Dresden 2006 – scientific programme

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AKB: Biologische Physik

AKB 30: Poster Session I

AKB 30.17: Poster

Monday, March 27, 2006, 15:30–18:00, P1

Negative thermal expansion and confomation changes in the smallest chiral amino acid, Alanine — •Heloisa Nunes-Bordallo¹, Dimitri Argyriou¹, Jörg Strempfer², Mariette Barthès ³, and Françoise Dénoyere⁴ — ¹Hahn-Meitner-Institut, Berlin. — ²Max-Planck-Institut für Festkörperforschung, Stuttgart. — ³Université Montpellier II, Montpellier, France — ⁴Université Paris XI, Orsay, France

Amino acid construction consists of a carboxylic acid (-COOH) and an amino (-NH₂) functional group attached to the same tetrahedral carbon atom, the α-carbon. Every amino acid, with the exception of glycine, comes in two forms, a left-handed (L) and a right-handed (D) version, which are identical mirror images of each other. We report on high resolution X-ray and neutron diffraction as well as quasi-elastic neutron scattering (QENS) studies on crystalline L- and D-alanine over a wide temperature range. Our aim is to verify the possibility that a phase transition, related to a break of the as C α-H bond, occurs in alanine. While no change in the space group symmetry was observed, a negative thermal expansion, by discrete steps, along the c- axis is observed till the melting point. Additional anomalies are also noticed in the a and b lattice constants at 170K. Moreover, the evolution of the mean-square displacement, obtained from the QENS, data shows a steadily increase on heating, but near 150K and again near 200K a deviation from the expected behavior is observed. The results suggest the excitation of new degrees of freedom, possibly due to a progressive conformational change of the NH³⁺ group.