Protonation and Hydrogen Exchange

Partial rate factors for exchange for a number of substituted aromatic compounds have been measiued. They reveal activation of ortho and para positions by electronreleasing groups. Some typical data are given in Table 10.8. The oi/ nz aroimd 

Data on hydrogen exchange have been of particular interest for comparison with theoretical predictions of aromatic reactivity. Because the electrophile is well defined and small, calculation of the stability of various competing intermediates by 

MO methods is feasible. We will return to the comparison of exchange data with theoretical predictions in Section 9.5. 

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The number of NMR parameters available for measurement is rather small, consisting of the chemical shift, relaxation rates (/1 and lo), scalar (J) couplings, dipolar (D) couplings, cross-relaxation rates (the NOE), and hydrogen exchange rates. All of these have been quantified for many of the amide protons of A131 A, and most of the data suggest the presence of little persistent structure. 

Amide proton temperature coefficients and hydrogen exchange rates can provide information about hydrogen-bonding interactions and solvent sequestration in unfolded or partly folded proteins (Dyson and Wright, 1991). Abnormally low temperature coefficients, relative to random coil values, are a clear indication of local structure and interactions. 

Roder, H. (1989). Structural characterization of protein folding intermediates by proton magnetic resonance and hydrogen exchange. In Methods in Enzymology (N. J. Oppen-heimer and T. L. James, eds.), Vol. 176, pp. 446-473. Academic Press, Orlando, FL. 

The distinction between proton and hydrogen atom (i.c., proton plus electron) transfer is clear-cut for the intermolecular transfer of one proton or hydrogen atom, when either the ionic or radical character of the reaction partners is changed. In the case of intramolecular transfers, however, the rearrangement of the electronic structure allows for partial charge compensations, and this distinction may become arbitrary (see. e g., Michl and Bonacic [2], p. 405). This is also true for the exchange of two protons between two molecules. 

Such fast rates at these low temperatures are not expected if only the isomerization mechanism is assumed to be responsible for the formation of prelumirhodopsin at least at low temperatures. Another mechanism, therefore, becomes feasible because of the very fast and constant rate of bathorhodopsin formation at low temperatures. This mechanism is proton translocation. To decide if indeed isomerization at the chromophore skeleton or proton translocation is the phenomenon observed, the rhodopsin was immersed in DjO and the normal procedure followed for substitution of all exchangeable protons with deuterium. Since it is well known that by the hydrogen on the retinal chromophore skeleton do not exchange, however, other rhodopsin protons including the proton near the Schiff base are exchanged for deuterium, the place where isomerization should occur is not altered. Therefore, if what we observe is isomerization, no noticeable effect in the rates of the formation between protonated and deuterium exchanged rhodopsin should be observed. However, a very strong effect is observed. 

Some areas which are not covered are isotope effects on proton and deuterium exchange with solvent, for example, the water-hydronium ion system (Saunders et al., 1984), deuterium isotope effects on acid and base strength (Halevi et al., 1979), on amino acids (Petersen and Led, 1979) and on hydration of cobalt (II) (Saunders and Evilia, 1985). Solvent-dependent isotope effects on equilibria involving hydrogen bonds in carbohydrates and... 

Henchman, M. J., Smith, D. and Adams, N. G. Proton-Transfer Reactions, Hydrogen-Exchange Reactions, and Proton-Transfer / Hydrogen-Exchange Reactions, presented at the 28th ASMS Conference, New York City, 1980, paper WAMOC 13 Ausloos, P. and Lias, S. G. 1981, J. Am. Chem. Soc. 

Roder, H. (1989). Structural characterization of protein folding intermediates by proton magnetic resonance and hydrogen exchange, Methods Enzymol. 176,446-473. 

Hydrogen exchange, in thiazole, especially deuteration, has been quantitatively investigated (379,380), but the mechanism of the reaction carried out at acidic or neutral pH corresponds to a protonation-deprotonation process (380), different from electrophilic substitution and is discussed in section I.3.E. 

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