Proton displacements

Table 1 Fitted parameters, chemical shifts and proton displacements ...

Proposed role of Zn2+ in carbonic anhydrase. Carbonic anhydrase catalyzes the reaction H20 + C02, HC03 + H+. In the enzyme, Zn2+ forms a complex with H20. Proton displacement generates an OH- ion still bound to the Zn2+. Nucleophilic attack of C02 by the OH- ion generates bicarbonate. 

Cory and Garroway [13] introduced the NMR pulsed gradient stimulated echo method to study compartments which are too small to be observed by conventional NMR imaging. They showed so-called proton displacement profiles of bulk water and dimethyl sulfoxide. The displacements are due to free diffusion and are Gaussian shaped. The profile of water in yeast cells showed restricted diffusion with a characteristic cell width of approximately 5 /xm. 

Figure 9 Possible protonation schemes of tris(catecholate) metal complexes. In path 1, the metal complex undergoes a series of two overlapping one-proton steps to generate a mixed salicylate-catecholate coordination. Further protonation results in the precipitation of a tris(salicylate) complex (e.g. enterobactin, MECAM). This differs from path 2, in which a single two-proton step dissociates one arm of the ligand to form a bis(catecholate) chelate. Path 3 incorporates features of paths 1 and 2. In this model, the metal again imdergoes a series of two overlapping one-proton reactions. However, unlike the case of path 1, the second proton displaces a catecholate arm, which results in a bis(catecholate) metal complex...

Table 6.8 Binding energies, H-bond lengths, and proton displacements of anions, computed at MP4/6-311+G(d,p) level. ...

In the D—[H]—A systems of Sections 4.4.1 to 4.4.9, little charge redistribution occurs within the interface upon electron transfer. For instance, in the symmetric dicarboxylic acid interfaces of Section 4.4.1, proton displacement from one side of... 

Fig. 22. A binary mixture of 2/3DN30C10 and DAP(Bn)22+ behaves in a manner analogous to an XNOR gate. In the absence of acid or base, a strong complex with pseudorotaxane geometry is formed, a Upon addition of trifluoroacetic acid (A), by virtue of the formation of a strong complex with the crown ether, protons displace the DAP(Bn)22+ from the cavity of 2/3DN30C10. b Neutralization of the solution with n-butylamine (B) results in the reformation of the pseudorotaxane. c If the initial mixture is treated with base, a strong interaction between n-butylamine and DAP(Bn)22+ leads to the destruction of the complex, d Neutralization of the solution with TFA leads to a simple acid-base reaction, and pseudorotaxane formation. If the two inputs are acid and base (A and B), with a 1 signaling presence and a 0 absence, this system behaves as an XNOR gate, if pseudorotaxane formation is represented by an output of 1...

This area was recently reviewed by Arnaut and Formosinho [12,13]. This chapter is strongly biased by personal experience and taste and will be concerned with some aspects of only PT reactions in which the proton displacement takes place by tunneling. The main emphasis will be on investigations by high-resolution optical spectroscopic methods, as developed in our and other laboratories. These reactions are frequently fully reversible and this will be the case for all reactions discussed here. General overviews of the field of low-temperature chemical dynamics can be found in several recent books and reviews [14,15]. 

The reorientation of partially deuterated rotors, as discussed in Section IV.A.5, is an example of proton displacements between positions that are distinguishable. Even when these positions are equivalent in the isolated molecule, the interaction with an environment of lower symmetry usually makes the positions inequivalent so that the PES becomes asymmetric. This situation applies also for proton translations in intra- and intermolecular PT processes, examples of which are discussed below (Sections IV.B.l and IV.B.2). Prototype examples for intramolecular transfers are the tautomer-ization of free base porphyrines and phthalocyanines and the PT in malonaldehyde, tropolone, 9-hydroxyphenalone, and so on. The tautomer-ization of carboxylic dimers is the best studied example of intermolecular transfer in a (near) symmetric PES. 

Zaromb (I6J) suggested that the formation or motion of ions by proton displacements may be coupled to rotations of neighboring molecules leading to the formation of valence defects. 

---