Let's solve the question step-by-step: ### **Step 1: Understanding the Problem** Fisherine is an amino acid with an ionizable R-group (side chain) that is **acidic**, with a pKa = 4.. - pKa of the **amino group** = 9.5 (will be +1 when protonated, when deprotonated) - pKa of the **carboxyl group** = 2.5 (will be when protonated, -1 when deprotonated) - pKa of the **R-group** = 4. (acidic, so when protonated, -1 when deprotonated) At different pH values, the groups will be protonated or deprotonated based on whether the pH is below or above their respective pKa values. ### **Step 2: Calculate Net Charge at Each pH** #### **a) pH 1.** - Carboxyl group (pKa 2.5): pH < pKa → **protonated ()** - Amino group (pKa 9.5): pH < pKa → **protonated (+1)** - R-group (pKa 4.): pH < pKa → **protonated ()** - **Net charge: + 1 + = +1** #### **b) pH 4.** - Carboxyl group (pKa 2.5): pH > pKa → **deprotonated (-1)** - Amino group (pKa 9.5): pH < pKa → **protonated (+1)** - R-group (pKa 4.): pH = pKa → about 50% deprotonated, but typically considered **halfway**, so for net charge, usually round to **average** between and -1, but for simplicity, let's use **-.5** - **Net charge: -1 + 1 + (-.5) = -.5** #### **c) pH 7.** - Carboxyl group (pKa 2.5): pH > pKa → **deprotonated (-1)** - Amino group (pKa 9.5): pH < pKa → **protonated (+1)** - R-group (pKa 4.): pH > pKa → **deprotonated (-1)** - **Net charge: -1 + 1 + (-1) = -1** #### **d) pH 11.5** - Carboxyl group (pKa 2.5): pH > pKa → **deprotonated (-1)** - Amino group (pKa 9.5): pH > pKa → **deprotonated ()** - R-group (pKa 4.): pH > pKa → **deprotonated (-1)** - **Net charge: -1 + + (-1) = -2** --- ### **Step 3: Isoelectric Point (pI)** The isoelectric point is the pH at which the amino acid has a net charge of zero. - At low pH: net charge is +1. - At high pH: net charge is -2. - At pH 4.: net charge is -.5. - At pH 7.: net charge is -1. The net charge passes through zero between pH 1. (+1) and pH 4. (-.5). The pI for an acidic amino acid is the average of the two lowest pKa values (carboxyl and R-group): \[ pI = \frac{pK_a(\text{carboxyl}) + pK_a(\text{R-group})}{2} = \frac{2.5 + 4.}{2} = 3.25 \] --- ### **Step 4: Similar Standard Amino Acid** The structure is most similar to **Aspartic Acid** or **Glutamic Acid**, which also have an acidic side chain. --- ### **Final Answers** a) pH 1.: Net charge = **+1** b) pH 4.: Net charge = **-.5** c) pH 7.: Net charge = **-1** d) pH 11.5: Net charge = **-2** e) Isoelectric point (pI): **3.25** f) Fisherine is most similar to: **Aspartic Acid** (or Glutamic Acid)