Design, synthesis, and calorimetric studies on protein-ligand interactions : apolar surface area, conformational constraints, and cation-[pi] interactions
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Because bimolecular interactions in water are poorly understood, three tactics commonly used to improve binding affinity in ligand design were investigated: (1) increasing apolar surface area, (2) introducing a conformational constraint, and (3) targeting a cation-[pi] interaction. Thermodynamic parameters of binding ligands to the Grb2 SH2 domain were determined by isothermal titration calorimetry (ITC), and structural data was obtained by X-ray crystallography. The apolar surface area of the pTyr+1 residue in Ac-pTyr-Acnc-Asn-NH₂ was varied by incrementally increasing the size of the cyclic Acnc residue from a 3-membered to a 7-membered ring. Increasing apolar surface area resulted in an increase in Ka due to a more favorable [delta]H⁰ that was dominated a less favorable [delta]S⁰. Structural analyses showed that all ligands bound in a similar mode, so differences in binding thermodynamics were attributed to the pTyr+1 residue. The thermodynamics of binding tripeptides wherein pTyr+1 was an n-alkyl group were studied. Ka increased when Ala was mutated to Abu, but additional methylene groups had no effect on Ka due to strong entropy-enthalpy compensation. While [delta]H⁰ was weakly correlated with buried surface area, there was no change in [delta]H⁰ between one methylene and two methylene groups, presumably because an enthalpic penalty is associated with a gauche interaction between C-[beta] and C-[gamma] of the Xaa side chain that was noted in the crystal structure. An olefin was installed in an attempt to alleviate the energetic penalty incurred from the gauche interaction, but the introduction of the constraint resulted in equipotent ligands. A putative cation-[pi] interaction between Arg67 and various aromatic groups was probed by varying the [pi]-donating capability of groups attached to a tripeptide scaffold. Although crystal structures demonstrated that three of the aryl groups were close enough to Arg67 to form a cation-[pi] interaction, only a modest increase in Ka was observed relative to analogues having only an N-acetyl group. Furthermore, a simple cyclohexyl group in place of aryl groups resulted in ligands that were equipotent with indolyl- and phenyl- derived analogues, so any cation-[pi] interaction is not significant.