1. Write the tetrapeptide structure in terms of ionizable groups

The tetrapeptide is: $\text{Gly-Glu-Asp-Tyr}$

In a peptide, the groups that can ionize are:

• N-terminal amino group of Gly
• C-terminal carboxyl group of Tyr
• Side chain of Glu: $-CH_2CH_2COOH$
• Side chain of Asp: $-CH_2COOH$
• Side chain of Tyr: phenolic $-OH$

The peptide bond amide groups are not ionized at this pH.

2. Check the state of each ionizable group at pH 12.5

At very high pH, acidic groups lose protons and become negatively charged.

(i) N-terminal amino group

Typical form: $-NH_3^+ \rightleftharpoons -NH_2 + H^+$ At pH $12.5$, it will be mostly: $-NH_2$ So, charge = 0.

(ii) C-terminal carboxyl group

$-COOH \rightarrow -COO^-$ So, charge = -1.

(iii) Glu side-chain carboxyl group

$-COOH \rightarrow -COO^-$ So, charge = -1.

(iv) Asp side-chain carboxyl group

$-COOH \rightarrow -COO^-$ So, charge = -1.

(v) Tyr side-chain phenolic group

Phenolic OH has $pK_a \approx 10$. At pH $12.5$, it is deprotonated: $-OH \rightarrow -O^-$ So, charge = -1.

3. Total negative charge

Now add all negative charges: $(-1) + (-1) + (-1) + (-1) = -4$

So the total number of negative charges is: $4$

4. Final answer

The total number of negative charges in the tetrapeptide at pH $12.5$ is: $\boxed{4}$

5. Comparison with stored correct answer

Stored correct answer = $4$

Our derived answer = $4$

Hence, they agree.