In a peptide, the amino acids are written from left to right with the
The left hand amino acid is called the N-terminus, and the right hand amino acid is called the C-terminus.
(b) Charged groups at pH 7
There are three points to remember:
#"pH = p"K_a#, the neutral and ionic forms are present in equal amounts.
#"pH < p"K_a#(i.e. more acidic), the protonated form will predominate.
#"pH > p"K_a#(i.e. more basic), the non-protonated form will predominate.
Both Met (methionine) and Thr (threonine) have neutral R-groups, and they are neither N-terminal or C-terminal amino acids, so we can ignore them in our calculations.
Glu (glutamic acid) has both a basic N-terminus and an acidic R-group.
The red line marks the pH 7 division mark.
For the R-group of Glu,
However, for the N-terminal group of Glu,
Thus, there are four ionized groups at pH 7.
(c) Net charge at pH 1
At pH 1, all groups have
For the C-terminal group of Gly and the R-group of Glu, the protonated forms are
For the N-terminal group of Glu and the R-group of Arg, the protonated forms are
Thus, at pH 1 the net charge is +2.
(d) Sequence using one-letter symbols
The one-letter symbols for the amino acids are