EFFECTS OF THE INTRODUCTION OF L-NUCLEOTIDES INTO DNA - SOLUTION STRUCTURE OF THE HETEROCHIRAL DUPLEX D(G-C-G-(L)T-G-C-G)CENTER-DOT-D(C-G-C-A-C-G-C) STUDIED BY NMR-SPECTROSCOPY

The effect of the substitution of a L-nucleoside for a D-nucleoside in the duplex d(G-C-G-T-G-C-G) .d(C-G-C-A-C-G-C) was studied by UV and NMR spectroscopy. These unnatural oligonucleotides have potential for antisense DNA technology [Damha, M. J., Giannaris, P. A., and Marfey, P. (1994) Biochemistry (preceding paper in this issue)]. The thermal stability of such duplexes is lower than that of the natural one and is dependent on the nucleotide type and/or sequence. Interestingly, inversion of the chirality of thymidine but not adenosine coincides with a large stabilizing enthalpy change. The structure of the heterochiral duplex d(G(1)-C-2-G(3)-(L)T-4-G(5)-C-6-G(7)).d(C-8-G(9)-C-10-A(11)-C-12-G(13)-C-14), where (L)T denotes the mirror image of the natural thymidine, has been determined by NMR spectroscopy. The sugar conformation was determined using the sum of coupling constants and the distances using a model free relaxation matrix approach. The torsion angles of the backbone follow from (3)J(HH), (3)J(HP), and (4)J(HP) coupling constants. The structure of the duplex was calculated by metric matrix distance geometry followed by simulated annealing. The structure is close to that of B-DNA. The base pair formed by (L)T and A is of the Watson-Crick type. All sugars adopt an S-type pucker. The incorporation of the L-sugar in the duplex is accomplished by changes in the backbone torsion angles around the phosphates and the glycosidic torsion angle of (L)T. The modification induces changes in the natural strand as well. The structure exhibits an unusual interaction between the aromatic rings of the (L)T-4.A(11) and G(3).C-12 base pairs, which provides a plausible explanation of the unusual thermodynamic properties of the duplex.