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Anisotropic rotational motions of poly( L ‐glutamic acid) in the α‐helix conformation
Author(s) -
Hahn U.,
Hanssum H.,
Rüterjans H.
Publication year - 1985
Publication title -
biopolymers
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.556
H-Index - 125
eISSN - 1097-0282
pISSN - 0006-3525
DOI - 10.1002/bip.360240704
Subject(s) - chemistry , rotational diffusion , methylene , anisotropy , relaxation (psychology) , helix (gastropod) , crystallography , molecule , rotation (mathematics) , diffusion , side chain , perpendicular , nuclear magnetic resonance , stereochemistry , thermodynamics , polymer , physics , organic chemistry , geometry , psychology , social psychology , ecology , mathematics , quantum mechanics , snail , biology
The anisotropic rotational motion of the backbone and the side chains of poly( L ‐glutamic acid) in the α‐helical structure was investigated using the 13 C‐ T 1 and T 2 relaxation times of all carbon atoms with directly attached protons, obtained at a 13 C‐Larmor frequency of 67.89 MHz. The evaluation of the nmr data was carried out according to the previously derived anisotropic diffusion model, in which the macromolecule is considered a rigid rod. The rotation of the backbone is characterized by two diffusion constants, D 1 and D 3 , describing the rotation perpendicular to and around the symmetry axis. The additional internal motion of the C β ‐methylene group is described as a jump process with a jump rate, k 1 , between two allowed rotametric states. Steric considerations indicate that the occupation of the third rotameric position is forbidden. The rotation of the C γ ‐methylene group is decribed as a one‐dimensional diffusion process around the C β –C γ bond. Investigation of the temperature dependence of the relaxation parameters led to the temperature dependence of the dynamic parameters. Activation energies were determined from these data. The dynamic parameters obtained for poly( L ‐glutamic acid) at 291 K are compared with the corresponding results of a previous study of poly( L ‐lysine). The development of an anisotropic diffusion model for the motions of the rod‐shaped poly( L ‐lysine) α‐helix and its application to the interpretation of the 13 C‐relaxation data of this molecule have already been published previously. In this model, both the overall molecular tumbling and the various internal motions have been characterized by diffusion constants or jump rates typical for each process. These dynamic parameters can be calculated from the spin–lattice relaxation times, the spin–spin relaxation times and the NOE factors of the C α , C β , and C γ nuclei of the polypetide. In the present paper, we describe the application of the above‐mentioned dynamic model to the interpretation of 13 C‐relaxation studies of a further homopolypeptide, poly( L ‐glutamic acid), in the α‐helical structure. Furthermore, we studied the temperature dependence of the relaxation times of this polymer and determined the anisotropic diffusion parameters at each temperature. From their temperature dependence and from comparison of our present results with the data of our previous study of poly( L ‐lysine), we were able to derive new insights into the intramolecular diffusion processes and the excitation of various motions.