**4. Conclusions**

In conclusion, we demonstrated that macrocycles containing benzoureido-15-crown-5-ether cation-binding head and bearing linear or branched alkyl side-chains with almost identical lipophilicity, present completely di fferent transport activities for translocations of cations across the lipid bilayer. The isomeric octyl-benzoureido-15-crown-5-ethers **1**, **r2**, **s2**, **3** described here, are very intriguing electrogenic macrocycles presenting K<sup>+</sup> over Na<sup>+</sup> kinetic selectivity of SK+/Na+ = 3 to 6.6. Specifically, we have demonstrated that simple structural variation from linear to branched octyl chains are strongly influencing the transport activities of K<sup>+</sup> cations when compared with Na<sup>+</sup> cations. The electrogenic macrocyclic compound-mediated K<sup>+</sup> influx is highly increased up to 27 fold by the addition of FCCP proton carrier with a special emphasis for the transport of K<sup>+</sup> cations, for which the SK+/Na<sup>+</sup> selectivity, is increasing from 6.6 to 45.5 in the case of **s2** and from 3.6 to 48.8 in the case of **3**. This is a significant step forward toward the development of electrogenic macrocycles with high K+/Na<sup>+</sup> selectivity.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2624-8549/2/1/3/s1, Scheme S1: Synthesis of compounds **1**, **r**, **s2**, **3**, Figure S1: 1H NMR spectrum of **1** (300 MHz, 298K, DMSO-d6), Figure S2: 13C NMR spectrum of **1** (75 MHz, 298K DMSO-d6), Figure S3 1H NMR spectrum of **r2** (300 MHz, 298 K, DMSO-d6), Figure S4 13C NMR spectrum of **r2** (75 MHz, 298 K, CDCl3), Figure S5 1H NMR spectrum of **s2** (300 MHz, 298 K, DMSO-*d*6), Figure S6 13C NMR spectrum of **s2** (75 MHz, 298 K, CDCl3), Figure S7 1H NMR spectrum of **3** (300 MHz, 298 K, DMSO-*d*6), Figure S8 13C NMR spectrum of **3** (75 MHz, 298 K, CDCl3), Figure S9 1H NMR dilution experiments in CDCl3 of **1** (0.01 to 0.16 M, from top to bottom), Figure S10 1H NMR dilution experiments in CDCl3 of **r2** (0.01 to 0.16 M, from top to bottom), Figure S11 1H NMR dilution experiments in CDCl3 of **3** (0.01 to 0.16 M, from top to bottom), Figure S12. Normalized I460/I403 for transporting K<sup>+</sup> across the bilayer membrane facilitated by di fferent amount of **1** and hill plot analysis of K+/H<sup>+</sup> antiport, Figure S13. Normalized I460/I403 for transporting Na<sup>+</sup> across the bilayer membrane facilitated by different amount of **1** and hill plot analysis of Na+/H<sup>+</sup> antiport, Figure S14 Normalized I460/I403 for transporting K<sup>+</sup> across the bilayer membrane facilitated by di fferent amount of **1** coupled with FCCP (0.1 mol%) and hill plot analysis of K+/H<sup>+</sup> antiport, Figure S15 Normalized I460/I403 for transporting Na<sup>+</sup> across the bilayer membrane facilitated by di fferent amount of **1** coupled with FCCP (0.1 mol%) and hill plot analysis of Na+/H<sup>+</sup> antiport, Figure S16 Normalized I460/I403 for transporting K<sup>+</sup> across the bilayer membrane facilitated by di fferent amount of **s2** and Hill plot analysis of K+/H<sup>+</sup> antiport, Figure S17 Normalized I460/I403 for transporting Na<sup>+</sup> across the bilayer membrane facilitated by different amount of **s2** and hill plot analysis of Na+/H<sup>+</sup> antiport, Figure S18 Normalized I460/I403 for transporting K<sup>+</sup> across the bilayer membrane facilitated by different amount of **s2** coupled with FCCP (0.1 mol%) and hill plot analysis of K+/H<sup>+</sup> antiport, Figure S19 Normalized I460/I403 for transporting Na<sup>+</sup> across the bilayer membrane facilitated by di fferent amount of **s2** coupled with FCCP (0.1 mol%) and hill plot analysis of Na+/H<sup>+</sup> antiport, Figure S20 Normalized I460/I403 for transporting K<sup>+</sup> across the bilayer membrane facilitated by different amount of **r2** coupled with FCCP (0.1 mol%) and hill plot analysis of K+/H<sup>+</sup> antiport, Figure S21 Normalized I460/I403 for transporting Na<sup>+</sup> across the bilayer membrane facilitated by different amount of **r2** coupled with FCCP (0.1 mol%) and hill plot analysis of Na+/H<sup>+</sup> antiport, Figure S22 Normalized I460/I403 for transporting K<sup>+</sup> across the bilayer membrane facilitated by di fferent amount of **3** and hill plot analysis of K+/H<sup>+</sup> antiport, Figure S23 Normalized I460/I403 for transporting Na<sup>+</sup> across the bilayer membrane facilitated by different amount of **3** and hill plot analysis of Na+/H<sup>+</sup> antiport, Figure S24 Normalized I460/I403 for transporting K<sup>+</sup> across the bilayer membrane facilitated by di fferent amount of **3** coupled with FCCP (0.1 mol%) and hill plot analysis of K+/H<sup>+</sup> antiport, Figure S25 Normalized I460/I403 for transporting Na<sup>+</sup> across the bilayer membrane facilitated by different amount of **3** coupled with FCCP (0.1 mol%) and hill plot analysis of Na+/H<sup>+</sup> antiport, Figure S26. HR-MS spectra of compounds **1**, **r2**, **s2**, **3**, Table S1 Pseudo first-order rate constants *k* (s−1) for the transport of K+/H<sup>+</sup> through LUVs at di fferent concentrations of the compounds to lipid without or with proton transporter FCCP, Table S2 Pseudo first-order rate constants *k* (s−1) for the transport of Na+/H<sup>+</sup> through LUVs at di fferent concentrations of the compounds to lipid without or with proton transporter FCCP. The initial rate for the blank has already been subtracted from all the rates.

**Author Contributions:** Conceptualization—M.B. Synthesis, Cation Transport—Y.-H.L. and S.-P.Z.; Investigation—D.W.; Writing—Original Draft Preparation—Y.-H.L. and M.B.; Supervision—M.B.; Funding Acquisition—M.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was also conducted within ANR-18-CE06-0004-02, WATERCHANNELS, and 1000 Talent Plan, WQ20144400255 of SAFEA, China. Y-H.L. wishes to thank the Innovation and Talent Introduction Base of Photoelectronic and Functional Molecular Solids Material (Grant No. 90002-18011002) from Sun Yat-Sen University, and also the China Scholarship Council (No. 201606380054) for the financial support. NSFC (National Natural Science Foundation of China, 21720102007), China. S.-P.Z. wishes to thank the China Scholarship Council for financial support.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
