularrotational diffusion) in thin polymer films is demonstrated. Since many molecules can be monitored in parallel,both the temporal and the spatial heterogeneity of polymer dynamics can be addressed.