17 March 2000 Ž . Chemical Physics Letters 319 2000 391–395 www.elsevier.nlrlocatercplett Intra ion–ion interactions in aqueous solution: b-alanine zwitterion and 1,2-ethenediammonium Ky-Youb Nam a,b , Jeong Hyeok Yoon b , Kyoung Tai No a,b, ),1 a Department of Chemistry, Soong Sil UniÕersity, Seoul 156-743, South Korea b CAMD Research Center, Soong Sil UniÕersity, Seoul 156-743, South Korea Received 24 May 1999; in final form 13 January 2000 Abstract To illuminate the behavior of ion pairs in aqueous solution, both ab initio SCRF MO calculation and MD simulation were performed. The free energy difference of each system was decomposed into contributing components in order to investigate the role of the water in aqueous solution. In aqueous solution, the dominant component contributing to the ion pair stability, is not a solute–solute interaction, but an ion pair–water interaction and the energy change that originated from the rearrangement of the water molecules around the ion pair. q 2000 Elsevier Science B.V. All rights reserved. 1. Introductions Electrostatic interactions play an important role in determining the structure and function of proteins w x 1–3 . Charged groups located on the exterior of the protein domain are crucial for expressing its surface properties, for example, biological activities as re- flected in active sites. Charged groups located in the interior of the protein domain are also important for understanding protein folding and stabilization of protein conformation. It was proposed that the reor- ganization of the polar groups around charge pairs is ) Corresponding author. Department of Chemistry, Soong Sil University, Seoul, 156-743, South Korea. Fax: q82-2-825-1745; e-mail: camdrc@camd.soongsil.ac.kr 1 Member of Center for Molecular Science, Korea. crucial in explaining the charge–charge interaction wx in proteins 4 . Since ionic and polar amino acid residues are responsible for enzyme activities and the residues around the active site control the strength of the activities, it is necessary to investigate the stabil- ity of ion–ion or polar–polar groups interactions systematically, in accord with its environments. wx Hendsch and Tidor 5 calculate the effect of salt bridges on the stability of proteins with a continuum electrostatic model. According to the results, all the salt bridges they studied were electrostatically desta- bilizing proteins by a substantial amount, ; 2.5–6.0 kcalrmol. wx In our previous work 6 , we investigated the stability of like- and oppositely-charged ion pairs, methyl ammonium, methyl acetate, and guanidinium, w x in aqueous solution with PCM 7–9 HF ab initio MO calculations. The attractive interaction between oppositely-charged ion pairs decreases, even becom- 0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž . PII: S0009-2614 00 00084-1