Carbon-13 nuclear relaxation measurements in adenosine monophosphate. Comment

Hd, Lüdemann; Röder, Oskar

doi:10.1021/ja00848a047

Cited by 3 publications

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“…Despite the fact that the polarities of 1È7 are not equivalent, their temperature-dependent experimental relaxation times are nearly identical within the T 1 experimental error ; hence temperature-dependent changes in solvent viscosity and the viscosity correction of the relaxation times have little e †ect on intramolecular energy barriers. 15,20 It is noteworthy that removal of the nucleobase in 6 and 7 gives comparable values, E a thus showing their insensitivity to the absence of heterocyclic aglycone in 6 and 7 in comparison with 1È5a.…”

Section: Temperature Dependence Of Longitudinal Relaxation Timesmentioning

confidence: 98%

Pseudorotation of the sugar moiety in pyrimidine nucleosides occurs on the same time-scale as the overall tumbling:2H NMR relaxation time study

et al. 1998

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The molecular motions of speciÐcally deuterated thymidines 1È5a and two C-3-deuterated allofuranoses 6 and 7 were studied through temperature-dependent 2H and 13C relaxation times. Deuterium relaxation times in 1È5a were found in the range 20È270 ms between 278 and 358 K, respectively. Interpretation of deuterium relaxation time, which is dominated by quadrupole mechanism, gave the e †ective rotational correlation time T 1 in the range from 62.5 ps in 2@,6-diDT (4) to 117.5 ps in 6-DT (3) at 278 K. The values are reduced to 9.3 ps (q eff ) q eff in 2@,6-diDT (4) to 16 ps in 6-DT (3) at 358 K. The values for 6 and 7 are 93.9 and 100.4 ps, respectively, at 278 q eff K, and are reduced to 19.8 and 22.6 ps, respectively, at 328 K. A deuterium quadrupole coupling constant (e2qQ/h) of 167 kHz was calculated from 2H and 13C relaxation times in 2@-DT (1). Arrhenius-type analysis of temperaturedependent deuterium relaxation times in 1È7 gave apparent activation energies in the range 20È23 (^0.9) kJ T 1 mol~1. Deuterium in anh-6-DT (5a) served as a reference for the evaluation of the activation energy of the overall molecular reorientation in the conformationally free counterparts 1È4 due to the fact that rotation across s and repuckering are prevented by its structure. The comparison of the apparent activation energies from the N H S conformationally free nucleosides 1È4 with those from the constrained analogue 5a and abasic sugars 6 and 7 suggests that their internal local motions are heavily coupled with the overall molecular reorientation, thus preventing the estimation of the pseudorotation energy barrier.

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Section: Temperature Dependence Of Longitudinal Relaxation Timesmentioning

confidence: 98%