Local Motions in Proteins as Investigated by the Thermal Coefficient of the Frictional Resistance to Rotation (Protein Dynamics, Fluorescence, Viscosity)
Scarlata, Suzanne Frances
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Permalink
https://hdl.handle.net/2142/70282
Description
Title
Local Motions in Proteins as Investigated by the Thermal Coefficient of the Frictional Resistance to Rotation (Protein Dynamics, Fluorescence, Viscosity)
Author(s)
Scarlata, Suzanne Frances
Issue Date
1984
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Physical
Abstract
The thermal coefficient of the viscosity (b) was measured through the fluorescence polarization of tryptophan and tyrosine both free in solution and as intrinsic protein chromophores. For free fluorophores it was found: (1) b is a function of the solvent only and is independent of the probe used in its measurement, (2) the values of b are equal to those determined by flow viscometry. In proteins two values of b are observed; one at lower temperatures, b(S) equal to the thermal coefficient of the solvent viscosity and at higher temperatures a second, reduced value (b(U)) is seen, the magnitude of which is distinctive of the individual protein. The transition from b(S) to b(U) occurs at the rotational amplitude at which the motions of the chromophore become limited by the surrounding peptide. The magnitude of b(U) appears to depend on the extent of coupling between the motions of the chromophore and the surrounding protein subdomain. This subdomain is characterized by only a few amino acids, indicating the importance of the immediate environment. Through a simple thermodynamic model, the enthalpy change for these coupling in motions can be calculated. In certain proteins containing two sets of fluorophores in very heterogeneous environments, the individual parameters for each can be discerned.
The study of b was used as an analytical tool to probe changes that occur in the protein neurophysin and its ligands upon binding and addition of lithium. The structure and fast motions of ocytocin and several of its analogs are greatly perturbed in the presence of this salt and become closer to those derived for vasopressin. The subunits of the neurophysin dimer are unchanged upon dissociation but in the ligated form, the rotational amplitude of tyrosine-49 in each protomer are very different. However, the binding sites as probed by the substrates are shown to be homologous. Lastly, energy transfer between the substrate and protein appears to exist.
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