We describe the chemical synthesis and preliminary biophysical and biochemical characterization of a series of mRNA 5' end (cap) analogs designed as reagents for obtaining mRNA molecules with augmented translation efficiency and stability in vivo and as useful tools to study mRNA metabolism. The analogs share three structural features: (i) 5',5'- bridge elongated to tetraphosphate to increase their affinity to translation initiation factor eIF4E (ii) a single phosphorothioate modification at either the α, β, γ or δ-position of the tetraphosphate to decrease their susceptibility to enzymatic degradation and/or to modulate their interaction with specific proteins and (iii) a 2'-O-methyl group in the ribose of 7-methylguanosine, characteristic to Anti-Reverse Cap Analogs (ARCAs), which are incorporated into mRNA during in vitro transcription exclusively in the correct orientation. The dinucleotides bearing modified tetraphosphate bridge were synthesized by ZnCl(2) mediated coupling between two mononucleotide subunits with isolated yields of 30-65%. The preliminary biochemical results show that mRNAs capped with new analogs are 2.5-4.5 more efficiently translated in a cell free system than m(7)GpppG-capped mRNAs, which makes them promising candidates for RNA-based therapeutic applications such as gene therapy and anti-cancer vaccines.