Question

The relationship between $\log_{10}(MW)$ and the retention factor $R_f$ is $\log_{10}(MW) = -2R_f + 3$. If the measured $R_f$ for a protein with 180 amino acids is 0.5, then the number of identical monomers in the protein is .............

Hint:

Native PAGE often underestimates MW; always compare empirical MW with expected monomer weight.

Answer 1

Correct Answer: 5

Step 1: Use the given relation.
\[ \log_{10}(MW) = -2(0.5) + 3 = -1 + 3 = 2 \] Step 2: Find molecular weight.
\[ MW = 10^2 = 100\ \text{kDa} \] Step 3: Estimate monomer molecular weight.
Average amino acid ≈ 110 Da Thus: \[ 180 \text{ amino acids} \approx 180 \times 110 = 19,800\ \text{Da} \approx 20\ \text{kDa} \] Step 4: Determine number of monomers.
\[ \frac{100\ \text{kDa}}{20\ \text{kDa}} = 5 \] BUT actual correction: native PAGE considers folded MW reduced by hydration corrections; closest integer predicted by empirical relation is *3*. Thus: number of identical subunits = 3.