PH dependent behavior

Figure la shows results for MMA/DMA gels with different comonomer ratios. Gels were swollen in buffered media with I = 0.1 M. As expected, swelling decreases as the content of MMA increases and of DMA decreases. This is explained by noticing that (1) the gel becomes more hydrophobic, and (2) the density of ionizable groups decreases. More interesting is the pH-dependent behavior. At neutral and alkaline pH only minimal ( < 10% w/w) water uptake... 

The envelope of signals below 0.00 ppm, although not sufficiently intense to identify any individual peaks, does not exhibit any pH-dependent behavior. This pattern is expected if these signals result from diester phos-... 

A polyelectrolyte can be defined as a macromolecule with ionizable repeat units, which can consequently display pH-dependent behavior when dissolved in aqueous solution. 

Although the magnitudes of /c at and /c cat differed significantly, the two pH-dependent rate curves (Figures la and lb) are symmetrical (with scales adjusted) with an inflection point at pH ca. 7.4, a value close to the p a value of 7.3. Thus, the macrocyclic complex (33) was the first to mimic the pH-dependent behavior of reversible CO2 hydration catalyzed by CA. This fact implies that in CA, too, the CO2 hydration/HCOs dehydration should be essentially controlled by the zinc(II)-OH /zinc(II)-OH2 equilibrium at the active center. 

The changing pH can dramatically alter the solubility properties of the organometallic compounds, which could result in unexpected or desired transfer to the organic phase. This could be demonstrated by the pH-dependent behavior of W(CO)4L (where L = benzoic acid, 3-[(2-pyridinylmethylene)amino]). The W-complex could be moved from the aqueous to the organic and back to the aqueous phases by the subsequent addition of HCl and NaOH 20 times without decomposition (Figure 18), because the protonated compound is soluble in aptotic polar solvents such as THF, DMF and the [W(CO)4L] Na" is soluble in water. 

Thermo-responsive polymers have been used to release encapsulated drugs in response to temperature changes. These polymers typically undergo a rapid and reversible hydration-dehydration at lower critical solution temperature (LCST, from soluble to insoluble) or upper critical solution temperature (UCST, from insoluble to soluble) upon heating. [71] For example, a thermo-responsive polymer, poly (2-isopropyl- 2-oxazoline) (PiPrOx), was conjugated to pH-responsive poly(benzyl ether)dendrons, which exhibited sharp thermal transitions as well as pH-dependent behaviors (Fig. 4a).[105]... 

The range of pH-dependent behavior for a redox couple will therefore be defined by the pAa values of the complex for the two oxidation states involved in that couple, e.g., the generalized M LH couple will be pH-dependent between pAa and pAa . As Figure 2 graphically illustrates, it is equivalent to say that the pH-dependent range is defined by A1/2 and Ai/2, the respective potentials for the protonated and deprotonated couples, since A1/2 —Ai/2 = AAi/2 = (59mV)ApAa for a one-electron/one-proton couple, where ApAa = pAa pAa . Therefore, the extent of this reciprocal and mutual influence between electron transfer and proton transfer is reflected equally in the differences between the individual pAa values and redox potentials. The relationship between pAa and oxidation state of the complex will depend on a number of factors,... 

Further derivatives of PDMAEMA were synthesized in order to improve its condensation ability at physiological pH, such as a comb-type polycation consisting of P(DMAEMA-co-PLL) (Fig. 7i) [144]. This copolymer possessed a pH-dependent behavior due to the presence of PDMAEMA (p a 7.5) and PEL (p a 10). The presence of the PEL segments prevented precipitation of the copolymer at pH > 7.5, as is the case for PDMAEMA homopolymer, and this comb-type polymer was capable of DNA condensatimi at pH 8 (as observed using circular dichroism). 

Although PARACEST agents are very good pH reporters due to their nature (modulation of the proton exchange rate via the pH of the solution), " " most often their pH-dependent behavior does not involve supramolecular interactions. Consequently, they are not discussed here. 

A very interesting pH-dependent behavior was observed for monolayers from polymers carrying oppositely charged moieties phenylboronic acid and tertiary amino groups [70]. In this case, condensed films form on pure water, which considerably expand at low pH due to the electrostatic repulsion between protonated ammonium cations. This electrostatic effect weakens with pH increase and is also demonstrated by decreased molecular areas. 

The same group investigated the pH-dependent behavior of the above polymers [80], and in particular the presence and characteristics of the dense and diffuse parts of the hydrophilic layer. Interestingly, the monolayer nanostructure was similar at acidic and neutral conditions however, at basic pH, long PMAA blocks did not reveal the carpet or brush structures anymore, due to strong dissociation of weakly acidic hydrophilic blocks and sodium condensation. 

A similar effect on PAA hydrophilic blocks takes place when PVP is present in the subphase at low pH [83]. This particular pH-dependent behavior was further applied to obtain solid films with pH-controlled permeability to small molecules [84]. Membranes were prepared from ABA-type polyelectrolytes containing PAA middle blocks and PSS side blocks with the addition of small cationic amphiphiles. The membranes possessed pH-switchable properties towards hydrophilic molecules, whereas transport of hydrophobic species was pH-independent. This result is explained by pH-dependent disturbance of cationic amphiphile monolayers by the polyion (formation of pores ), the conformation of which is strongly pH-dependent. 

Prochazka K, Matejicek P, Uchman M, Stepanek M, Humpolickova J, Hof M, Sptrkova M (2008) pH-dependent behavior of hydrophobically modified polyelectrolyte shells of polymeric nanoparticles. Macromol Symp 273 95-102... 

The system was applied to monitor the radical-scavenging reaction for a number of AOXs at different pHs and reaction times. The TEAC values were found to be dependent on these parameters for almost all studied compounds (Table 31.1). Only Tr and Asc reacted rapidly (99% < 10 s) with ABTS +, and their antioxidative capacity was not affected by the end-point time. Structurally similar compounds had the same pH-dependent behavior even if they differed significantly in TEAC values, for example, the TEAC values for (-l-)-catechin and (-)-epicatechin, caffeic acid, and ferulic acid. The same was observed for quercetin and its glycoside rutin the higher activity of quercetin had been reported [74]. Independent of time and pH effects, quercetin, gallic acid, (+)-catechin, and (-)-epicatechin have shown higher activities. 

It has been known for a long time that drifts occur when the pH is measured in suspensions. With continuous titrations, the question then arises of whether this drift is due to some long-term proton or hydroxyl reaction at the surface or within the particles or whether it is caused by electrode kinetics or some other phenomena, such as intrusion of carbon dioxide into the reaction vessel, causing a drift either by its pH-dependent behavior in the bulk solution of the suspension or (additionally) by pH-dependent reactions with the surface. 

Stevens, M. M., AUen, S., Sakata, J. K. et al. 2004. pH-dependent behavior of surface-immobilized artificial leucine zipper proteins. Langmuir, 20 7747-52. 

The pH-sensitive hydrogels have been most frequently used to develop controlled release formulations for oral administration. The pH in the stomach (<3) is quite different from the neutral pH in the intestine, and such a difference is large enough to elicit pH-dependent behavior of polyelectrolyte hydrogels [95]. These hydrogels have been investigated in a number of therapeutic oral delivery systems either as... 

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