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Investigations of local dynamic properties and lattice instability in L-alanine crystal

Investigations of local dynamic properties and lattice instability in L-alanine crystal B. Rakvin Ruđer Bošković Institute, P.O. Box 180, 10002, Zagreb, Croatia Investigation of local dynamic properties and lattice instability in crystalline amino acids are of interest for their ferroelectric, piezoelectric and nonlinear optical properties. Moreover, the intrinsic motions of various structural fragments give a fundamental base for understanding dynamic properties of the proteins. Generally, various dynamical effects of these fragments are displayed over a large range of time scale, from femtoseconds (individual bond vibrations) to picoseconds (small group fluctuations) to microseconds and longer (collective motions). Properties of crystalline L-Alanine, as one of simplest amino acids, have been the subject of such extensive investigations. In the crystal, the molecule is zwitterionic (+H3N-C2H4COO-) and the nitrogen atom is coupled to three networks of intermolecular hydrogen bonds of unequal strength. Despite that X-ray and neutron single-crystal diffraction measurements support the same crystal structure at low and at room temperature, there are several other experimental evidences for lattice instability at around 220 K. For example: birefringence and light depolarization measurements showed some subtle symmetry breaking as well as observation of the change in slope of the T1 relaxation time of amine group in 1H NMR measurements around the same temperature. A possible explanation for this phenomenon is the strong dynamic Jahn-Teller effect due to the NH3+ charge-lattice coupling 1. As one possible approach to study of local dynamic properties of such systems various paramagnetic centers introduced in the lattice as local probe can be employed. Detailed EPR studies of individual probes (as specific fragments) provided additional information on the intermolecular dynamics and help to better understand lattice instability in crystalline amino acids. In this presentation stable L-alanine paramagnetic centers (radicals ; •CHCH3COOH and •CNH3CH3COOH) denoted as, SAR1 and SAR2, respectively2, have been address. They were employed as paramagnetic probes and examined by various electron paramagnetic resonance techniques (CW-EPR, FT-EPR, X-band and W-band, and ENDOR) in order to provide new experimental parameters for dynamic behavior of various molecular fragments in the vicinity of 220 K. In the temperature region of lattice instability it was shown that motional dynamics of NH3+ strongly modulated motional dynamics of CH3 group and Hα of SAR1 and CH3 group of SAR2, respectively. The obtained results3 indicates the presence of dynamic Jahn-Teller effect due to the NH3+ charge-lattice coupling in the lattice and also they can be used to explain microscopic origin of time dependent disorder in the L-alanine lattice. References 1. M. Barthes, H.N. Bordallo, F. Denoyer, J.-E. Lorenzo, J. Zaccaro, A. Robert, and F. Zontone, Eur. Phys. J. B 37 (2004) 375 2. E. Sagstuen, E. O. Hole, S. R. Haugedal, and W. H. Nelson, J. Phys. Chem. A 101 (1997) 9763. 3. B. Rakvin, N. Maltar-Strmečki, D. Kattnig and G. Grampp, Molec. Phys. 105 (2007) 2087.

L-alanine; paramagnetic centres

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