The amino acid containing an indole part is

Which among the following amino acids has the lowest molar mass?

Consider the following sequence for aspartic acid:
$H_3N^+-CH(COOH)-CH_2COOH \leftrightarrow{pK_1=1.88} H_3N^+-CH(COO^-)-CH_2COOH \leftrightarrow{pK_R=3.65} H_3N^+-CH(COO^-)-CH_2COO^- \leftrightarrow{pK_2=9.60} H_2N-CH(COO^-)-CH_2COO^-$
The $pI$ (isoelectric point) of aspartic acid is:

The general structural formula of $\alpha$-amino acid is $NH_2-CH(R)-COOH$. The group $R$ in tryptophan $(X)$ and histidine $(Y)$ are respectively:

The peptide bond is represented by:

In fibrous proteins,the polypeptide chains are held together by.......