Promoters, water molecules

The collapse of the proteolytic tetrahedral intermediate of the promoted-water pathway requires a proton donor in order to facilitate the departure of the leaving amino group. Rees and Lipscomb (1982) considered Glu-270, but favored Tyr-248 for this role, but Monzingo and Matthews (1984) fully elaborated on a role for Glu-270 of carboxypeptidase A and Glu-143 of thermolysin as intermediate proton donors. This proposal for carboxypeptidase A is corroborated by the near-normal activity observed for the Tyr-248- Phe mutant of rat carboxypeptidase A (Garden et al, 1985 Hilvert et al, 1986) and is reflected in the mechanistic scheme of Fig. 31 (Christianson and Lipscomb, 1989). Mock (1975) considered Glu-270 a proton donor in the carboxypeptidase A mechanism, but his mechanism does not favor a Glu-270/zinc-promoted water molecule as the hydrolytic nucleophile. Schepartz and Breslow (1987) observed that Glu-270 may mediate an additional proton transfer in the generation of the Pi product carboxylate. 

A combination of the promoting effects of Lewis acids and water is a logical next step. However, to say the least, water has not been a very popular medium for Lewis-acid catalysed Diels-Alder reactions, which is not surprising since water molecules interact strongly with Lewis-acidic and the Lewis-basic atoms of the reacting system. In 1994, when the research described in this thesis was initiated, only one example of Lewis-acid catalysis of a Diels-Alder reaction in water was published Lubineau and co-workers employed lanthanide triflates as a catalyst for the Diels-Alder reaction of glyoxylate to a relatively unreactive diene . No comparison was made between the process in water and in organic solvents. 

Some people think that 48% aq, HBr is good enough and, until recently, some underground chemistry texts agreed. But it is not The 48% is only strong enough to promote what is called acid catalyzed hydration (don t ask) when the medium it is in is water. That means that a water molecule (an OH) will add instead of a Br. 

Human growth hormone is a very complex molecule biologically. Several diverse biological activities such as anaboHc, insulin-like, diabetogenic, and lactogenic activities have been ascribed to hGH, which also appears to promote water and salt retention. An in-depth discussion of these activities may be found in several excellent reviews available in the Hterature (34—36). 

In this case water is effectively acting as a catalyst for the reaction by lowering the energy of activation. These catalytic water molecules are more likely to participate in the reaction under supercritical conditions because their high compressibility promotes the formation of solute-solvent clusters. 

Surfactant blends of interest will exhibit clouding phenomena in aqueous solutions undergoing a phase transition from a one phase system to a two phase system at a discrete and characteristic temperature, referred to as the Cloud Point (CP). This value indicates the temperature at which sufficient dehydration of the oxyethylene portion of the surfactant molecule has occurred and this results in its "displacement" from solution. The addition of lyotropic salts will depress the CP, presumably due to the promotion of localised ordering of water molecules near the hydrophilic sheath of the surfactant molecule (8). Furthermore, the addition of different oils to surfactant solutions can induce either an elevation or a depression of the recorded CP and can be used to qualitatively predict the PIT (8x9). 

The effect of crystal field splitting is easily seen by studying the absorption spectrum of [Ti(H20)6]3+ because the Ti3+ ion has a single electron in the 3d orbitals. In the octahedral field produced by the six water molecules, the 3d orbitals are split in energy as shown in Figure 17.3. The only transition possible is promotion of the electron from an orbital in the t2g set to one in the eg set. This transition... 

Irradiation of Ti02 results in an electron being promoted to the CB and an electron from a water molecule being withdrawn into the low-lying vacancy in the Ti02 VB ... 

Similarly, irradiation of GaP promotes an electron to the CB which is then passed to a water molecule ... 

In this case, the reaction of water molecules with palladium complexes is promoted by heat, so no addition of soda is required. The global chemical reaction is expressed by... 

---