Naslov
Internal dynamics of crystalline L-alanine ; An ENDOR study

Autori
Rakvin, Boris ; Maltar-Strmečki, Nadica ; Kattnig, Daniel ; Grampp, Günter

Vrsta, podvrsta i kategorija rada
Sažeci sa skupova, sažetak, znanstveni

Izvornik
XXIII International EPR Seminar, April 23-25, 2009, Bad Gottleuba, Germany / - Bad Gottleuba, 2009, 30-30

Skup
XXIII International EPR Seminar

Mjesto i datum
Bad Gottleuba-Berggießhübel, Njemačka, 23.04.2009. - 25.04.2009

Vrsta sudjelovanja
Predavanje

Vrsta recenzije
Nije recenziran

Ključne riječi
alanine ; ENDOR ; α-proton
(alanin ; ENDOR ; α-proton)

Sažetak
Properties of crystalline L-Alanine, as one of simplest amino acids have been the subject of extensive investigations. Frequently, it has been studied as model for the examination of a wide range of intermolecular interactions. 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 is the strong dynamic Jahn-Teller effect due to the NH3+ charge-lattice coupling. In the present study the first stable L-alanine radical ( CHCH3COOH), SAR1, was employed to detect dynamic behavior of NH3+ group in the vicinity of 220 K. An α -proton isotopic hyperfine coupling constant was estimated in the temperature region around 220 K by employing ENDOR spectroscopy. The temperature dependence of the isotropic coupling was described in terms of harmonic oscillations. The obtained frequency 522 ± ; 28 cm-1 (751 ± ; 41 K) could be correlated with rocking vibration of CO2- group (525 cm-1) or with the first excited level of torsion oscillation of NH3+ group (475 cm-1). Moreover, significant broadening of the ENDOR α-proton line at higher temperatures (T>250 K) leads to activation process of Δ E = 2730 K (1897 cm-1) which is comparable with expected deformation modes δ (NH3+) of amine group in the crystal (~ 1700 cm-1). Obtained results indicate symmetrical bending of all bonds to Cα from planar geometry of SAR1 center. This bending is driven by small distortions of the NH3+ group below 220 K. The deformation does affect hydrogen bonds in the lattice and contribute of small variation of dipole moment of each molecule which would be aloud by symmetry of the lattice and it would not be the order parameter of a conventional phase transition. The above finding additionally explains a complex dynamic behavior of SAR1 center in the L-alanine lattice and support evidence for properties related to lattice instability like the onset of depolarization in the transmitted light at around 220 K.

Izvorni jezik
Engleski

Znanstvena područja
Fizika, Kemija

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