Subject protonation states

Protonation State. At optimal pH for enzymatic activity ( 5-6) [101, 102, 105], the Asp dyad can in principle exist in three protonation states, a deprotonated, a mono-protonated or a doubly protonated form. Because hydrogen atoms cffe invisible in the X-ray structure, evidence for a specific protonation state must be inferred indirectly by spectroscopic or titration measurements. Up to now, the existence of the doubly protonated, neutral form hfree enzyme. The existence of the deprotonated, doubly negative form is supported by a recent NMR study [102] at pH 6. However, this study has been subjected to criticism [106] and it is not conclusive. Our ab initio simulations of this form show that the Asp dyad is unstable even in the ps timesccde because of the strong Asp-Asp repulsion, which turns out to be -t-30 kcal/mol as estimated with a simple electrostatic model [100]). Thus, our calculations do not support the existence of this form. 

J(PQ) and J(PN)). Trends in J(P170) have been used to study the protonation of phosphoryl tribromide, phosphoryl trifluoride and dif1uorophosphoric acid. Nitric acid had a greater protonation power than Hammett acidity functions indicated.77 This coupling has been the subject of a semiempirical sum over states theoretical study.7 ... 

In a subject with the breadth of solid state electrochemistry, and with the inevitable constraints on the length of a textbook, some topics have been omitted. In particular, less emphasis on proton conduction has been given than on the transport of other ions, in part because of the existence of a recent book on this topic in the same series. Proton Conductors, Ed. Philippe Colomban, Cambridge University Press (1992). Also the important topic of intercalation into graphite has been largely omitted. Several excellent texts on this subject are already available. 

The a-methylbenzyl cation (1) can be approached from the alcohol dehydration direction or the alkene protonation direction, as shown, and both of these processes have been the subject of ab initio molecular orbital calculations. It was found that the alcohol dehydration has a transition state about half way between the two stmctures shown, with the transition state and the carbocation having about the same amount of 7T-orbital overlap. However, the alkene protonation has an earlier transition state with less effective 7r-orbital overlap than that in the cation. This is held to explain the different Yukawa-Tsuno r+ values found for the two processes, 0.7-1.1 for alkene... 

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