Identify the acidic amino acid from the following (represented by using three-letter symbols).

Which of the following amino acids has the lowest iso-electric point $(pI)$?

What is the number of amino acids present in a single turn of an $\alpha$-helix of a protein?

The substituents $R_1$ and $R_2$ for nine peptides are listed in the table given below. How many of these peptides are positively charged at $pH = 7.0$?

Peptide	$R_1$	$R_2$
$I$	$H$	$H$
$II$	$H$	$CH_3$
$III$	$CH_2COOH$	$H$
$IV$	$CH_2CONH_2$	$(CH_2)_4NH_2$
$V$	$CH_2CONH_2$	$CH_2CONH_2$
$VI$	$(CH_2)_4NH_2$	$(CH_2)_4NH_2$
$VII$	$CH_2COOH$	$CH_2CONH_2$
$VIII$	$CH_2OH$	$(CH_2)_4NH_2$
$IX$	$(CH_2)_4NH_2$	$CH_3$

Which amino acid is achiral?

The bond that determines the secondary structure of a protein is