Potent anti-hepatitis B viral activity and inhibition of bacteriophage T7 RNA polymerase by a "fat" nucleoside and its 5'-triphosphate derivative: synthetic, biochemical, and biological studies of 4,8-diamino-6-imino-6H-1-beta-D-ribofuranosyl-imidazo[4,5-E] [1,3]diazepine-5'-triphosphate.