The predicted molar extinction coefficient at 280 nm for the peptide GEEFHISFLLIMFGAWSTHMYRTYWFIHEMISTRY is _________ M$^{-1}$cm$^{-1}$ [Molar extinction coefficients for phenylalanine, tryptophan and tyrosine at 280 nm are 200, 5600 and 1400 M$^{-1}$cm$^{-1}$, respectively]
Predicting Peptide Molar Extinction Coefficient at 280 nm
The molar extinction coefficient ($ \epsilon_{280} $) of a peptide at 280 nm is calculated based on the sum of contributions from its aromatic amino acid residues: Tryptophan (W), Tyrosine (Y), and Phenylalanine (F).
Step 1: Count Aromatic Residues
Identify and count the occurrences of W, Y, and F in the peptide sequence GEEFHISFLLIMFGAWSTHMYRTYWFIHEMISTRY:
- Tryptophan (W): 2
- Tyrosine (Y): 2
- Phenylalanine (F): 3
Step 2: Note Given Molar Extinction Coefficients
The molar extinction coefficients ($ \epsilon $) at 280 nm for these residues are:
- $ \epsilon_F = 200 \, \text{M}^{-1}\text{cm}^{-1} $
- $ \epsilon_W = 5600 \, \text{M}^{-1}\text{cm}^{-1} $
- $ \epsilon_Y = 1400 \, \text{M}^{-1}\text{cm}^{-1} $
Step 3: Calculate Total Molar Extinction Coefficient
The total molar extinction coefficient is the sum of the products of the counts and their respective coefficients:
$ \epsilon_{peptide} = (\text{Count}_W \times \epsilon_W) + (\text{Count}_Y \times \epsilon_Y) + (\text{Count}_F \times \epsilon_F) $
$ \epsilon_{peptide} = (2 \times 5600 \, \text{M}^{-1}\text{cm}^{-1}) + (2 \times 1400 \, \text{M}^{-1}\text{cm}^{-1}) + (3 \times 200 \, \text{M}^{-1}\text{cm}^{-1}) $
$ \epsilon_{peptide} = 11200 + 2800 + 600 \, \text{M}^{-1}\text{cm}^{-1} $
$ \epsilon_{peptide} = 14600 \, \text{M}^{-1}\text{cm}^{-1} $
The calculated molar extinction coefficient for the peptide is 14600 M$^{-1}$cm$^{-1}$.
Important Questions from Enzyme Assays Molar Extinction Coefficient
- A solution shows a transmittance of 20% when taken in a cuvette of 2.5 cm path length. If the molar absorption coefficient of the solution is $12000 \text{ dm}^3/\text{mol.cm}$, the concentration of the solution is ________ $\times 10^5 \text{ mol/dm}^3$ (rounded off to two decimal places).
- A solution containing GTP has molar extinction coefficient of $1.55 \times 10^4$ $mol^{-1}dm^3cm^{-1}$ at a given wavelength. The concentration of GTP solution is $1.290 \times 10^{-5}$ $mol$ $dm^{-3}$. The absorbance of GTP solution in 1 cm cuvette at the same wavelength will be .................
- An enzyme preparation has activity of 2 Units per 20 $\mu$l, and protein concentration 0.4 mg/ml. The specific activity (Units/mg) of this enzyme will be ________
- Measurement of the absorbance of a solution containing NADH in a path length of 1cm cuvette at 340 nm shows the value of 0.31. The molar extinction coefficient of NADH is $6200 M^{-1} cm^{-1}$. The concentration of NADH in the solution is ________ $\mu M$ (correct to integer number).
- If a $10$ mM solution of a biomolecule in a cuvette of path length $10$ mm absorbs $90\%$ of the incident light at $280$ nm, the molar extinction coefficient of the biomolecule at this wavelength is ________ $M^{-1}cm^{-1}$. (Round off to two decimal places)
