react with complementary centers on the substrate. The model will also demonstrate the termolecular complex and, if vacancies are two layers thick, the compulsory order often found in the formation of a ternary complex; it is clearly seen how the binding of the "second" substrate will block that of the "first". The model also provides a simple demonstration of the conversion of binding energy into "strain" energy. When the active-site cleft has a shape or size not precisely complementary to that of the substrate the non-directional forces may caused the substrate to bind but in so doing become strained by the steric hindrance in the cleft and activated. Provided the model is composed of sufficient Styrofoam balls i t mav easilv be seen how the close-packing prevents entry of substrate molecules into the inteGor of inenzyme; the reason for active-sites as clefts a t the surface of enzymes then becomes self evident. The model is rigid as is the enzyme by virtue of its close-packing and the interatom bonds: in the former case glue and in the latter disulfide and peptide links. A detailed account of close-packing in enzymes and its consequences is available from the author.
Acetic Acld In Toluene: A Safer Conductivity Experiment John W. Hill University of Wisconsin River Falls, Wisconsin 54022 Many beginning laboratory courses involve the testing of a solution of dry hydrogen chloride in benzene for electrical conductivity. Distilled water is added, and the solution is tested again. The experiment can be faulted on two counts: (1)The benzene-HC1solution is difficult to prepare, store, and handle, and (2) benzene is quite toxic. A safer system involves glacial acetic acid and toluene. Neither reagent conducts electricity when dry, nor does amixture of the two. Addition of distilled water gives a two-layer solution. The lower (aqueous) layer readily conducts electricity. This svstem illustrates the same principles as the benz e n e - ~ ~ l s o l u t i oItn has . the advantages thatareticacid in lens irritatinz and easier u, handle than gaseous hydrogen chloride and that toluene is considerably leis toxic than benzene.
Volum 53, Number 12, December 1976 / 779
