Figure 1

The standard G-tetrad with reference numbering. When the glycosidic bond angle is anti the chain progresses over the page, when it is syn the chain progresses below the page. According to Webba da Silva [56] the sign of the loop topology is positive when the first stem is progressing towards the viewer and the second stem is found rotating clockwise, and negative when it is found rotating anti-clockwise. E. g., when the glycosidic bond angle is anti the topology of a loop connecting the stem of base 1 to the stem of base 2 would be marked with - sign, and the topology of a loop connecting the stem of base 1 to the stem of base 4 would be marked with + sign.