Binding mode of xanthine-based compounds at the four AR subtypes.

<p>Compound <b>36</b>, in dark green, was selected as reference to show the proposed binding mode at the four AR subtypes. The crystallographic coordinates of caffeine, in magenta, bound to hA_2A AR are reported superimposed to the binding mode of compound <b>36</b>. The xanthine core of compound <b>36</b> is oriented in a similar manner to the crystallographic data. Residues particularly important in the binding are reported as light grey sticks.