engleski

Low frequency vibration mode and lattice disorder in L-alanine detected by Electron Spin Echo spectroscopy

As one of the smallest naturally occurring chiral amino acid, L-alanine 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 picosecond time-resolved coherent Raman scattering (CARS), and inelastic neutron scattering. The peaks corresponding to these modes are very sharp (Γ ~ 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 employment 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 electron spin echo (ESE) spectroscopy which can operate in ~ 4 ns steps with ~ 80 ns dead-time for detection. In particular, ESE decay time, TM, of organic free radicals, introduced into the lattice as probes, could be the basis of such a measurement. In this lecture, the possibility of using ESE technique to detect the presence of disordered low frequency modes in the L-alanine and its deuterated analog L-alanine-d7 will be presented. 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.

EPR; L-alanine; Spin lattice

nije evidentirano