J. Phys. Chem. 1994, 98, 10742-10749
10742
Determination of Proton Transfer Energies and Lattice Energies of Several Amino Acid Zwitterions Kyoung Tai No,*>~~S Kwang Hwi Cho,'?S Oh Young Kwon,' Mu Shik JhonJJ'and Harold A. Scheraga*9 Departments of Chemistry, Soong Si1 University, 1-1 Sang Do 5 Dong, Dong Jak Gu, Seoul 156-743, Korea, and Korea Advanced Institute of Science and Technology, 373-1 Kusung-dong, Yusung-gu, Taejon 305-701, Korea, and Baker Laboratory of Chemistry, Come11 University, Ithaca, New York 14853-1301 Received: May 30, 1994; In Final Form: August 9, I994@
The gas-phase proton transfer energies of several amino acids, Gly, Ala, Val, Ser, Thr, Cys, &His, €-His, and 4-OH-Pro were calculated with ab initio 6-31G** optimized geometries. For accurate descriptions of electrostatic interaction energies and the transferability of point charges from one molecule to another, a method for calculating empirical point charges, the Mulliken-Population-Constrained-Potential-Derived (MPCPD) method is proposed. The MPCPD point charges accurately reproduce ab initio electrostatic potentials and have good transferability. Lattice energies of molecular crystals of amino acids in the zwitterionic form were calculated from the proton transfer energies and heats of sublimation data. The interaction energies other than electrostatic energy which contribute to the lattice energies were also estimated using the electrostatic energies calculated from several point-charge sets.
Introduction Determination of the thermodynamic properties for proton transfer in amino acids is very important for understanding the activity of biological and for calculating lattice energies of amino acids using the Born-Haber cycle. However, proton transfer enthalpies of gas-phase amino acids are not known from experiment. Therefore, a number of quantum mechanical calculations have been reported for proton transfer in vacuo,8-12in water cluster environments, and in aqueous sol~tion.'~-*~ Heats of sublimation data and structural information from molecular crystals contain important information for investigating the interactions between molecules, especially if the geometries of gas-phase molecules do not change appreciably compared with those of the molecules in crystals. Kitaigorodskii21,22 developed potential energy functions for some relatively small organic molecules using equilibrium conditions. Momany et al.22,24and Hagler et al.25,26developed empirical potential energy functions for peptides using equilibrium conditions and heats of sublimation of molecular crystals which are good approximations of lattice energies. Most amino acids exist in zwitterionic form in aqueous solution and in crystals and in neutral form in the gas phase. Therefore, amino acid zwitterions in crystals are changed to the neutral form upon sublimation, and neutral gas-phase amino acids are changed to zwitterions upon solvation in water. For this reason, heats of sublimation of amino acid molecular crystals could not simply be converted into lattice energies by correcting for the difference in energy partitioned into molecular motions between the gas-phase molecule and the molecule in a crystal. For the development of accurate potential energy functions for amino acid zwitterions, it is necessary to have accurate
* To whom correspondence should be addressed.
' Member of the Center for Molecular Science, Korea.
* Soong Si1 University.
Cornell University. Korea Advanced Institute of Science and Technology. @Abstractpublished in Advance ACS Abstracts, September 15, 1994.
estimations of lattice energies. For this purpose, Voogd et aL9 carried out extensive computations of the proton transfer energies for several amino acids and of the contributions of electrostatic energies to the lattice energies of amino acid molecular crystals. Molecular orbital calculations were carried out with CND0/2 and ab initio methods with several basis sets, minimal, split, double-5, and polarized double-
