Positron annihilation lifetime measurements on aqueous solutions of sodium dodecylsulfate (NaLS) were performed over a wide range of temperatures and surfactant concentrations. The sensitivity of the positronium formation process toward changes in physical properties accompanying micellization is demonstrated. The critical micelle concentration (CMC) is associated with a sharp decrease in positronium formation. The CMC of 8.4±O.5 mM at 20"C in this system is in agreement with literature values. At higher temperatures the CMC is shifted to slightly higher surfactant concentrations (iO.4±O.7 mM at 90 "C). The formation probability for thermal positronium as expressed by 1 2 , the intensity of the long· lived component in the lifetime spectra, gradually decreases above the CMC with increasing surfactant concentration reaching a limiting value. A model is suggested involving the reactions of "hot" positrons or positronium atoms with micellar aggregates leading to the observed reduction of thermalized positronium atoms.

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references cited therein; (d) H. Basch, Theor. Chim. Acta, 28, 151 (1973). We thank Professor C. Trindle (University of Virginia) for this reference.(3) For reviews, see (a) G. L. Closs in Top. Stereochem., 3, 193 (1968); (b) P.

The structural features of carbonic anhydrase (carbonate hydro-lyase; EC 4.2.1.1) in aqueous solution were probed by the positron annihilation technique. The data obtained under varying conditions of temperature, pH, and enzyme concentration were interpreted in terms of the free volume model. The change of enzymic activity with temperature is accompanied by a change in free volume of the protein. Upon thermal denaturation an irreversible change in free volume of the molecule occurred. At low temperatures the protein-water interactions were investigated. These results are discussed in terms of current concepts of structure-function relationships in proteins. This study shows the sensitivity of the positron annihilation method toward the structure of proteins related to their overall conformation and to the nature of bound water.

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