Three dinucleoside monophosphates containing 8-bromoguanosine (br8G), (2'-5')C-br8G, (3'-5')C-br8G, and dC-br8G, were synthesized and characterized by UV absorption, CD, and 1H NMR spectroscopy. The 1H NMR data show that all the br8G residues in these dimers take a syn glycosidic conformation. At low dimer strand concentration (5 X 10(-5) M), the UV hypochromicity data suggest that the degree of base stacking decreases in the following order, (2'-5')C-br8G greater than C-G approximately equal to dC-br8G greater than (3'-5')C-br8G. The CD data also suggest little stacking in (3'-5')C-br8G. At high dimer strand concentration (5 X 10(-3) M), only (3'-5')C-br8G shows duplex formation in 0.1 M NaCl. The duplex is assumed to take a left-handed helical structure similar to that of Z-DNA. The Tm of this duplex is surprisingly high for a dimer (about 35 and 45 degrees C at 5 X 10(-3) and 10(-2) M dimer strand concentration, respectively). The above results and the similarity between the CD spectra of (3'-5')C-br8G and poly(G-C) suggest the possible existence of Z-form structure in ribooligo- and ribopolynucleotides with alternating purine-pyrimidine sequences.