Ureas buffering action

A typical procedure involves adding a suitable amount of urea to an acidified solution of, e.g., aluminum ions. A weak acid, such as formic or acetic acid, is also added to provide buffering action so that the pH does not rise too rapidly as the urea hydrolyzes. The solution is then heated until the pH rises to the desired value, which is 4 for quantitative precipitation of hydrated aluminum oxide. Homogenously precipitated hydrated aluminum oxide is superior to that precipitated by addition of ammonia in that it is purei denser, more compact, and more easily filtered. Some reagents used in PFHS are listed in Table 2. 

Urea, ranging from 5 to 9 M, is the most common choice of chaotropic agent, and disruption of hydrogen and hydrophobic bonding between proteins is the main mode of action. When used in conjunction with thiourea (from 0 to 2 M), the two can aid solubilization of poorly soluble samples. The use of urea or urea-thiourea mixtures in SD buffers has no effect on the charge of the proteins within the sample. This is ideal for use in separating proteins according to their p in the first... 

Fig. 9. Experimental shift of the activity-external pH curve due to the feed-back action of pH-active products, (a) The product is a base immobilized urease (0-0) urease in stirred solution (x-x) substrate urea, product (NH4)2 CO3 (From Sel gny et al. [40].) (b) papain immobilized in a collodion membrane (J-J) papain in stirred solution ( ) substrate benzoyl-L-arginine ethyl ester / iC00R2->i iC00"+H++i 20H (From Goldman et al. [53] and Katchalski et al. In both cases concentrated buffer or grinding to powder of membranes strongly decreases the differences observed between soluble and insoluble enzymes. 

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