2-THIOZEBULARINE: BASE MODIFIED NUCLEOSIDE FULLY CONSTRAINED IN C3′-endo CONFORMATION IN SOLUTION

2-Thiopyrimidinone ribofuranoside (2-thiozebularine, s(2)zeb) was synthesized by the adaptation of silyl method of N-glycosidic bond formation and using thionation of protected 2-oxonucleoside derivative (zebularine, zeb) with Lawesson reagent. The X-ray crystal structure of s(2)zeb and NMR determined conformations of s(2)zeb and zeb in solution were compared with structures of 2-thiouridine (s(2)U) and uridine (U). In the solid state s(2)zeb molecule adopts conformation typical for ribonucleosides: C3'-endo C2'-exo twist type of ribofuranose pucker, anti of N-glycosidic bond and trans around C4'-C5' bond. In aqueous solution, however, almost 100% population of s(2)zeb exhibits C3'-endo ribofuranose pucker. The population of N-conformer of s(2)zeb is about 20% higher than for zeb (analogously to pair of s(2)U and U nucleosides) indicating similar influence of steric effect of bulky sulfur atom on stabilization of N-type ribose conformation. Interestingly, the absence of 4-carbonyl function in zeb and s(2)zeb raises the population of C3'-endo conformation by about 30% in comparison to U and s(2)U as a result of significant anomeric effect. Additive action of both effects makes the 2-thiozebularine almost fully constrained in C3'-endo conformation in aqueous solution. Cytotoxic properties of s(2)zeb are less pronounced in comparison to zebularine, with IC50 > 100 mM for HeLa and K562 cancer cells and for HUVEC non-cancerous cells.