Molecular correlation time enhancement

N-protonation the absolute magnitude of the Ad values is larger than for Af-methylation <770MR(9)53>. Nuclear relaxation rates of and have been measured as a function of temperature for neat liquid pyridazine, and nuclear Overhauser enhancement has been used to separate the dipolar and spin rotational contributions to relaxation. Dipolar relaxation rates have been combined with quadrupole relaxation rates to determine rotational correlation times for motion about each principal molecular axis (78MI21200). NMR analysis has been used to determine the structure of phenyllithium-pyridazine adducts and of the corresponding dihydropyridazines obtained by hydrolysis of the adducts <78RTC116>. 

Enhancement of relaxation is also dependent on the effective correlation time rc which depends on molecular motion, on the rate of relaxation of the electron spins and on the exchange rate between free and bound water. rc is given by the equation (4). 

T2, and the nuclear Overhauser enhancement, NOE, comprise a set of parameters which characterize molecular motions. In the case of simple isotropic motion, the dependence is in terms of a single correlation time characterizing the exponential decay of the autocorrelation function. However, in many instances, the assumption of isotropic motion is not valid. For rigid systems, the relaxation behavior can then often be predicted by assuming simple anisotropic motion (1 ). Often, superposition of two or more independent motions must be used to satisfactorily interpret observed relaxation behavior. Recently, however, the wide-... 

Thus when (cua + (Ux)Tr is not much less than 1, where is the molecular rotational correlation time, the potential maximum enhancement is not attained and it is not possible directly to use the size of the observed NOE to deduce the relative contribution of the dipole-dipole mechanism to the overall longitudinal relaxation of the observed nucleus. (228) The extreme narrowing condition is more stringent at higher measuring fields, but even for protons at say 300 MHz it is easily fulfilled (229) by small molecules for which Tr 10 -10 s. However, with large molecules such as globular... 

C Spin lattice relaxation times, Tj, spin-spin relaxation times, T2, and nuclear Overhauser enhancements, NOE, for the a-carbons of PBLG of various molecular weights have been used to study transitions from rigid to flexible forms of this polymer (Allerhand and Oldfield, 1973). Effective rotational correlation times, reff, calculated from 7 - and NOE-values, for the a-carbons were 24-32 nanoseconds for the helical form and approximately 0-8 nanoseconds for the random coil (Allerhand and Oldfield, 1973). The transition from the a-helix to the random-coil of PLM causes the resonances of the a- and carbonyl carbons to move upfield 2-3 and 3-4 ppm respectively (Tadokoro et al., 1973), which is consistent with results obtained for PBLG and PCBO. Further work is required before the reasons for the chemical shift differences between the corresponding carbons in the helical and random-coil forms in deuterochloroform-TFA systems can be elucidated. Plots of chemical shifts and relaxation times vs. pH have been used to study the helix-coil transition of poly-L-lysine hydrochloride in aqueous solution (Saito and Smith,... 

It is not so well known that nOes also depend on molecular mobility (correlation times Xg) and observation frequency (ca) in a l-cox fashion. This factor is almost null when one observes molecules of ca 1000 Daltons at 400 MHz. Saponins are thus not ideal molecules for the observation of strong nOes. NOes may be negative even though in some articles the nOfi stand for enhancement (29, 30) The first negative nOes of saponins were recognized as such in a 1989 article by I. Kitagawa et al.(61). 

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