Oxidative cleavage of carotenoids catalyzed by enzyme models and beta-carotene 15,15´-monooxygenase

The enzyme that catalyzes the central cleavage of β-carotene is an iron monooxygenase. The protein was isolated from chicken intestinal mucosa and overexpressed in two different cell lines. Inductively coupled plasma (ICP) emission analysis revealed that the hydrophobic 60.3-kDa enzyme contains one iron/mole protein. The substrate specificity was investigated, and the reaction mechanism elucidated incubating α-carotene in the presence of highly enriched 17O2 and H218O. A supramolecular enzyme model was synthesized, binding carotenoids Ka > 106 mol-1, which mimics the regiospecific enzymatic cleavage of carotenoid

Resonant hyper-Raman scattering in spherical quantum dots

A theoretical model of resonant hyper-Raman scattering by an ensemble of spherical semiconductor quantum dots has been developed. The electronic intermediate states are described as Wannier-Mott excitons in the framework of the envelope function approximation. The optical polar vibrational modes of the nanocrystallites (vibrons) and their interaction with the electronic system are analized with the help of a continuum model satisfying both the mechanical and electrostatic matching conditions at the interface. An explicit expression for the hyper-Raman scattering efficiency is derived, which is valid for incident two-photon energy close to the exciton resonances. The dipole selection rules for optical transitions and Fr\"ohlich-like exciton-lattice interaction are derived: It is shown that only exciton states with total angular momentum $L=0,1$ and vibrational modes with angular momentum $l_p=1$ contribute to the hyper-Raman scattering process. The associated exciton energies, wavefunctions, and vibron frequencies have been obtained for spherical CdSe zincblende-type nanocrystals, and the corresponding hyper-Raman scattering spectrum and resonance profile are calculated. Their dependence on the dot radius and the influence of the size distribution on them are also discussed.Comment: 12 pages REVTeX (two columns), 2 tables, 8 figure

Spin dynamics in semiconductor nanocrystals

Time-resolved Faraday rotation is used to study both transverse and longitudinal spin relaxation in chemically-synthesized CdSe nanocrystals (NCs) 22-80 Angstroms in diameter. The precession of optically-injected spins in a transverse magnetic field occurs at distinct frequencies whose assignment to electron and exciton spins is developed through systematic studies of the size-dependence and theoretical calculations. It is shown that the transverse spin lifetime is limited by inhomogeneous dephasing to a degree that cannot be accounted for by the NC size distribution alone. Longitudinal spin relaxation in these NCs occurs on several distinct timescales ranging from 100 ps-10 microseconds and exhibits markedly different dependencies on temperature and field in comparison to transverse spin relaxation.Comment: 25 pages, 11 figures, tabl

From insects to bioinspired catalysis

The author describes the events leading to his PhD studies at the University of Zurich under the supervision of Prof. Hans Schmid and the evolution of his research interests into the field of biol. chem. resulting in the completion of his habilitation at UZH and ultimately a professorship at the University of Basel

The Central Cleavage of ?,?-Carotene – A Supramolecular Mimic of Enzymatic Catalysis

The enzyme ?,?-carotene 15,15?-dioxygenase which cleaves ?,?-carotene to retinal (provitamin A) has been identified for the first time in chicken intestinal mucosa and subsequently sequenced and expressed in two different cell lines. To mimic this unusual metabolism a supramolecular receptor has been synthesized which binds ?,?-carotene with Ka = 8.3 × 106 M?1 and it was shown that the corresponding oxo-ruthenium complex catalyzes the selective cleavage of carotenoids

In Memoriam André S. Dreiding: Obituary

In his talk at the 49th Bürgenstock Conference on Stereochemistry the author paid tribute to André S. Dreiding, the founder of this event

Structural and functional mimics of cytochromes P450

A review on the competitive effort by inorg., org. and phys. chemists who synthesized/used enzyme models as a source to understand heme-thiolate monooxygenase catalysis.  In general, the natural structure has been simplified in various ways: the peripheric Me and vinyl substituents were omitted as well as in most cases the difficult-to-handle thiolate ligand.  For most of the complexes the meso-positions are substituted by Ph rings to prevent oxidn. at the meso-C atoms and further to provide a handle for functionalization.  In some cases the 3-pyrrole positions have been substituted with Me groups and Bu chains to render the complexes more sol. in org. solvents and to restrict conformational mobility of the Ph rings at the meso-positions