engleski

ELECTRON SPIN ECHO AND HEAT CAPACITY EVIDENCE FOR LOW FREQUENCY VIBRATIONAL MODES AND LATTICE DISORDER IN L-ALANINE AT CRYOGENIC TEMPERATURES

L-alanine, as the smallest naturally occurring chiral amino acid, is often employed as a model for the investigation of a wide range of intermolecular interactions, which are expected to be present in more complex and biologically relevant molecules. For example, it is expected that low frequency lattice vibrations contain information connected with supramolecular self-assembly in the biomolecular processes. The nonlinear behavior of such low frequency vibrations in L-alanine have been previously studied by Raman, IR, picosecond time-resolved coherent Raman scattering (CARS), and inelastic neutron scattering. Several of these techniques employed picosecond time resolution, which is necessary to detect low frequency fluctuations. One of the lowest frequency vibronic modes (42 cm-1 torsional mode) has a lifetime on the order of 5 ns at temperatures around 1.1 K. The peaks corresponding to these modes are very sharp (&#915 ; ~ 2.4´10-3 cm-1 ~70MHz) and the CARS detected relaxation rate is temperature independent below 10 K. From this evidence, one expects that the utilization of a spectroscopic technique with a timescale longer than that of the CARS, say around nanosecond, may prove fruitful in the detection of such low frequency modes, and this provided the motivation for the present undertaking. A technique appropriate to the nanosecond timescale is pulsed electron paramagnetic resonance spectroscopy (Pulsed-EPR), which can operate in ~ 4 ns steps with ~ 80 ns dead-time for detection. In particular, the electron spin echo (ESE) decay time, TM, of organic free radicals could be the basis of such a measurement. In this work, we have utilized the ESE technique to detect the presence of disordered low frequency modes in the L-alanine and its deuterated analog L-alanine-d7. An unusually sharp decrease in the spin-echo relaxation rate of the probe with decreasing temperature indicates the presence of such low frequency vibrational modes within the system, which have activation energy of around 78 K in the 5-20 K temperature region for L-alanine and about 26 K for L-alanine-d7. Additionally, we measured the heat capacity, CP, for both lattices over the 1.8 &#8211 ; 20 K temperature range. The CP/T3 vs. T plots exhibit sharp peaks and a deuteration effect. Detailed measurements using single crystals and powders, as well as various cooling rates are consistent with a disordered or glassy lattice structure in these compounds in the 4 K range, and help to understand the difference in the CP data between our study and an earlier report.

alanine; low frequency vibration; relaxation; epr

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