Info

OH- (equivalents)

OH- (equivalents)

FIGURE 3-10 Titration of an amino acid. Shown here is the titration curve of 0.1 m glycine at 25 °C. The ionic species predominating at key points in the titration are shown above the graph. The shaded boxes, centered at about pK1 = 2.34 and pK2 = 9.60, indicate the regions of greatest buffering power.

From the titration curve of glycine we can derive several important pieces of information. First, it gives a quantitative measure of the p^a of each of the two ionizing groups: 2.34 for the —COOH group and 9.60 for the —NH+ group. Note that the carboxyl group of glycine is over 100 times more acidic (more easily ionized) than the carboxyl group of acetic acid, which, as we saw in Chapter 2, has a p^a of 4.76—about average for a carboxyl group attached to an otherwise unsub-stituted aliphatic hydrocarbon. The perturbed p^a of glycine is caused by repulsion between the departing proton and the nearby positively charged amino group on the a-carbon atom, as described in Figure 3-11. The opposite charges on the resulting zwitterion are stabilizing, nudging the equilibrium farther to the right. Similarly, the p^a of the amino group in glycine is perturbed downward relative to the average p^a of an amino group. This effect is due partly to the electronegative oxygen atoms in the carboxyl groups, which tend to pull electrons toward them, increasing the tendency of the amino group to give up a proton. Hence, the a-amino group has a p^a that is lower than that of an aliphatic amine such as methylamine (Fig. 3-11). In short, the p^a of any functional group is greatly affected by its chemical environment, a phenomenon sometimes exploited in the active sites of enzymes to promote exquisitely adapted reaction mechanisms that depend on the perturbed p^a values of proton donor/acceptor groups of specific residues.

How To Bolster Your Immune System

How To Bolster Your Immune System

All Natural Immune Boosters Proven To Fight Infection, Disease And More. Discover A Natural, Safe Effective Way To Boost Your Immune System Using Ingredients From Your Kitchen Cupboard. The only common sense, no holds barred guide to hit the market today no gimmicks, no pills, just old fashioned common sense remedies to cure colds, influenza, viral infections and more.

Get My Free Audio Book


Post a comment