Abstract

Ab initio molecular orbital calculations were performed on neutral and protonated 2,4-diamino-6-methylpteridine. The computations were repeated for the N-1-protonated molecule in a highly simplified model of the methotrexate binding site of Escherichia coli dihydrofolate reductase. The strengths of the ionic bond to Asp 27, the hydrogen bond to Thr 113, and the interaction with the peptide bonds between Ile 5 and Ala 6 and between Ala 6 and Ala 7 were calculated to be 100, 10, 5, and 1 kcal/mole, respectively, in the absence of solvent. These strengths suggest that the ionic bond is the most important component of the binding of protonated methotrexate. The energy calculations also provide a semiquantitative explanation of the hitherto unexplained shift to longer wavelength of the lowest-frequency UV absorbance band of N-1-protonated methotrexate upon binding to dihydrofolate reductase. This shift is due to a direct effect of the electrostatic field of the carboxylate ion of Asp 27 upon the molecular orbitals of the methotrexate.