Spin lattice conformation

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). 

The proton spin-lattice relaxation-rate (R,) is a well established, nuclear magnetic resonance (n.m.r.) parameter for structural, configurational, and conformational analysis of organic molecules in solution. " As yet, however, its utility has received little attention in the field of carbohydrate chemistry,... 

The significance of n.m.r. spectroscopy for structural elucidation of carbohydrates can scarcely be underestimated, and the field has become vast with ramifications of specialized techniques. Although chemical shifts and spin couplings of individual nuclei constitute the primary data for most n.m.r.-spectral analyses, other n.m.r. parameters may provide important additional data. P. Dais and A. S. Perlin (Montreal) here discuss the measurement of proton spin-lattice relaxation rates. The authors present the basic theory concerning spin-lattice relaxation, explain how reliable data may be determined, and demonstrate how these rates can be correlated with stereospecific dependencies, especially regarding the estimation of interproton distances and the implications of these values in the interpretation of sugar conformations. 

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. 

There has been extensive effort in recent years to use coordinated experimental and simulation studies of polymer melts to better understand the connection between polymer motion and conformational dynamics. Although no experimental method directly measures conformational dynamics, several experimental probes of molecular motion are spatially local or are sensitive to local motions in polymers. Coordinated simulation and experimental studies of local motion in polymers have been conducted for dielectric relaxation,152-158 dynamic neutron scattering,157,159-164 and NMR spin-lattice relaxation.17,152,165-168 A particularly important outcome of these studies is the improved understanding of the relationship between the probed motions of the polymer chains and the underlying conformational dynamics that leads to observed motions. In the following discussion, we will focus on the... 

CNMR Spin-Lattice Relaxation and Conformational Dynamics in a 1,4-Polybutadiene Melt. 

Poly(benzyl ether) dendrimers synthesized by Frechet el al. have been studied with many techniques in order to reveal their conformational properties. Size exclusion measurements performed by Mourey et al. [154], rotational-echo double resonance (REDOR) NMR studies by Wooley et al. [155] and spin lattice relaxation measurements by Gorman et al. [156] reveal that back-folding takes place and the end-groups can be found throughout the molecule. The observed trends are in qualitative agreement with the model of Lescanec and Muthukumar [54],... 

Strong evidence of the dominant Influence of molecular conformation on the properties of coals Is Implicit In the several data sets which show an extremum In the measured property when plotted against carbon rank. Examples are the extrema which occur In the solid state properties of mass density (22,23) and proton spin-lattice relaxation rate (24) as well as In solvent swelling and extractablllty ( ). 

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. 

The reversible conformational change was seen by the disappearance of unresolved magnetic HFS of Mossbauer spectra after the illumination (decrease of spin-lattice relaxation time) and its restoration after the illumination of bleached samples at 77°K by light of 575 nm wavelength. 

In ribonuclease A 13C spin-lattice relaxation of the carbonyl and a and / carbon atoms is slower in the denaturated protein than in the native sample [177]. Apparently, the skeleton of this macromolecule becomes more flexible on denaturation, probably owing to conformational changes. However, the s carbons of lysine in the native protein exhibit relatively large T, values which change only insignificantly on denaturation [177]. This behavior is attributed to a considerable segmental mobility of the lysine side chain (Table 3.17 [177]). 

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],... 

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. 

Since the different types of transitions have different energies of activation, the line width sometimes decreases discontinuously with varying temperature. At characteristic transition temperatures the spin-lattice relaxation time T reaches a minimum. Comparative study of this NMR method of measuring transition temperatures with the dynamic mechanical and dielectric methods is sometimes very valuable to the understanding of structural or conformational effects. 

---