Christian Spitz

Polarized absorption and anomalous temperature dependence of fluorescence depolarization in cylindrical J-aggregates

We study chiral J-aggregates of the amphiphilic dye 1A that are spontaneously and asymmetrically generated from achiral dye monomers. These aggregates occur in two types. One type possesses a threefold split J-absorption band and forms micrometer-sized superhelices. The other type has a twofold split J-absorption band and forms smaller nanoparticles. We show that the analysis of optical experiments with polarized light in terms of an exciton model yields strong indications that the smaller aggregates have a cylindrical structure as well; the lower exciton band is polarized along the cylinder axis, while the higher band is polarized perpendicular to it. Our analysis allows for an estimate of the number of molecules around the circumference of the cylinder. Fluorescence–polarization excitation spectra at room temperature confirm the cylinder model. At low temperature, these spectra reveal a surprising loss of fluorescence polarization upon excitation in the higher exciton band. Possible explanations for this observation are discussed. 2002 Elsevier Science B.V. All rights reserved.

Multiple binding sites of fluorescein isothiocyanate moieties on myoglobin: photophysical heterogeneity as revealed by ground- and excited-state spectroscopy.

Fluorescein isothiocyanate (FITC)-myoglobin conjugates were synthesized with a binding stoichiometry of one to three fluorophores per protein. FITC binding sites were determined by matrix-assisted laser desorption-ionization time-of-flight mass spectrometry (MALDI-TOF MS). Five lysine residues and the N-terminal amino group were identified as preferential binding sites. The ground and excited-state absorption spectra and the fluorescence decay of the conjugates in the native and denatured state of the carrier protein were analyzed. For comparison, unbound FITC and FITC covalently bound to a polysaccharide (dextran) were studied. For FITC, FITC-dextran and the FITC-myoglobin conjugates, only one FITC absorption peak was obtained in the ground state spectrum. Similarly, the excited state absorption (ESA) spectra of unbound FITC and of FITC-dextran showed only one single maximum whereas two maxima were detected for the native FITC-myoglobin conjugates. One of these sub-bands disappeared following urea treatment of the conjugate. We conclude that ESA measurements of extrinsic fluorophores on proteins can be used to monitor different micro-environments of the fluorophore and to distinguish between different conformational states of the labeled protein. This method can be a useful tool for analysing coexisting protein conformations.

Free Volume Nanocavities in PMMA Revealed by Rotational Dephasing of Paraterphenyl in a Transient Grating Experiment

The excited state decay behaviour of p-terphenyl in cyclohexane solution and in a rigid poly(methyl methacrylate) (PMMA) film is reported as measured in a ps polarization dependent transient grating experiment. In both cases, the diffracted intensity shows a biexponential decay. The slower component is attributed to excited state lifetime. In the liquid solution, the faster decay component is described by the hydrodynamical Stokes–Einstein–Debye theory for rotational reorientation under slip boundary conditions. In the rigid matrix it is interpreted in terms of a free volume effect.

Biochemical diagnostics by excited state absorption spectroscopy

Excited state absorption (ESA) spectroscopy test electronic transitions from the first excited states to higher ones. These transitions are often much more sensitive to the environment of the specimen under investigation than ground state absorption spectra as well as fluorescence spectra. Therefore ESA on intermolecular interactions becomes a candidate for efficient biochemical sensors. In addition, many applications where fluorescence probes are used today may be improved in future regarding sensitivity and velocity by the use of ESA, because absorptive measurements principally allow much faster detection in comparison to fluorescence. Two examples are given: on one hand photoinduced intramolecular electron transfer reactions in special pyrenylbenzene compounds were proven to be strongly dependent on environmental conditions are polarity, for example. Moreover, intermolecular electron transfer to the neighborhood was identified by this technique. On the other hand the denaturation of myoglobin is observed by ESA spectroscopy on fluorescein probes covalently bound to be protein. During unfolding the protein the local environment of the probes changes, which si reflected effectively in the spectra. In both examples the ESA method is compared to traditional fluorescence techniques. Besides the confirmation of the traditionally obtained results, valuable new information is yielded.

Layer-by-layer coated capsules for efficient and directed electron transfer

We present a concept, where this effect is used to implement a stepwise electron transfer with the direction given by a polarity gradient i.e. increasing solvatation. The layer-by-layer deposition of polyelectrolytes can be used to build such a structure if dye-doped polyelectrolytes with increasing polarity are deposited onto charged colloids. Finally the basic colloids are removed, providing the electron transfer (ET)-system on nanometer scale on capsules with a diameter of 2 /spl mu/m.

Optically induced and polarity driven electron transfer in mesoscopic multilayer systems

The authors present organic films that are prepared using a new technique from layers of polyelectrolytes, which have a high imprinted polarity gradient across the film thickness of 20 nm. These films could load with covalently bond pyrene molecules with a mean distance of 2 nm. Thus a high probability of overlapping wavefunctions of neighboring molecules should appear and we expect the electronic states to form a cascade that is bent by the polarity gradient.

Observation and characterization of nonlinear processes in organic and biological samples by excited state absorption (ESA) spectroscopy

By excited state absorption (ESA) spectroscopy, electronic transitions from the first excited states to higher ones are probed. These transitions are often much more sensitive to the environment of the specimen under investigation than ground state absorption spectra as well as fluorescence spectra. Therefore ESA spectroscopy on intermolecular interactions becomes a candidate for efficient biochemical sensors. In addition, many applications where fluorescence probes are used today may be improved in future regarding sensitivity and speed by the use of ESA, because absorptive measurements principally allow much faster detection in comparison to fluorescence.