3) Consider the structures of two amino acids, glutamate and lysine, in their neutral and ionized forms. HO O O- O NH2 NH3+ H2N COOH H2N COOH H2N COOH H2N COOH acid conjugate base base conjugate acid glutamate, pKa = 4.1 lysine, pKa = 10.5 At pH 13, which forms of glutamate and lysine should be prevalent? Briefly explain your answer.
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Amino acids [general formula $\mathrm{NH}_{2} \mathrm{CH}(\mathrm{R}) \mathrm{COOH} ]$ can be considered polyprotic acids. In many cases, the R group contains additional amine and carboxyl groups. (a) Can an amino acid dissolved in pure water have a protonated COOH group and an unprotonated $\mathrm{NH}_{2}$ group $\left(K_{\mathrm{a}} \text { of } \mathrm{COOH}\right.$ group $=4.47 \times 10^{-3} ; K_{\mathrm{b}}$ of $\mathrm{NH}_{2}$ group $=6.03 \times 10^{-5} ) ?$ Use glycine, $\mathrm{NH}_{2} \mathrm{CH}_{2} \mathrm{COOH}$ , to explain why. (b) Calculate $\left[^{+} \mathrm{NH}_{3} \mathrm{CH}_{2} \mathrm{COO}^{-}\right] /\left[\mathrm{NH}_{3} \mathrm{CH}_{2} \mathrm{COOH}\right]$ at pH 5.5 (c) The R group of lysine is $-\mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2}\left(\mathrm{p} K_{\mathrm{b}}=3.47\right)$ Draw the structure of lysine at pH 1 , physiological pH $(\sim 7),$ and $\mathrm{pH} 13$ (d) The R group of glutamic acid is $-\mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COOH}\left(\mathrm{p} K_{\mathrm{a}}=\right.$ 4.07$) .$ Of the forms of glutamic acid that are shown below, which predominates at $(1) \mathrm{pH} 1,(2)$ physiological pH $(\sim 7),$ and (3) $\mathrm{pH} 13 ?$
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