Conformation from spin-lattice relaxation times

Indeed, 13C spin-lattice relaxation times can also reflect conformational changes of a protein, i.e. helix to random coil transitions. This was demonstrated with models of polyamino acids [178-180], in which definite conformations can be generated, e.g. by addition of chemicals or by changes in temperature. Thus effective molecular correlation times tc determined from spin-lattice relaxation times and the NOE factors were 24-32 ns/rad for the a carbons of poly-(/f-benzyl L-glutamate) in the more rigid helical form and about 0.8 ms/rad for the more flexible random coil form [180],... 

If the amount of the sample is sufficient, then the carbon skeleton is best traced out from the two-dimensional INADEQUATE experiment. If the absolute configuration of particular C atoms is needed, the empirical applications of diastereotopism and chiral shift reagents are useful (Section 2.4). Anisotropic and ring current effects supply information about conformation and aromaticity (Section 2.5), and pH effects can indicate the site of protonation (problem 24). Temperature-dependent NMR spectra and C spin-lattice relaxation times (Section 2.6) provide insight into molecular dynamics (problems 13 and 14). 

Makriyannis, A., and Knittel, J. J. (1979) The conformational analysis of aromatic methoxyl groups from carbon-13 chemical shifts and spin-lattice relaxation times. Tetrahedron Lett., 2753-2756. 

Three parameters are readily obtainable from FiMR spectra which may be useful in studying binding interactions the chemical shift [jS], the linewidth (Av) or the apparent or effective spin-spin relaxation time (T2 ), and the spin-lattice relaxation time (Ti). C chemical shifts can reflect steric strain and change in the electronic environment within a molecule when it hinds to another species. Spin-lattice and spin-spin relaxation times can yield information on the lifetimes, sizes and conformations of molecular complexes. 

Spin-lattice relaxation times and 13C chemical shifts were used to study conformational changes of poly-L-lysine, which undergoes a coil-helix transition in a pH range from 9 to 11. In order to adopt a stable helical structure, a minimum number of residues for the formation of hydrogen bonds between the C = 0 and NH backbone groups is necessary therefore for the polypeptide dodecalysine no helix formation was observed. Comparison of the pH-dependences of the 13C chemical shifts of the carbons of poly-L-lysine and (L-Lys)12 shows very similar values for both compounds therefore downfield shifts of the a, / and peptide carbonyl carbons can only be correlated with caution with helix formation and are mainly due to deprotonation effects. On the other hand, a sharp decrease of the 7] values of the carbonyl and some of the side chain carbons is indicative for helix formation [854]. 

Scheme 65). The conformational state and barrier to rotation of methyl groups have attracted much theoretical and experimental interest. The barriers to rotation of methyl in various aza aromatics have been determined in the solid state from H-spin-lattice relaxation times (85JOC2972). Such barriers for 84a-i are given (in kJ mol-1) in Scheme 66. 

C NMR studies on [3-13C]Ala-labeled bR reveal that the C terminal residues, 226-235, participate in the formation of the C terminal a-helix as manifested from the peak position of 15.91 ppm with reference to the conformation-dependent 13C chemical shifts.68 69 81 82 103 The presence of the a-helix was also proved in view of the corresponding conformation-dependent displacement of peaks from [2-13C] and [l-13C]Ala-bR.81 Only part of this a-helix was visible by X-ray diffraction,25 owing to the presence of motions with correlation times of the order of 10-6s, as judged from the carbon spin-lattice relaxation times, 7jc, and spin-spin relaxation times, T2C, under CP-MAS conditions.81... 

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