THEORETICAL STUDIES OF THE REACTIONS OF THE SULFUR-SULFUR BOND .2. BINDING AND REACTIONS WITH GROUP 1A AND 2A METALS

Ab initio model studies of the binding and reactions of disulfides with alkali and alkaline earth metals are reported. The model systems considered are H2S2Mn+ and H2OMn+ with Mn+ = Li+, Na+, K+, Be2+, Mg2+, and Ca2+. The monovalent metals were found to bind to one of the sulfur atoms, while the alkaline earth metals additionally may form bridged bonds between both S atoms. The mechanisms of the reactions of the metals with RSSR are studied, and the traditional electrophilic scission is predicted to be of limited interest compared to the concomitant electrophilic and nucleophilic S-S scission. The mechanism is thus more like a metal-assisted nucleophilic substitution reaction leading to S-S bond fission rather than a purely electrophilic cleavage as is commonly assumed. The alkali metals will probably not react appreciably with RSSR, neither in the gas phase nor in solution. For alkaline earth metals, especially Be2+, there are several reactions possible depending on the availability of nucleophiles, all leading to S-S bond fission in case of nucleophilic excess. Competition with H+ may require high pH in the reactions while competition between the disulfide and H2O for the metal most likely will prevent Ca2+ and to some extent Mg2+ from reacting in solution. Thus among the metals considered, those which are predicted not to react with S-S bonds to any extent are also those metals naturally present in living systems. On the other hand, Be2+ is the metal predicted to react most strongly with S-S bonds. This metal is also known to be very toxic. These results and suggestions have biological implications of interest. © 1979, American Chemical Society. All rights reserved.