Theoretical studies on the conformation of protonated dopamine in the gas phase and in aqueous solution

A trans. (T) and two gauche (G1 and G2):conformers have been identified for protonated dopamine in the gas phase upon ab initio calculations up to the QCISD(T)/6-31G*//HF/6-31G* and MP2/6-311++G**// MP2/6-311++G** levels and based on B3LYP/6-31G* optimizations in DFT. Free energy differences at 298 K and 1 atm were calculated to-be 3.2-5.6 kcal/mol between T and G1, and about 0 kcal/mol between the G2 and G1 conformers. The OH groups are nearly coplanar with the benzene ring and form an O-H ... O-H intramolecular hydrogen bond:in their most stable arrangement. Using the free energy perturbation method through Monte Carlo:simulations, relative solvation free energies were evaluated in:aqueous solution at T = 310 K and 1 atm. Ab initio/Monte Carlo torsional potential curves were calculated along pathways where Small rotations about the C(ring)-C(beta) and C(beta)-C(alpha) axes were allowed. No stable rotamers but the gas-phase optimized structures were identified. The T - G1 and G2 - G1 relative solvation free energies were calculated at -2.63 +/- 0.31 and 1.34 +/- 0.43 kcal/mol, respectively,The calculated T - G1 total free energy difference is at least 0.6 +/- 0.3 kcal/mol in aqueous solution, predicting a G:T ratio of at least 75:25 as compared to the experimental value of 58:42. The calculated result is sensitive:both to the applied basis set and to the level of the electron correlation considered upon obtaining the internal energy. When dopamine acts in a biological environment, its protonated form is presumably surrounded by counterions, mainly by chloride anions. If a chloride counterion, set at N ... Cl separation of 6 Angstrom to estimate the upper bound of the counterion effect on a solvent separated DopH(+) ion, is also considered in the solution simulations, the T - G1 relative solvation free energy takes a value of -0.55 +/- 0.95 kcal/mol. Computer modeling shows that a close chloride ion. largely modifies the solution structure in the immediate vicinity of protonated dopamine. The effect is different for the gauche and the trans conformers and leads, to a decrease of the solvation preference for the trans form. Although the DopH(+...)Cl(-) ion pair separated by a single water molecule is not favored in the bulk aqueous solution, such arrangement is possible in more restricted regions, e.g., in a receptor cavity or when passing lipid membranes. At such places one could expect an increase in the G conformer over the T form at pH = 7.4 and T = 310 K as compared to the G:T ratio found in D(2)O solution of dopamine at pH = 7.