Spacer chain length dependence in hydraphile channels

Five benzyl-terminal tris(macrocycle)s were prepared in an effort to determine the optimal length for hydraphile channels. They are represented in shorthand as PhCH 2 C n C n CH 2 Ph, in which is diaza-18-crown-6 and n is 8 ( 3 ), 10 ( 4 ), 12 ( 5 ), 14 ( 6 ), or 16 ( 7 ). As the length increased by ∼5 A for 6 and 7 (10 A) compared to 5 , it was expected that flux would eventually diminish owing to excessive conformational flexibility. Shortening of the spacer chains by a corresponding increment ( 3 , 4 ) was expected to lead eventually to a compound that was too short and thus incapable of transporting Na + on the timescale of the analytical method. Cation transport rates were measured by dynamic 23 Na NMR and normalized to the activity of Dn C 12 C 12 Dn ( 2 , Dn=dansyl), used as a standard. The observed activity was in the order 5 (C 12 ) ∼ 6 (C 14 )> 4 (C 10 ) ∼ 7 (C 16 ). No cation transport activity was observed for 3 (C 8 ). We estimate the optimal channel length to be ∼45 A, the same as reported for the pore of the KcsA K1 channel.