Several conclusions can be drawn when considering the transport results obtained for all twelve BU derivatives. Changes (up to two orders of magnitude) in the BU anion binding strength do not seem to affect the transport significantly. However, the overall high affinity of the fluorinated BU macrocycles to anions might be important for effective transport. Conversely, a too high lipophilicity or a too large size cause a decrease in the transport activity. The preferred conformation of the BU substituents could be affected by the fluorinated groups and may also be a contributing factor, as it should have an impact on the shape and lipophilic surface of the individual BU derivatives. Unfortunately, this is not readily evaluated, especially as the lipid membrane is a complex environment and the substituents are flexible and relatively free to rotate.