FORCE-FIELD PARAMETERS FOR SULFATES AND SULFAMATES BASED ON AB-INITIO CALCULATIONS - EXTENSIONS OF AMBER AND CHARMM FIELDS

Ab initio self-consistent field (SCF) Hartree-Fock calculations of sulfates R-O-SO3(-1) (A = Me, Et, i-Pr) and sulfamates R-NHSO3(-1) (R = H, Me, Et, i-Pr) were performed at the 4-31G(*S*N)//3-21G(*S*N) basis set levels, where asterisks indicate d functions on sulfur and nitrogen atoms. These standard levels were determined by comparing calculation results with several basis sets up to MP2/6-31G*//6-31G*, Several conformations per compound were studied to obtain molecular geometries, rotational barriers, and potential-derived point charges. In methyl sulfate, the rotational barrier around the C-O bond is 1.6 kcal/mol at the MP2 level and 1.4 kcal/mol at the standard level. Its ground state has one of three HCOS torsion angles trans and one of three COSO torsion angles trans. Rotation over 60 degrees around the single O-S bond in the sulfate group costs 2.5 kcal/mol at the MP2 and 2.1 kcal/mol at the standard level. For ethyl sulfate, the calculated rotational barrier in going from the ground state, which has its CCOS torsion angle trans, to the syn-periplanar conformation (CCOS torsion angle cis) is 4.8 kcal/mol. However, a much lower barrier of 0.7 kcal/mol leads to a secondary gauchelike conformation about 0.4 kcal/mol above the ground state, with the CCOS torsion angle at 87.6 degrees. Again, one of the COSO torsion angles is trans in the ground state, and the rotational barrier for a 60 degrees rotation of the sulfate group amounts to 1.8 kcal/mol. For methyl sulfamate, the rotational barriers are 2.5 kcal/mol around the C-N bond and 3.3 kcal/mol around the N-S bond. This is noteworthy because sulfamate itself has a calculated rotational barrier around the N-S bond of only 1.7 kcal/mol. These and other data were used to parameterize the well-known empirical force fields AMBER and CHARMm. When the new fields were tested by means of vibrational frequency calculations at the 6-31G*//6-31G* level for methyl sulfate, sulfamate, and methyl sulfamate ground states, the frequencies compared favorably with the AMBER and CHARMm calculated frequencies. The transferability of the force parameters to beta-D-glucose-6-sulfate and isopropyl sulfate appears to be better than to isopropyl sulfamate. (C) 1995 by John Wiley & Sons, Inc.