43 research outputs found
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
and vibrational modes with angular momentum 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 Ă— 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