J.R. Harbridge et al. / Journal of Magnetic Resonance 159 (2002) 195–206
205
findings for irradiated a-amino isobutyric acid agree
with the CW results obtained by Box and Freund [60].
The activation energies for rotation of the methyl group
that dominates T1e between 10 and 65 K are approxi-
mately the same for dimethyl-malonic acid (170 K,
1.4 kJ/mol) and a-amino isobutyric acid (160 K, 1.3 kJ/
mol), indicating that the barrier to rotation may have
larger contributions from steric effects within the radi-
cal, rather than contributions imposed by the lattice.
The barrier to rotation of the second methyl group is
much higher in a-amino isobutyric acid (600 K, 5.0 kJ/
mol) than in di-methyl-malonic acid (250 K, 2.1 kJ/mol),
which indicates that steric interaction with the lattice has
a larger impact for this methyl group than for the other
methyl group.
constant obtained by fitting the CW-SR curves to a
single exponential or the intermediate value of a three-
component fit are consistent with the values of T1e re-
quired to model the ELDOR curves. The large effects of
dynamic processes on electron spin relaxation could be
distinguished from relaxation processes that are not
frequency dependent by comparing values of T1e ob-
tained at S-band and X-band. Activation energies for
methyl and amino group rotation between 160 (1.3 kJ/
mol) and 1900 K (16 kJ/mol) were obtained by analysis
of the temperature dependence of 1=T1e in temperature
intervals where the dynamic process dominates T1e.
Acknowledgments
The radical in irradiated L-valine has two distinctly
different barriers to methyl group rotation: Ea ¼ 165
(1.4 kJ/mol) and 900 K (7.5 kJ/mol). NMR studies be-
tween 130 and 500 K showed the effects on the tempera-
ture dependence of 1=T1n due to one methyl group with
Ea ¼ 1360 K (11.3 kJ/mol) [12,16]. Rotation of the me-
thyl group in the radical with the lower energy barrier
may be too fast to detect in the NMR studies. For the
second methyl group the higher barrier to rotation ob-
served by NMR (1360 K, 11.3 kJ/mol) than by EPR
Support of this work by NIH GM 21156 is gratefully
acknowledged. We thank Cobe Laboratories (Golden,
CO) for irradiating the samples. Prof. George Rinard
assisted in the development of the Mathcad routines
based on the model in Fig. 2. Richard Quine wrote the
software to implement the programmable timing unit
for the pulse sequences used in these experiments.
(900 K, 7.5 kJ/mol) may indicate that c-irradiation of L-
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