Adenine Nucleosides in Solution: Stabilisation of the anti‐Conformation by C‐5′ Substituents

Abstract

Proton magnetic resonance spectra of 30 adenine nucleosides have been obtained in dilute (0.001 to 0.05 M) aqueous or dimethyl sulfoxide solutions and have been analysed with respect to structuredependent variations of the purine proton chemical shifts. In 2Hz0 the resonance signal of H-8 shows a marked dependence upon the nature of substituents linked to C-4' of adenine ribofuranoside derivatives and upon the presence of a 2': 3'-O-isopropylidene group while H-2 is relatively unaffected.Therefore the chemical shift difference, 6H-8 -6H-2 = Ad, may serve as an indicator of intramolecular ribose-base interact ons. d6 varies with the chemical nature of C-4' or C-5' substituents in the order: ammonium, sulfonium (0 ppm) < hydrogen < hydroxyl amino < thio < halogen < phosphate, carboxylate (0.4 ppm), and decreases with increasing chain length of these substituents. These results suggest the existence of electrostatic and dipolar repulsion or, in the majority of cases, attractive forces between the C-5' and C8-H regions of adenine nucleosides which have been previously demonstrated for 5'-nucleotides only. They imply a strong preference of most adenosine derivatives with unsubstituted 2' and 3'-hydroxyl groups for the anti-conformation range and the gauche, gauche C-5' conformation. In dimethyl sulfoxide, basically the same situation exists but different substituents exhibit smaller individual A6 variations than in water, indicating different solvation shells. In this solvent two additional conformation-stabilizing contributions can be recognized, viz. an interaction of the bridge oxygen (0-4') of ribose with adenine, and a hydrogen bond between H-8 and a 5'-carboxylate anion. This energetic stabilisation of the anti and g,g-conformers by most types of adenosine C-5' substituents supports the "rigidity" concept of nucleotides, extending it to the nucleoside level. The only exception found is 5'-nitromethyladenosine in which ultraviolet, circular dichroism and magnetic resonance spectra suggest rotation of the adenine base to a "non-classical" conformation due to base-chain n-electron interaction. The anti-conformation of the adenosine derivatives used in this study can in some cases be correlated with their enzymatic deamination by adenosine and AMP aminohydrolase.The biochemical function of nucleosides and nucleotides as substrates or effectors of the degrading and polymerizing enzymes of nucleic acid metabolism depends critically upon the molecular conformation which they assume in solution. The three substructures of a typical nucleoside, viz. the heterocyclic base, the pentofuranose ring and the exocyclic 5'-carbon atom carrying various substituents, are known to be arranged in certain relative orientations which have been Part 1 of a series entitled Forces Stabilizing the Conformation of Nucleosides in Solution.

Adenine Nucleosides in Solution: Circular Dichroism Studies and Base Conformation

“…polarisation of C -H at the imidazole and protonation at the pyrimidine ring, should shift the adenine dipole moment in the same direction.) Finally, compounds of group V have strong CD effects but exceptionally small chemical shift differences (XXI: 0.07; XXVII: 0.06 ppm) of their purine protons; this is in agreement with the concept of "stacked" x-electron systems [17].…”

“…and related properties of nucleosides not only depend on the chemical nature of C-5' substituents but contributions from the electronegative 0-4' (or 0-1 ') oxygen atom of ribose which is close to the aglycone can also be expected. Some experimental evidence for this assumption could be seen in our previous PMR studies [17], and in the substantial changes of Cotton effects in 4'-thiouridine and 4'-thioadenosine when compared with the parent nucleosides [3,15]. We here describe in more detail the PMR (Table 3) and CD characteristics (Fig.…”

“…The compounds studied here have also been investigated by PMR spectroscopy [17] and a marked preference for the anti-conformation was found which is due to intramolecular forces; more recent studies of various 5'-nucleotides [29,30] = -2800 to -6400 deg. cm2 dmol-' ; because this chain-base interaction is rather strong and, in the case of adenosine 5'-carboxylate, approaches a hydrogen bond which in turn would affect the adenine chromophore, this correlation cannot be fortuitous.…”

“…We have recently presented evidence, based upon systematic proton magnetic resonance (PMR) studies, that in solution most adenosine derivatives with an unsubstituted adenine base and in the absence of 2' : 3'-isopropylidene groups strongly favor a "rigid" anti-conformation due to weak but noticeable dipolar or electrostatic interactions of C-5' substituents with the imidazole ring (CH-8) of the purine system [17]. It was pointed out that comparison of a large number of closely related compounds is essential in such studies in order to evaluate the extent to which a spectroscopic method reflects changes in the base torsion angle or other chemical or physical properties of the molecules.…”

“…This type of spectrum is produced by the nitromethyl anion (XXI) [17], toluene sulfonyl (XXVI), or benzyl ester group (XXVII). It can be distinguished from group IV in that the molar ellipticities of both long-wavelength and short-wavelength bands are much larger (> 10000 deg.…”