Based on the above and on the remarkable properties of the
2
′-O,
4
′-C-methylene bridged LNA monomers it was decided to synthesise oligonucleotides comprising one or more
2
′-O,′-C-methylene-&bgr;-D-xylofuranosyl nucleotide monomer(s) as the first stereoisomer of LNA modified oligonucleotides. Modelling clearly indicated the xylo-LNA monomers to be locked in an N-type furanose conformation. Whereas the parent
2
′-deoxy-&bgr;-D-xylofuranosyl nucleosides were shown to adopt mainly an N-type furanose conformation, the furanose ring of the
2
′-deoxy-&bgr;-D-xylofuranosyl monomers present in xylo-DNA were shown by conformational analysis and computer modelling to prefer an S-type conformation thereby minimising steric repulsion between the nucleobase and the
3
′-O-phopshate group (Seela, F.; Wömer, Rosemeyer, H.
Helv. Chem. Acta
1994, 77, 883
). As no report on the hybridisation properties and binding mode of xylo-configurated oligonucleotides in an RNA context was believed to exist, it was the aim to synthesise
2
′-O,
4
′-C-methylene-&bgr;-D-xylofuranosyl nucleotide monomer and to study the thermal stability of oligonucleotides comprising this monomer. The results showed that fully modified or almost fully modified Xylo-LNA is useful for high-affinity targeting of complementary nucleic acids. When taking into consideration the inverted stereochemistry at C-
3
′ this is a surprising fact. It is likely that Xylo-LNA monomers, in a sequence context of Xylo-DNA monomers, should have an affinity-increasing effect.
基于上述内容和2'-O,4'-C-亚甲基桥联LNA单体的显著特性,决定合成包含一个或多个2'-O,4'-C-亚甲基-β-
D-木糖呋喃核苷酸单体的寡核苷酸作为LNA修饰寡核苷酸的第一立体异构体。建模清楚地表明
木糖-LNA单体被锁定在N型
呋喃糖环构象中。而2'-去氧-β-
D-木糖呋喃核苷主要采用N型
呋喃糖环构象,xylo-DNA中的2'-去氧-β-
D-木糖呋喃单体的
呋喃糖环经构象分析和计算机建模表明更倾向于S型构象,从而最小化核碱基和3'-O-
磷酸基团之间的立体排斥(Seela, F.; Wömer, Rosemeyer, H.Helv. Chem. Acta1994, 77, 883)。由于在RNA上下文中尚不存在关于xylo构型寡核苷酸的杂交性质和结合模式的报告,因此旨在合成2'-O,4'-C-亚甲基-β-
D-木糖呋喃核苷酸单体并研究包含该单体的寡核苷酸的热稳定性。结果表明,完全修饰或几乎完全修饰的Xylo-LNA可用于高亲和力靶向互补核酸。考虑到C-3'的反式立体
化学,这是一个令人惊讶的事实。很可能Xylo-LNA单体在Xylo-DNA单体的序列环境中具有增加亲和力的效果。