Phenylalanine scan of the aromatic core. (A) All F substitutions eliminate αP197A modes except for αY93F (two-site, adult AChRs; 30 µM ACh; −100 mV; open is down). Calibration, 1 s/7 pA (low resolution traces) and 250 ms, 5 pA (high resolution clusters). (B) F substitutions at one-site, αδ-only AChRs (3 mM ACh; 70 mV; open is up). Clusters from αY190F, αY198F, and δW57F are homogeneous.