We note, however, small deviations of this general trend for the different probe compounds used. In experiments with methionine, 18 O-AKIEs approach a higher intrinsic KIE 3 of 1.031 (dashed line in Figure ) than with histidine (1.026, solid line). O-AKIEs from experiments with tyrosine even decrease slightly from 1.0275 to 1.0240 as forward commitment decreases.