Exchange Capacity of the Cotton Fibers

The number of exchangeable hydroxyl groups (exchange capacity) of the cotton fibers was determined by shaking 0.18 g of cotton in 10 ml of a solution of 1 M NaOH for 24 h to ensure that a maximum of the cotton OH sites were exchanged (substitution of H+ by Na+). The cotton was then rinsed thoroughly to remove the excess NaOH. It was then shaken for 24 h in 10 ml of a solution of 1 N HCl to exchange Na+ ions with H+. The difference in pH (or difference in Na+ concentration) before and after exchange allowed us to calculate the number of active sites (or the exchange capacity) of the

Table 1. Quantities of enzyme in the different phases. Initial quantity of enzyme: 2 mg in 4 ml; m(cotton) = 0.1 g

Enzyme Solution after Washing Immobilized Yield (%)

contact (mg) solutions (mg) quantity (mg) Pepsin 0.0677 1.857 0.075 3.8 Trypsin_0.0235_1.889_0.087_4.4

Table 2. Enzyme exchange capacity of the cotton. m(cotton) = 0.1 g which corresponds to a surface of 5.5 x 1020 A ; molecular mass (MM) pepsin = 36,000 g/mol, MM trypsin = 23,000 g/mol


Number of enzyme molecules

Number of enzyme molecules

Yield (%)

fixed/g of cotton

fixed/Â2 of cotton


125.3 x 1014

2.28 x 10-6



228.8 x 1014

4.16 x 10-6


Pepsin Trypsin

Fig. 9. Areas taken up by a molecule of pepsin (left) and trypsin (right)

Pepsin Trypsin

Fig. 9. Areas taken up by a molecule of pepsin (left) and trypsin (right)

cotton fibers. The value found was 6.56x 1018 protons exchanged on 0.18 g of cotton, which corresponds to 6x 10-3 active sites/À2 (i.e., about 5 meq of protons/g of cotton), in turn, to an exchange capacity of 3.6 x 1019 protons/g of cotton (0.18 g of cotton used corresponds to a surface of 9.9x102°À2). This value was used for calculating the yield of enzyme fixation.

For enzyme immobilization we used 0.1 g cotton, which corresponds to 5.5x102°À2, taking into account the specific area of the cotton given above. Table 1 gives the different quantities of enzymes fixed on the cotton fibers, assuming that the specific activity of the immobilized enzyme was the same as that in solution. Table 2 gives the exchange capacity values obtained for both proteases calculated using the value of the specific area of cotton fibers given above. Pepsin and trypsin molecules occupy a mean area of 438,000 À2 and 240,000 À2, respectively (Fig. 9).

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