OH ch3 CH (24) A consequence of the existence of I could be its collapse with perchlorate ion to produce a covalent ester. The stability of this ester is anticipated to be very low owing to the leaving-group characteristics of perchlorate ion and because the ester would be tertiary. Even methyl perchlorate has verylimited stability in aqueous solution: D. N. Kevill and G. M. L. Lin, Tetrahedron Lett., 949-952 (1978). The NMR spectra of our reaction solutions do not show any resonances in addition to pinacolone and pinacol. If such an ester is formed, it must be below the detectable level.pivotal role in this mechanism in that it can be transformed to products by two paths. One path (path a) is the spontaneous ring opening to a carbonium ion; the other (path b) is an SN2-like process involving a water molecule. A change in mechanism from path b to path a is observable in solutions with high salt content. Greater transition-state stabilization due to ionic aggregation may make path a more favorable than path b. It is also possible that the acidity dependence of path a may be greater than that for path b.25 In either case, an effect of this nature observable in water is striking. It is analogous to the effects found in acetic acid and in aqueous acetone solvolysis studies where the ion-pair mechanistic schemes were first developed. Most studies in aqueous media, however, have not been designed to exceed the "limiting laws" for solution behavior, and thus have not uncovered unusual effects of salts as has been done here. Even with the knowledge that salt effects of this type could be observed the choice of the substrate is probably a critical factor in revealing them. Tetramethylethylene oxide is very unusual in that its modes of reaction lie so near the mechanistic demarcation between SN1 and SN2 processes.The phenomenon of mechanistic switching as explored here is very interesting and is under active investigation with a number of epoxides. Whether the epoxide ring itself imparts intrinsic sensitivity to the reaction environment or whether the extent of substitution engenders such sensitivity are questions currently being addressed.(25) We are indebted to the referees for directing our attention to the possibility that a differential acidity dependence may be the origin of the mechanistic switch.

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The x-ray structures of 20 proteins have been examined and each of the residues in these proteins was assigned to the inside or outside of the molecules and to a conformational state. The data obtained confirm that polar groups are generally found on the outside of proteins and nonpolar residues are generally found on the inside. Seven of the amino acids (Ala, Arg, Cys, His, Pro, Ser, Tyr) have inside/outside preferences which are not consistent with their usual assignment as either polar or nonpolar residues; explanations are given for these apparent inconsistencies. Of the three types of backbone structure considered here (extended, alpha helix, and nonregular), extended structures have the greatest preference for the inside of proteins, and nonregular structures have the greatest preference for the outside. It is suggested that differences in entropy play an important part in the inside/outside preferences of backbone structures. There are generally significant changes in the conformational preferences of the residues in going from the inside to the outside of proteins; environmental (rather than local) solute-solvent interactions seem to be the predominant cause of these changes in conformational preferences.

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