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Struktur und Dynamik Poster: Mi., 14:00–16:30 M-P121
Hydrogen in N-Methylacetamide: Positions and dynamics of the hydrogen
atoms using neutron scattering
H. N. Bordallo 1,2 , W. Kalceff 3 , D.N. Argyriou 1 , M. Barthès 4 , T. Seydel 2 ,
C. Fehr 1 , F. Juranyi 4,5
1 Hahn-Meitner-Institut, Glienicker Str., 100 Berlin 14109, Germany – 2 Institut Laue-
Langevin, BP 156 - 38042 Grenoble Cedex 9, France – 3 Department of Applied Physics,
University of Technology Sydney, Australia – 4 Université Montpellier II, cc 26 -
34095 -Montpellier cedex 05, France – 5 Physical Chemistry, Saarland University, 66123
Saarbrucken, Germany – 6 Laboratory for Neutron Scattering, Paul Scherrer Institut,
5232Villigen, Switzerland
A comparison with several molecular mechanics force fields widely used in molecular
dynamics simulations of proteins, nucleic acids, and small molecules reveals systematic
deviations from electronic structure calculations and protein structure statistics. These
results point to the importance of understanding the anharmonic mode coupling of the
amide group, present in the hydrogen bonds found in protein side chains and main
chains. It is a non-trivial problem to choose a simplistic hydrogen bonded system
that accurately mimics hydrogen bonds found in proteins. One of the long-standing
building block models of the repeated peptide linkage of polypeptides and proteins is Nmethylacetamide,(CH3CONHCH3,
hereafter NMA), a single amide containing methyl
groups at both extremities.
NMA, which is highly hygroscopic and melts just above room temperature (301 K),
undergoes a structural phase transition around room temperature (T ′
=274 K), and
recent structural results support the idea of a glassy-type disorder below T ′
, as well as
the occurrence of a second phase in the vicinity of 230-240 K. From 13 C NMR studies,
down to 200 K, the coexistence of typical broad lines with very narrow bands indicates
that dynamic disorder is still observed well below the structural transition.
In order to get further information on the relationship between local dynamics and
the molecular structure of NMA, new QENS measurements were performed using FO-
CUS at PSI and IN10 at the ILL. Also, neutron diffraction measurements on the fully
deutereted NMA, using the Fine Resolution Powder Diffractometer E9 at HMI and
the High Resolution Powder Diffractomer (HRPD) at ISIS, were also carried out. Here
we find that systematic deviation from the room temperature structure in the model
system NMA influences the local dynamics, giving rise to a glass-type structure.