ISSN 1995-0780, Nanotechnologies in Russia, 2010, Vol. 5, Nos. 3–4, pp. 165–176. © Pleiades Publishing, Ltd., 2010. Original Russian Text © P.V. Komarov, V.G. Alekseev, S.D. Khizhnyak, M.M. Ovchinnikov, P.M. Pakhomov, 2010, published in Rossiiskie nanotekhnologii, 2010, Vol. 5, Nos. 3–4. 165 INTRODUCTION The study of binding aqueous solutions and organic solvents by adding low-molecular-weight thickeners [1] is one of the most urgent tasks in contemporary colloidal chemistry, supramolecular chemistry, and disordered systems physics. The interest in this topic arises mainly due to the fact that understanding the hierarchy of structure formation in these systems may provide the key to managing the self-organization pro- cesses of other molecular systems, which is important in the context of developing new technologies for pre- paring nanostructured materials by their self-assem- bling methods. An aqueous solution of cysteine and silver nitrate is a unique example of a nanodisperse system where thixotropic gel forms at an ambient temperature. Structure formation in the solution occurs even at a very low concentration of low-molecular-weight com- ponents of about ~0.01% [2]. This system is of interest from both the fundamental and applied viewpoint. In the first place, it is a relatively simple and inexpen- sively prepared model system for studying self-organi- zation processes in a dilute aqueous solution; sec- ondly, it can serve as a matrix for the design of different nanostructures and pharmaceuticals. The latter appli- cation is provided by the biological activity of the sys- tem due to the presence of silver ions showing antisep- tic properties. In spite of the fact that a range of substances (acids, certain electrolytes, polymers, etc.) showing a strong influence on gelation was determined through numer- ous experiments [2–8] and observed spatial structures were studied in detail, many processes resulting in the formation of a gel spatial structure remain unclear. We have previously enunciated several hypotheses on the mechanism of structure formation in a cysteine–silver solution (CSS). At the very beginning, when cysteine is mixed with silver nitrate excess, silver mercaptide (SM) molecules appear. This fact is confirmed by UV spectroscopy by the emergence of an absorption band at ~200 nm assigned to the –S–Ag group and by IR spectroscopy by the disappearance of the absorption band at 2544 cm –1 , which corresponds to the stretch- ing vibrations of the thiol group (–SH) in a cysteine molecule [2]. There is evidence, both theoretical and experimen- tal, to believe that SM molecules can associate to form –Ag–S···Ag–S– oligomeric chains. Indeed, sulfur atoms in an SM molecule have two lone pairs of elec- trons, and the electrons of one of the pairs can be shared by involving free orbitals of silver ion. Spectral data also indicate such a possibility. Thus, the UV spectrum of a very diluted aqueous solution of SM shows two absorption bands at 280 and 355 nm which are probably associated with the energetic spectrum of a shared electron system enclosed in a one-dimen- sional potential well. The formation of SM oligomers was previously discussed in the literature [9]. Oligo- meric chains of SM molecules associate at a concen- tration sufficient enough to form dense micelles or a precipitate. However, additional Ag + ions interact with oligo- meric chains if there is an excess of silver ions. One can Study of Silver Mercaptide Nanocluster Formation in Cystein–Silver Solution by Atomistic Molecular Dynamics Simulation P. V. Komarov a, b , V. G. Alekseev a , S. D. Khizhnyak a , M. M. Ovchinnikov c , and P. M. Pakhomov a a Tver State University, ul. Zhelyabova 33, Tver, 170100 Russia b Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, ul. Vavilova 28, Moscow, 119991 Russia c Tver State Medical Academy, ul. Sovetskaya 4, Tver, 170642 Russia e-mail: pv_komarov@mail.ru Received July 28, 2009 Abstract—The formation of nanosized silver mercaptide aggregates in a cysteine–silver solution (CSS) at the early stage of gelation was studied by atomistic molecular dynamics simulation. Low-molecular-weight ions , Ag + , and H 3 O + in the solution were taken into account in an explicit form by the designed model. The structure and UV absorption spectra of resultant complexes of silver mercaptide (SM) with silver ions were studied by ZINDO/1 quantum chemistry. DOI: 10.1134/S1995078010030031 NO 3 – ARTICLES