Abstract
Focal point of this thesis is the investigation of photo-induced
radical pair reactions using the method of chemically induced dynamic nuclear
polarization (CIDNP). The CIDNP method is a magnetic resonance technique
having as observable non-thermally generated nuclear spin polarization
and using for detection NMR spectroscopy. The spin polarization caused
by coupling of the nuclear spins to neighboring electron spins allows the
study of magnetic interaction parameters such as exchange coupling J, hyperfine
coupling a, or electronic g-factor, in addition it can be used to generate
high levels of nuclear spin polarization. Both aspects are demonstrated
by use of selected radical pair systems.
In order to fully exploit the power of the method a novel high resolution
NMR spectrometer was designed featuring mechanical field cycling. It allows
to investigate CIDNP generation for the first time in a magnetic field
range between 0 and 7 Tesla. By employing step motor controlled positioning
of a full NMR probe the dynamic polarization is created at variable magnetic
field while NMR detection is performed at a fixed field of 7 Tesla.
The study comprises studies on rigidly bridged electron transfer systems,
flexible cycloketones, and bimolecular amino acid-dye radical pair complexes.
The electron transfer systems are of the type donor-spacer-acceptor
and serve as models of biological systems showing charge separation after
photoexcitation (as e.g. in photosynthesis of bacteria and green plants).
For the first time the electronic exchange interaction is successfully
determined while avoiding restrictive model assumptions, for spacers of
variable length but keeping donor and acceptor group unchanged. Based on
this approach the full reaction kinetics of the photocycle is elucidated.
A second class of reaction systems investigated are cycloketones of
variable cycle size that serve as model for the interplay of spin and molecular
dynamics in reactions with paramagnetic intermediates. Utilizing the extended
magnetic field range it has become possible to question the theoretical
approach accepted so far for modeling the reaction kinetics and to develop
the needed qualitative amendments.
As the third subject polarization behavior is studied at radical pairs
of the type amino acid-dye molecule. Particular relevance is due to selective
polarization of few amino acid residues in macromolecules such as
peptides and proteins opening an interesting method for studying structural
features and dynamic processes such as protein folding. Based upon field
dependent CIDNP measurements as performed here the specificity of individual
residues in the polariza-tion process can be better understood and utilized
to find optimum conditions for polarizing nuclear spins.
Furthermore, interesting polarization effects at zero magnetic field
are observed and interpreted in frame of an uncomplicated model. |
