Abstract

This paper reports the crystal structures of free acid and ammonium salt of adenosine 2′- monophosphate (2′-AMP). 2′-AMP crystallizes in the hexagonal space group P6₅22 with a = 9.530(3) Å, c = 73.422(2) Å, and Z= 12. 2′-AMP.NH₄ crystallizes in the trigonal space group P3₁, with a = 9.003(2) Å, c = 34.743(2) Å and Z= 6. Both the structures were solved by direct methods and refined by full matrix least- squares method to final R factors of 0.080 and 0.038 for 2′-AMP and 2′-AMP.NH₄ respectively. The adenine bases of both the structures are in syn conformation contrasting with the anti geometry in 3′-AMP, 5′-AMP and the enzyme bound state. Ribose moiety of 2′-AMP is in C2′ -endo conformation. However, the ribose moieties of both the nucleotide molecules display C2′-endo-C3′-exo twist conformation in 2′- AMP.NH₄ structure. Both structures demonstrate g⁺ conformation about C4′ -C5′ bond. 2′-AMP and one of the nucleotide molecules of 2′-AMP.NH₄ are protonated at N1 and the ammonium ion is involved in a bifurcated hydrogen bond with 03′ B and 03A atoms. A characteristic feature of both the structures is the intramolecular 05′ -N3 hydrogen bond. Our crystallographic results on 2′-AMP corroborates the earlier conclusion that the enzyme- bound state is not the lowest energy state of this nucleotide. 2′ -AMP displays base-ribose 04′ stacking not seen in the 2′-AMP.NH₄ structure. Theoretical and experimental studies on 2′-, 3′- and 5′-AMP structures have been discussed.