engleski

ENDOR STUDY OF LATTICE INSTABILITY IN GAMMA IRRADIATED L-ALANINE CRYSTAL PROBED BY SECOND STABLE RADICAL

Studies of dynamical response of amino acids in a wide temperature range attracted considerable attention because of their suitability as model systems and building blocks of proteins and large biomolecules as well as due the fact that their dynamical properties play main roles in regulating protein behaviour. Until now extensive investigations of L-alanine crystal have been undertaken, pointing out very interesting physical properties. Despite the fact that x-ray and neutron single-crystal diffraction measurements support the same crystal structure at low (20 K) and at room temperature, there are several other experimental evidences for lattice instability [1] at around 220 K such as birefringence and light depolarization measurements and 1H NMR measurements around the same temperature. Radiation induced alanine radicals have been explored extensively using different EPR spectroscopy techniques over many years. Despite of these investigations, molecular environment of the three known radicals and the dynamic processes associated with the radicals motion are still not fully understood [2]. Detailed EPR studies of individual probes, as specific fragments of the radicals, provide additional information on the intermolecular dynamics and help to better understand lattice instability in crystalline L-alanine [3, 4]. Such investigation of local dynamic properties and lattice instability in crystalline amino acids are of interest for understanding dynamic properties of the proteins as well. In this study second stable L-alanine radical, R2, was use as a probe for investigation of the lattice instability. The ENDOR study focuses on the dynamical properties of three amino protons of R2 radical in the temperature range from 120-300 K. The motional dynamics of the amino group is important due to its involvement in intermolecular interactions through the hydrogen bond network in the crystal lattice. The results of the temperature dependence ENDOR study show phase transition around 220 K for all three amino protons and additionally confirm the strong dynamic Jahn-Teller effect in the L-alanine lattice and support evidence for properties related to lattice instability at around 220 K.

alanine; EPR; lattice instability

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