The m easurem ents were perform ed in a Bruker M SL-300 spectrom eter operating at a field o f 7.05 Tesla. The spin lattice relaxation times were deter m ined by the inversion-recovery pulse sequenceThe tem p eratu re range studied was from 203 K to 373 K for the deuteron -T x and from 251 K to 373 K for the c a rb o n -13 data. The tem peratures were controlled to ± 0 .5 K by a metal sheathed C hrom el/A lum el therm ocouple. The carbon-13-spectra were obtained under continuous broadb an d -p ro to n decoupling.T he accuracy o f the data is considered better th an ± 5 % , except for the lowest tem peratures w here the e rro r increases to ± 10% , due to the low signal to noise ratio and because o f the steep tem p eratu re dependence o f T v
TheoryIn the high tem perature range all different c a r bons o f the tw o sugars yield individual carbon-13-signals in the 75.4 M H z spectra. A t constant co n centration and tem perature all carbons carry ing a single p ro to n yield identical spin lattice relax atio n times. Since the individual C -H bonds point into different directions in a molecule fixed axis system this result is a strong indication, th a t the ro tatio n a l m obility o f the two disaccharides is iso tropic. This conclusion was also derived by A ller h and et al.
The ribose conformations of formycin, 2-azaadenosine, nebularine, 8-bromoinosine, and 8-bromo- xanthosine have been studied using proton magnetic resonance in ND3 solutions, in D2O solutions, and in pyridine solutions. The temperature was varied between - 60 °C and +40 °C in ND3 and between +10 °C and +60 °C in D2O solutions. In solution, 2-azaadenosine and nebularine have a conformational behaviour similar to that of the common purine (β) ribosides. This is in agreement with the conformations observed in the solid state. The conformations of formycin and formycin B have strong analogies with those of the 8-bromopurineribosides and differ significantly from those of the 8-azapurineribosides since they adopt preferentially the syn-S-g+ conformation. This conformation is very probably stabilized by an intramolecular hydrogen bond between O (5′) and N (3).
From the analysis of the HRNMR spectra of two tricyclic analogues of adenosine and guanosine, 4,5-diamino-9-(β-D-ribofuranosyl) pyrimido[5,4-f]pyrrolo[2,3-d]pyrimidine (adenosine-adenosine, AA) and 4,7-diamino-9-(β-ᴅ-ribofuranosyl)pyrimido[5,4-f]pyrrolo-[2,3-d]pyrimidin-5-one (adenosine-guanosine, AG), dissolved in liquid ND3 the preferred conformations of the ribose moiety are derived in the temperature range between + 40 and -60 °C. The analysis is based on the two state N ↔ S model of the furanoside ring proposed by Altona and Sundaralingam. Both compounds show a pronounced stabilization of the S-conformer of the sugar ring ([S] ~ 0.8). The van't Hoff enthalpy for the S ↔ N equilibrium is - 3 kJ mol-1. The syn ↔ anti equilibrium is even at -60 °C fast compared to the HRNMR time scale.
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