title: A sharp thermal transition of fast aromatic-ring dynamics in ubiquitin.
abstract: Aromatic amino acid side chains have a rich role within proteins and are often central to their structure and function. Suitable isotopic-labelling strategies enable studies of sub-nanosecond aromatic-ring dynamics using solution NMR relaxation methods. Surprisingly, it was found that the three aromatic side chains in human ubiquitin show a sharp thermal dynamical transition at approximately 312 K. Hydrostatic pressure has little effect on the low-temperature behavior, but somewhat decreases the amplitude of motion in the high-temperature regime. Therefore, below the transition temperature, ring motion is largely librational. Above this temperature, a complete ring-rotation process that is fully consistent with a continuous diffusion not requiring the transient creation of a large activated free volume occurs. Molecular dynamics simulations qualitatively corroborate this view and reinforce the notion that the dynamical character of the protein interior has much more liquid-alkane-like properties than previously appreciated.
authors.affiliation: Department of Biochemistry & Biophysics, University of Pennsylvania, Philadelphia, PA 19104 (USA).
authors.forename: Vignesh
authors.initials: V
authors.lastname: Kasinath
authors.forename: Yinan
authors.initials: Y
authors.lastname: Fu
authors.forename: Kim A
authors.initials: KA
authors.lastname: Sharp
authors.forename: A Joshua
authors.initials: AJ
authors.lastname: Wand
mesh_terms: D006801:Humans; D056004:Molecular Dynamics Simulation; D019906:Nuclear Magnetic Resonance, Biomolecular; D011487:Protein Conformation; D013696:Temperature; D025801:Ubiquitin / Q000737:chemistry*
chemical_list: D025801:Ubiquitin
keywords: NMR spectroscopy; conformational analysis; protein dynamics; relaxation; thermal activation