28. are questions with numerical answer.
The predicted molar extinction coefficient at 280 nm for the peptide
GEEFHISFLLIMFGAWSTHMYRTYWFIHEMISTRY is ____ M⁻¹cm⁻¹.
[Molar extinction coefficients for phenylalanine, tryptophan and tyrosine at 280 nm are 200,5600
and 1400M⁻¹cm⁻¹, respectively]

The predicted molar extinction coefficient at 280 nm for the peptide GEEFHISFLLIMFGAWSTHMYRTYWFIHEMISTRY is 17100 M⁻¹cm⁻¹, calculated using the provided values for aromatic amino acids.

Step-by-Step Calculation Method

Count the occurrences of phenylalanine (F), tryptophan (W), and tyrosine (Y) in the peptide sequence, as these contribute to absorbance at 280 nm. Phenylalanine appears 5 times, tryptophan 2 times, and tyrosine 2 times. Multiply each by its molar extinction coefficient (F: 200, W: 5600, Y: 1400 M⁻¹cm⁻¹) and sum: (5 × 200) + (2 × 5600) + (2 × 1400) = 1000 + 11200 + 2800 = 17100 M⁻¹cm⁻¹.

Why These Amino Acids Matter

Only F, W, and Y have significant absorbance at 280 nm due to their aromatic rings; other residues like histidine contribute negligibly per standard approximations. The provided values align with common biochemical references, where tryptophan dominates (5600 M⁻¹cm⁻¹), followed by tyrosine (1400 M⁻¹cm⁻¹) and phenylalanine (200 M⁻¹cm⁻¹). This additive method assumes no disulfide bonds or conformational effects altering ε.

Common Pitfalls in Prediction

Errors often arise from miscounting residues or using incorrect ε values (e.g., some sources list Trp as 5500 or 5690 M⁻¹cm⁻¹). Here, sticking to problem-specific values ensures precision; phenylalanine’s low ε (200) is sometimes overlooked despite multiple occurrences. Verification: sequence has exactly 5F, 2W, 2Y, confirming 17100 M⁻¹cm⁻¹.