PH memory effect

Dousova et al. [142] found that calcined Mg/Al LDHs were effective in removal of As (V) compounds from aqueous solutions at 293 K and neutral pH utilizing the memory effect . More than 70 % of As (V) compoimds were removed from aqueous solution at low sorbent-solution ratios. Parida et al. also studied the affinity of Mg/Fe LDHs toward the removal of inorganic selenite (SeOs ) from aqueous media [143]. The results indicated that the efficacy of SeOs removal increases with a decrease in either pH or temperature. 

While enzymes, as a rule, essentially lose their normal activity and specificity, they possess new useful features 1) utilization of substrates non-soluble in water 2) their ability to change substrate and inhibitor selectivity and specificity 3) they alternate of reactions thermodynamics and kinetics reactions so that desirable products are favoured 4) improvements of enzyme stability and 5) the possibility to fix enzymes and reaction intermediates at states of certain pH and ionic strength in both solution and crystal form ( molecular memory effects ). 

A new area of research is the control of pH for nonaqueous enzyme reactions using organic pH buffers. - These buffer materials, which are soluble in nonaqueous media, strongly control the effect of pH on the reaction, and are able to override the pH memory of the enzyme before lyophilization or immobilization from an aqueous solution. - Typically, buffer salts employed are oppositely charged (e.g., / -COOH and / -COONa+, where R is hydrophobic in nature). It is the ratio of these two forms that controls the enzymatic rate. The buffers are believed to function by displacing hydrogen atoms of carboxylic acid groups on the surface of the enzyme, . - ... 

Neumann [2] first used the term molecular hysteresis to describe the phenomenon which was hysteresis on precipitation and dissolution of the polymers in pH changes of aqueous solutions. However, because segments of the molecules interact, as do molecules in condensed form, the hysteresis he describes is still a collective phenomenon. Frieden [3] proposed the idea of a hysteretic enzyme in 1970 and Hand and Carpenter [4] reported that phosphofructokinase, an important regulatory enzyme, could be a hysteretic enzyme. The concept is of a simple mechanism consisting of two parts fast binding between the enzyme and a substrate, and a slow conformational change in the complex enzyme. The resulting conformation is retained for a considerable time, behavior that constitutes a memory effect but not hysteresis. 

It is well known that pH has a major influence on the behavior of enzymes in aqueous media. Most who use enzymes under low-water conditions are aware of the phenomenon known as pH memory . The activity and other properties of the enzyme are affected by the pH of the last aqueous phase to which it was exposed before drying for use in low-water conditions. This phenomenon is usually attributed to the relative rigidity of enzyme molecules at low hydration, by analogy with the effects of co-drying with additives, (see Sect. 8.2.5)... 

Ion mobility spectrometers with hydrated proton ion chemistry is exquisitely sensitive to ammonia, and two teams have sought to engineer IMS-based analyzers for ammonia in water." ° " In the earlier development," ammonia was thermally purged through a silicone membrane, and the limit of detection in this approach was 1.2 mg/L. The membrane was free of memory effects, and pH was controllable and permitted determination of ammonium ion concentrations. The system was engineered to avoid biofouling. 

There are a number of techniques that have been successfully applied to synthesize modified hydrotalcites (48). The most commonly method used is the co-precipitation of two metal salts in alkaline solution at a constant pH value of about 10. Another method uses the classical ion exchange process in which the guest anions are exchanged with the anions in the interlayer spaces of preformed layered double hydroxides to produce specific anion intercalated modified hydrotalcites. StiU another method is a lattice reconstruction after heating, i.e., calcination, which is based on the structural memory effect of these materials, due to which the original structure is reproduced after rehydration. 

---