PDBbind provides for each complex an experimentally-determined value of the inhibition constant K i , the dissociation constant K d , or the half-maximal inhibitory concentration IC 50 , in decreasing order of preference (e.g. if both K i and K d values are available, PDBbind reports the measurement of K i ). The refined set includes only measurements of K i and K d , while the general set also includes data for which only IC 50 measurements were available. For our purposes, these values are used interchangeably and are collectively denoted by the binding constant, K. We used the negative base-10 logarithm of K, commonly denoted as pK: pK = -log 10 K