Exciton bimolecular annihilation dynamics in supramolecular nanostructures of conjugated oligomers

We present femtosecond transient absorption measurements on $\pi$-conjugated supramolecular assemblies in a high pump fluence regime. Oligo(\emph{p}-phenylenevinylene) monofunctionalized with ureido-\emph{s}-triazine (MOPV) self-assembles into chiral stacks in dodecane solution below 75$^{\circ}$C at a concentration of $4\times 10^{-4}$ M. We observe exciton bimolecular annihilation in MOPV stacks at high excitation fluence, indicated by the fluence-dependent decay of $1^1$B$_{u}$-exciton spectral signatures, and by the sub-linear 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

Origin of spectral broadening in pi-conjugated amorphous semiconductors

We present a study of the picosecond fluorescence dynamics of pi-conjugated semiconducting organic dendrimers in the solid state. By varying the degree of branching within the dendrons, referred to as the dendrimer generation, a control of intermolecular spacing of the emissive core and therefore of the lattice parameter for Forster-type energy transfer is achieved. This allows a distinction between spectral diffusion and excimer formation as the two main sources of spectral broadening in organic semiconductors. Whereas Forster-type dispersive spectral relaxation is independent of temperature but strongly dependent on the interchromophore distance, excimer formation is also strongly thermally activated due to temperature-dependent conformational changes and the influence of thermally activated dynamic disorder. The rapid spectral diffusion allows a determination of the excimer rise in the emission, which is shown to have a profound impact on the steady state luminescence properties of dendrimer films. We show that the dendrimer generation not only allows a microscopic control of intermolecular interactions but also a direct control of the rate of spectral diffusion. Implications for the design of novel materials for optoelectronic devices are discussed

Substrate specificity of a peptidyl-aminoacyl-l/d-isomerase from frog skin

In the skin of fire-bellied toads (Bombina species), an aminoacyl-l/d-isomerase activity is present which catalyses the post-translational isomerization of the l- to the d-form of the second residue of its substrate peptides. Previously, this new type of enzyme was studied in some detail and genes potentially coding for similar polypeptides were found to exist in several vertebrate species including man. Here, we present our studies to the substrate specificity of this isomerase using fluorescence-labeled variants of the natural substrate bombinin H with different amino acids at positions 1, 2 or 3. Surprisingly, this enzyme has a rather low selectivity for residues at position 2 where the change of chirality at the alpha-carbon takes place. In contrast, a hydrophobic amino acid at position 1 and a small one at position 3 of the substrate are essential. Interestingly, some peptides containing a Phe at position 3 also were substrates. Furthermore, we investigated the role of the amino-terminus for substrate recognition. In view of the rather broad specificity of the frog isomerase, we made a databank search for potential substrates of such an enzyme. Indeed, numerous peptides of amphibia and mammals were found which fulfill the requirements determined in this study. Expression of isomerases with similar characteristics in other species can therefore be expected to catalyze the formation of peptides containing d-amino acids

Comparative effects of melittin and its hydrophobic and hydrophilic fragments on bilayer organization by Raman spectroscopy.

For bilayer systems consisting of 1,2-dimyristoyl phosphatidylcholine (DMPC) incubated with melittin, a polypeptide capable of integrating itself within the membrane, temperature profiles derived from Raman spectroscopic data indicate the existence of an immobilized lipid annulus surrounding the polypeptide. In particular, temperature profiles derived from C--H, C--D and C--C stretching mode parameters for 25:1, 14:1 and 10:1 lipid:protein mole ratios exhibit two order-disorder transitions. The primary (lower) gel to liquid crystalline phase transition is depressed when polypeptide concentration is increased. The concentration-independent higher temperature transition is associated with a fluidization of the immobilized boundary lipids present at the lipid-polypeptide interface within the bilayer. We estimate that five to seven lipids are involved in this discrete boundary layer around the inserted membrane component. The behavior of the intrinsic hydrophobic (residues 1-19) and of the extrinsic hydrophilic (residues 20-26) portions of melittin in the bilayer is compared with the properties of the intact polypeptide. We emphasize evidence that both intrinsic and extrinsic components immobilize lipids contiguous to the polypeptide

BSTI, a trypsin inhibitor from skin secretion of Bombina bombina related to protease inhibitors of nematodes

From skin secretions of the European frog Bombina bombina, a new peptide has been isolated that contains 60 amino acids, including 10 cysteine residues. Its sequence was determined by automated Edman degradation and confirmed by analysis of the cDNA encoding the precursor. A search in the databanks demonstrated that the pattern of cysteine residues in this skin peptide is similar to the ones found in protease inhibitors from Ascaris and in a segment of human von Willebrand factor. The 3D structure of the trypsin inhibitor from Ascaris suum could be used as a template to build a model of the amphibian peptide. In addition, we have demonstrated that this constituent of skin secretion is indeed an inhibitor of trypsin and thrombin, with K(i) values in the range of 0.1 to 1 microM. The new peptide was thus named BSTI for Bombina skin trypsin/thrombin inhibitor