Puromycin and some of its analogues: conformational properties in solution. A 360-MHz nuclear-magnetic-resonance investigation.

Abstract

The intramolecular conformation of puromycin in 2H2O solution at 25 °C and 65 °C was studied using vicinal coupling constants and chemical shifts obtained at 360 MHz. From vicinal coupling constants a 73% preference for the N‐type (C‐3′‐endo) ribose conformer is deduced. The reference compound, 6‐N‐dimethyl‐3′‐deoxy‐3′‐(acetamido)adenosine, displays even greater preference (80% N). Around the C‐4′– C‐5′ bond of the sugar residue and the C‐α–C‐β bond of the amino acid moiety the gauche+ rotamer (backbone notation) is preferred by 78% and 63% at 25 °C and 73% and 60% at 65 °C, respectively.From the proton magnetic resonance parameters of the equilibrating species 6‐N‐dimethyl‐3′‐(2′)‐O‐(methoxyacetyl)adenosine it is deduced that the gauche+ orientation of the exocyclic 5′‐CH2OH group is again preferred, but the nature and the position of the substituent exert an influence on the ribose conformational equilibrium. The change from 3′‐amide to 3′‐ester decreases the percentage of N‐type ribose to 34% in the latter whereas the 2′‐ester shows 55% preference for type N.Eight puromycin conformers are found to occur in significant amounts in solution, the most abundant conformer (36% at 25 °C and 31% at 65 °C) is the one identical with the solid‐state conformation of puromycin dihydrochloride. Calculated time‐averaged shielding values due to the phenyl ring of the amino acid moiety are in reasonable agreement with experiment and are dominated by the preferred conformer which is characterized by the proximity of the phenyl ring to the H‐4′, H‐5′ and H‐5” protons of the ribose moiety. The results substantiate the Remin and Shugar assignment of the C‐5′ methylene hydrogens in purine nucleosides. The possible biological significance of these findings is discussed.