MM3(96) parameterization for camptothecin analogs: an ab initio and molecular mechanics study

Area of Science:

• Computational Chemistry
• Molecular Modeling
• Medicinal Chemistry

Background:

• Camptothecin (CPT) is a natural product with significant anticancer activity.
• Accurate molecular modeling is crucial for understanding CPT analogs and designing new drugs.
• Existing molecular mechanics force fields, like MM3(96), lacked parameters for certain atoms in CPT.

Purpose of the Study:

• To derive new torsional parameters for the MM3(96) force field specific to camptothecin (CPT).
• To validate these new parameters by comparing MM3 calculations with ab initio quantum mechanics results.
• To utilize the improved MM3 parameters for conformational analysis and quantitative structure-activity relationship (QSAR) studies of CPT analogs.

Main Methods:

• Ab initio quantum mechanics calculations (Gaussian 92, RHF/6-31G**, 4-31G**) were performed on representative compounds.
• Potential energy curves were computed for dihedral angles involving atoms with missing MM3 parameters.
• MM3 torsional parameters (V1, V2, V3) were optimized to reproduce the ab initio torsional profiles.

Main Results:

• New MM3(96) torsional parameters were successfully derived for atom types found in camptothecin.
• MM3 calculations using the new parameters yielded molecular structures in good agreement with ab initio results.
• Conformational analyses revealed two distinct 'boatlike' conformations for the alpha-hydroxy lactone moiety in CPT analogs.

Conclusions:

• The developed MM3 parameters enhance the accuracy of molecular modeling for camptothecin and its analogs.
• The predicted low-energy lactone conformation aligns with experimental X-ray crystallographic data and ab initio structures.
• These findings facilitate more reliable conformational analyses and QSAR studies for CPT-based drug discovery.