BUs 1 and 2 were clearly the most active compounds of the series of BUs with meta-substituted benzyl groups (red and green curves in Figure ). No clear difference could be made in the activity of BUs 1 and 2 at a concentration of 0.02 mol%. Therefore, the transport experiment was repeated at a 10-fold lower BU concentration (0.002 mol%) indicating that BU 1 was more active than 2 (Figure , Table ). BU 2 has a higher lipophilicity and binding strength than 1, and the impacts of small changes in these two parameters on transport activity might be counteracting. The higher lipophilicity of BU 2 could enhance the diffusion rate through the apolar membrane interior. However, the very high binding strength of BU 2 could slow down the release of the anion at the membrane interface. We note that for compound 1 anion release was previously found to be rate limiting in the transport process.