$25\mathrm{.}40$ Draw a bond$-$line structure showing the zwitterionic form of each of the following amino acids:

$($a$)$ L$-$Valine

$($b$)$ L$-$Tryptophan

$($c$)$ L$-$Glutamine

$($d$)$ L$-$Proline

$25\mathrm{.}44$ Draw all stereoisomers of L$-$isoleucine. In each stereoisomer, assign the configuration $(R$ or $S)$ of all chiral centers.