title: Direct evidence of the amino acid side chain and backbone contributions to protein anharmonicity.
abstract: Elastic incoherent neutron scattering has been used to study the temperature dependence of the mean-square displacements of nonexchangeable hydrogen atoms in powders of a series of homomeric polypeptides (polyglycine, polyalanine, polyphenylalanine and polyisoleucine) in comparison with myoglobin at the same hydration level (h = 0.2). The aim of the work was to measure the dynamic behavior of different amino acid residues separately and assess the contribution of each type of side chain to the anharmonic dynamics of proteins. The results provide direct experimental evidence that the first anharmonic activation, at approximately 150 K, is largely due to methyl group rotations entering the time window of the spectrometer used; however, contributions on the order of 10-20% from the motions of other groups (e.g., the phenolic ring and the methylene groups) are present. Our data also indicate that the dynamical transition occurring at approximately 230 K can be attributed, at least at the hydration level investigated, mainly to motions involving backbone fluctuations.
authors.affiliation: Department of Physical and Astronomical Sciences, University of Palermo, via Archirafi 36, I-90123 Palermo, Italy.
authors.forename: Giorgio
authors.initials: G
authors.lastname: Schiró
authors.forename: Chiara
authors.initials: C
authors.lastname: Caronna
authors.forename: Francesca
authors.initials: F
authors.lastname: Natali
authors.forename: Antonio
authors.initials: A
authors.lastname: Cupane
mesh_terms: D000596:Amino Acids / Q000737:chemistry*; D056004:Molecular Dynamics Simulation; D009211:Myoglobin / Q000737:chemistry; D010455:Peptides / Q000737:chemistry*; D017433:Protein Structure, Secondary; D011506:Proteins / Q000737:chemistry*
chemical_list: D000596:Amino Acids; D009211:Myoglobin; D010455:Peptides; D011506:Proteins; C019529:polyalanine; C011080:polyglycine; C012001:polyphenylalanine; C043801:polyisoleucine