Interpret the peptide structure

A peptide sequence is written from N-terminus to C-terminus.

So, we must identify the amino acid residues in the given tetrapeptide from left to right as:

• first residue = N-terminal amino acid
• last residue = C-terminal amino acid

Decode the one-letter symbols provided

The relevant one-letter codes are:

• $F$ = Phenylalanine
• $L$ = Leucine
• $D$ = Aspartic acid
• $Y$ = Tyrosine
• $I$ = Isoleucine
• $Q$ = Glutamine
• $P$ = Proline

Read the tetrapeptide residues

From the given tetrapeptide structure, the residues in order are:

• 1st residue: Phenylalanine $(F)$
• 2nd residue: Leucine $(L)$
• 3rd residue: Aspartic acid $(D)$
• 4th residue: Tyrosine $(Y)$

Hence the peptide sequence is: $\boxed{FLDY}$

Check options

• A: YQLF $\rightarrow$ incorrect
• B: FIQY $\rightarrow$ incorrect
• C: PLDY $\rightarrow$ incorrect
• D: FLDY $\rightarrow$ correct

Comparison with stored answer

Our derived answer is D: FLDY, which matches the stored correct answer.