Additionally, compound 4 provided suitable crystals for analysis following soaking into preformed crystals of MutSβ bound to mismatch DNA (MSH2 ATP binding site occupied by ADP, PDB: 8R7E). Ligand density was observed in the MSH3 ATP pocket, consistent with the biochemical binding mode and UV cross-linking data. The asymmetric unit of the crystal once again contained two MutSβ complexes and two ligand poses were also observed for this compound, although the ligand was not fully resolved in one of these molecules (Figure ). A crystallographic sulfate ion located at the α-phosphate position was observed again in both cases. The aminopyrrolopyrimidine core was observed to make a tridentate interaction with the backbone of the nucleotide binding pocket. Surprisingly, we did not observe any interaction with Asp875 even though this is one of the most dominant interactions for pteridine 1 (Figure ). An in silico analysis of 4 was also performed to verify the protein ligand interactions using FMO. The X-ray structure of 4 in MSH3 (PDB: 8R7E) was combined with the MSH2 loop from the pteridine 1 X-ray structure (PDB: 8R7C) and the resulting hybrid model used in the analysis (the MSH2 loop was not resolved in the X-ray structure for 4 in MSH3). From this analysis, we were able to identify and quantify the interaction of 4 with Asp875 (Figure ), showing that the strongest interaction of 4 in the MSH3 ATP binding site is calculated to be the salt bridge between the piperazine and the Asp875, as expected based on the interactions of pteridine 1.