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The molecular electrostatic potential (MEP) is well-known as a rigorously defined expectation quantity which is measured as the first-order interaction between he molecular charge (electrons and nuclei) distribution and a positive unit charge at any point in space surrounding the molecule. In spite of the time-consuming computation required by SCF Hartree-Fock wavefunction, the isopotential maps have proven useful in the analysis of long-range non-covalent interactions in complex biological systems, proton affinities, solvation processes, and in the evaluation of electrostatic charges for molecular mechanics and dynamics studies[26]. In tabulating the wavefunction data from MOAPC(PM3), electrostatic potential (ESP) (Fig. 3c) of CIT is apparently different from that of OA (Fig. 3b). This modelling agrees with the results from QSAR, in which substituents at m-position are important for OA agonist-receptor interaction due to their electronic nature(for a detailed electrostatic data of atoms in these compounds, see also Table 6-8. and the related contents there). In Fig. 3 (the magnitude decreases with the spectral ordering of the colors by white, red, yellow, green, cyan, blue, and violet charcoal), the electrostatic regions appear consistent with the QSAR data. The red regions refer to an area where interactions with negative charges are more favorable. One of these regions lies to one side of the plane of the benzyl ring and C(16,18) of 29 and CIT as shown in Fig. 3. In the case of CIT, the S(22) atom of the hetero-ring is more positive rather than the negative O(22) of 29. A negative charge on the molecules in benzyl ring and N(15) of oxazolines and thiazolines would be favorable to hydrogen bond donation. The blue regions interact extensively with a positive charge and thus would be an area to suspect hydrogen bond acceptor groups on the protein. Fig. 3. Continuous electrostatic potential mapped onto electron density surface of a) 29, b) OA, and c) CIT. Fig. 3a Continuous electrostatic potential mapped onto electron density surface of 29 Fig. 3b Continuous electrostatic potential mapped onto electron density surface of OA Fig. 3c Continuous electrostatic potential mapped onto electron density surface of CIT Next page /Previous page /Table of Contents