Synthetic hydraphile channels were systematically varied at the side arm position and tested under neutral and acidic conditions against the Gram-negative bacterium E. coli. Side arm structure is a critical determinant of channel activity within the bilayer and greatly influences toxicity. The biological activity data correlate remarkably well with the results of Na + transport studies conducted using synthetic liposome systems. Experiments to determine minimum inhibitory concentrations (MIC) were conducted under both neutral and acidic conditions. Hydraphile toxicity was diminished at pH = 5.5, compared to that at pH = ∼7, providing an example of a pH-dependent, synthetic, non-peptide ion channel.