th  - 1988 - 51st ENC Conference

126 I
EXPERIMENTAL EVALUATION OF NMR IMAGING PROBES
S. L. Talagala* and L. D. Hall, Department of Chemistry,
University of British Columbia, Vancouver, B.C., Canada
Several probe designs suitable for high field NMR Imaging have been
suggested in the recent years to overcome the deficiencies of traditional
solenoid and saddle shaped coils. The variety of designs proposed demonstrate
the difficulty in optimizing all necessary criteria with a single design.
This study presents an experimental evaluation of the performance of four
commonly used probe designs.
The four designs chosen for evaluation, the birdcage design, Alderman &
Grant design, split-ring resonator, and the saddle coil were all constructed
with the same diameter (7.5cm) and length for direct comparison. The criteria
chosen to evaluate the performance of each design included the probe 0-factor
(loaded and unloaded), the effect of sample on the probe resonance frequency
and the homogeneity of the rf magnetic field inside the probe. In addition,
the signal-to-nolse ratio and the 90 ° pulse width for a "point" phantom were
also determined.
Experimental results obtained indicate the relative merits and demerits of
different probe designs. In summary, it is seen that higher 0 values are
afforded by the resonator probe designs (birdcage, Alderman-Grant and
split-ring) and that the split-ring resonator gives the best results. In
general, the dielectric losses associated with the resonator probes are lower
than that of the saddle coil, and the Alderman-Grant design provides the best
performance with respect to this parameter. Qualitative estimates show that
the resonators generate higher inductive losses compared to the saddle coil.
The best rf homogeneity in all three axes is provided by the split-ring
resonator. The rf homogeneity of the saddle coil is comparable to that of the
Alderman-Grant design in the transverse plane and the former offers better
performance along the longitudinal axis.
127 I HETERONUCLEAR TWO-DIMENSIONAL NNR METHODS FOR THE DETERM]NATION
OF THE PRIMARY STRUCTURE OF PEPTIDES
VoLker Bornemann, A. Scott Chesnick, Gregory Helms, Richard E.
Moore and Walter P. Miemczura*
Department of Chemistry, University of Hawaii, Honolulu, HI 96822.
The common method of determining the primary structure of peptides and
small proteins using RMR spectroscopy involves a combination of homonuctear
experiments in both protonated and deuterated solvents. These experiments are
generally the most straight-forward method for solving this problem. There are
several drawbacks to this approach. Data must be obtained in the presence of
protonated solvents. Also, key information is obtained from proton-proton nOe
experiments which can be weak or sometimes absent due to chemical exchange. Finally,
the method breaks down for non-standard amino acids such as beta amino acids or those
which are chemically modified. Recently the improved sensitivity of modern
spectrometers and the introduction of proton detected direct and long-range
heteronuctear chemical shift correlation experiments has opened a whole new avenue
towards solving the problem of peptide sequencing with NNR spectroscopy. Now
hitherto unavailable information concerning the direct and remote connectivities
between carbon or nitrogen and hydrogens can now be obtained on reasonable quantities
of material. With the aide of tow level isotopic enrichment (ca 1OX uniform N-15
throughout the molecule) even carbon-nitrogen correlations can be determined. Our
work involves the isolation and identification of biologically active natural
products from blue-green algae. In many instances these compounds are peptidat in
nature. Because of the modified nature of these molecules the standard NNR
sequencing techniques are often inadequate. We have found that a broad range of not
only homonuclear but also heteronuctear experiments are often needed in order to
completely characterize these new compounds. By combining direct and tong-range
correlation of carbon and nitrogen to protons with carbon-nitrogen correlation
experiments it is possible to determine the primary sequence of peptides using one
end two bond correlation experiments. We will present two recent peptidat compounds
which could not be sequenced with homonuctear two-dimensional experiments due to
structural modifications or ambiguous homonuctear data. For both of these compounds
the availability of data from the heteronuclei was essential in solving the unknown
structures. ALl information wilt be provided for the experiments presented. ]n
addition, hardware and probe modifications necessary for performing the carbon-
nitrogen correlation experiments wilt be discussed.
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