Conformational preferences of anticodon 3'-adjacent hypermodified nucleic acid base cis-or trans-zeatin and its 2-methylthio derivative, cis-or trans- ms(2)zeatin

Conformational preferences of the hypermodified nucleic acid bases N6-(Delta(2)-cis-hydroxyisopentenyl)adenine, cis-io(6)Ade also known as cis-zeatin, and N(6)-(Delta(2)-trans-hydroxyisopentenyl)adenine, trans-io(6)ade or trans-zeatin, and 2-methylthio derivatives of these cis-ms(2)io(6)Ade or cis-ms(2)zeatin, and trans-ms(2)io6Ade or trans-ms(2)zeatin have been investigated theoretically by the quantum chemical Perturbative Configuration Interaction with Localized Orbitals (PCILO) method. Automated geometry optimization using quantum chemical MNDO, AM1 and PM3 methods has also been made to compare the salient features. The predicted most stable conformation of cis-io(6)Ade, trans-io(6)Ade, cis-ms(2)io(6)Ade and trans-ms(2)io(6)Ade are such that in each of these molecules the isopentenyl substituent spreads away (has "dista" conformation) from the five membered ring imidazole moiety of the adenine. The atoms N(6), C(10) and C(11) remain coplanar with the adenine ring in the predicted preferred conformation for each of these molecules. In cis-io(6)Ade as well as cis-ms(2)io(6)Ade the hydroxyl oxygen may participate in intramolecular hydrogen bonding with the H-C(10)-H group. In trans-io(6)Ade the hydroxyl group is oriented towards the H-C(2) instead. This orientation is retained in trans-ms(2)io(6)Ade, possible O-H...S hydrogen bonding may be a stabilizing factor. In all these four modified adenines C(11)-H is favourably placed to participate in intramolecular hydrogen bonding with N(1). In cis-ms(2)io(6)Ade as well as trans-ms(2)io(6)Ade the 2-methylthio group preferentially orients on the same side as C(2)-N(3) bond, due to this non-obstrusive placing, orientation of the hydroxyisopentenyl substituent remains unaffected by 2-methylthiolation. Thus the N(1) site remains shielded irrespective of the 2-methylthiolation status in these various cis-and trans-zeatin analogs alike. Firmly held orientation of hydroxyisopentenyl substituent in zeatin isomers and derivatives, in contrast to adaptable orientation of isopentenyl substituent in i(6)Ade and ms(2)i(6)Ade, may account for the increased efficiency of suppressor tRNA and reduced codon context sensitivity accompanied with the occurrence of ms(2)-zeatin (ms(2)io(6)Ade) modification.