Exponential regression of odor threshold concentration [<i>O</i>] and average X–C distance <i>r</i> (X = C, Si, Ge, Sn).

<p><i>In vivo</i> data points [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0182147#pone.0182147.ref071" target="_blank">71</a>] are given as blue diamonds, regression curves as black lines. Error bars depict the standard deviation. The data points of <i>rac</i>-<b>1a</b>–<i>rac</i>-<b>1d</b> deviate from an exponential form, while the data for <b>2a</b>–<b>2d</b> follow an exponential form.