Acid-base balance case study

Experimental alkaptonuria has also been produced in rats on a diet de-ficent in sulfur-containing amino acids (295). Similar excretory patterns were produced after additional phenylalanine, tyrosine, or their corresponding keto acids, and the condition was relieved on gi dng cysteine, but not ascorbic acid (644). Moreover the p-hydroxyphenylpyruvate excretion was much lower, relative to the homogentisic acid excretion, than in the type of ascorbate-dependent alkaptonuria studied by Sealock in the guinea pig (rats cannot in any case be made ascorbic acid-deficient). Xeuberger and Webster (644) also showed that this second type of experimental alkaptonuria could be produced in many types of amino acid imbalance, or in protein deficiency, and that the threshold intake of phenylalanine or tyrosine required to produce the condition varied with the nutritional state and also with the acid-base balance, acid urines being associated with a decreased homogentisic acid excretion (cf. also 150, 273, 787). 

Another case where the test reaction has been used to correlate catalytic behavior and physicochemical properties, is the interesting vanadium-containing system studied by Trikalitis and Pomonis (1995). They studied the transformation of isopropanol over Lai -xS VOb. This solid contains vanadium in at least two oxidation states, Vm and VIv. The systematic variation of the Sr content modifies the acid-base balance on the surface... 

Due to their better biomimetic properties, phospholipids have been proposed as an alternative to 1-octanol for lipophiiicity studies. The use of immobilized artificial membranes (lAM) in lipophiiicity determination was recently reviewed and we thus only briefly summarize the main conclusions [108]. lAM phases are silica-based columns with phospholipids bounded covalently. lAM are based on phosphatidylcholine (PC) linked to a silica propylamine surface. Most lipophiiicity studies with lAM were carried out using an aqueous mobile phase with pH values from 7.0 to 7.4 (log D measurements). Therefore, tested compounds were neutral, totally or partially ionized in these conditions. It was shown that the lipophiiicity parameters obtained on I AM stationary phases and the partition coefficients in 1-octanol/water system were governed by different balance of intermolecular interactions [109]. Therefore the relationships between log kiAM and log Poet varied with the class of compounds studied [110]. However, it was shown that, for neutral compounds with log Poet > 1, a correspondence existed between the two parameters when double-chain lAM phases (i.e., lAM.PC.MG and IAM.PC.DD2) were used [111]. In contrast, in the case of ionized compounds, retention on lAM columns and partitioning in 1 -octanol / water system were significantly different due to ionic interactions expressed in lAM retention but not in 1-octanol/water system and due to acidic and basic compounds behaving differently in these two systems. 

Table 11 shows some representative results from the cathodic reduction of some aromatic hydrocarbons. These include cases with Ei j2 near the cathodic limit or in the discharge region of the SSE (benzene, toluene) and cases with Ex j2 at considerably more positive potential (naphthalene, anthracene again we must anticipate the discussion of reactivity and refer to Table 21). Reactions nos. 1, 2, 6, and 7 immediately demonstrate one difficulty with such studies in that the catholyte of a divided cell becomes strongly basic as electrolysis progresses. In sufficiently basic medium, the initial product, a 1,4-dihydro derivative (cf. the Birch reduction Birch and Subba Rao, 1972), will rearrange to a conjugated system which, in contrast to the 1,4-dihydro derivative, is further reducible to the tetrahydro product (nos. 1 and 6). In a non-divided cell the acid production at the anode balances the base production and thus only a little rearrangement occurs. It is therefore not a trivial problem to find out if the tetrahydro product is formed from the conjugated dihydro product, formed directly or by rearrangement [eqn (78)].

Diesters of phosphinic acids have also received attention. When both phosphonic and phosphinic ester moieties are present in the same substrate molecule, the question is naturally raised as to which of the two phosphinoyl moieties will be expelled and which will remain attached to carbon. Perhaps the simplest system to be studied is also the most recently reported. Thus, in reactions between 171 and aliphatic aldehydes or benzaldehyde in diethyl ether with MgBr2 and Et3N, it is the phosphonate moiety which is preferentially eliminated to leave the methylphosphinic esters 1V although with other bases such as NaH, KOBu or BuLi, all in thf, the balance of this reaction and that giving the alternative 173 is more equitable. For the slightly more complex case of 174 with RCHO (R = Ph or Pr ), the preferential fission occurs to liberate the phosphinate moiety... 

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