Q.35 The absorbance of a 5 × 10−4 M solution of tyrosine at 280 nm wavelength is 0.75.
The path length of the cuvette is 1 cm.
The molar absorption coefficient at the given wavelength in M−1cm−1,
correct to the nearest integer, is _____.
Molar Absorptivity of Tyrosine at 280 nm: Detailed Calculation Guide
The molar absorptivity (ε) of tyrosine at 280 nm is calculated using Beer’s Law, where absorbance (A = 0.75), concentration (c = 5 × 10-4 M), and path length (l = 1 cm) are given. Rearranging Beer’s Law ε = A / (c × l) yields ε = 0.75 / (5 × 10-4 × 1) = 1500 M-1 cm-1, rounded to the nearest integer.
Beer’s Law Explanation
Beer’s Law (A = ε c l) relates absorbance to concentration and path length, with ε as the constant molar absorptivity specific to the solute and wavelength. For tyrosine, aromatic rings absorb UV at 280 nm, primarily from phenolic groups. Substituting values confirms ε = 1500 M-1 cm-1, matching standard literature values of 1490–1500 M-1 cm-1 for tyrosine.
Step-by-Step Solution
- Absorbance A = 0.75
- Concentration c = 5 × 10-4 M
- Path length l = 1 cm
- ε = A / (c l) = 0.75 / (0.0005 × 1) = 1500 M-1 cm-1
This direct calculation aligns with protein spectroscopy where tyrosine contributes ~1490 M-1 cm-1 per residue at 280 nm.
Verification with Literature
Standard ε for tyrosine at 280 nm is consistently reported as 1490 or 1500 M-1 cm-1, derived from amino acid analysis methods like Gill and von Hippel. Variations (e.g., 1280 or 1450) occur due to solvent or measurement conditions, but 1500 fits neutral aqueous solutions best.
Proteins and amino acids like tyrosine absorb UV light at 280 nm due to aromatic residues, making molar absorptivity tyrosine 280 nm a key parameter in biochemistry for concentration determination. This guide solves a CSIR NET-style problem: absorbance of 0.75 for a 5×10-4 M tyrosine solution in a 1 cm cuvette, finding ε to the nearest integer.
Why 280 nm for Tyrosine?
Tyrosine’s phenolic ring causes strong absorption near 280 nm, with ε ≈ 1490–1500 M-1 cm-1 in water. Tryptophan dominates at higher values (~5500), but isolated tyrosine matches this query. Beer’s Law ensures precise quantification in spectrophotometry.
Detailed Calculation Using Beer’s Law
Apply A = ε c l:
| Parameter | Value | Unit |
|---|---|---|
| Absorbance (A) | 0.75 | – |
| Concentration (c) | 5 × 10-4 | M |
| Path length (l) | 1 | cm |
| ε (calculated) | 1500 | M-1 cm-1 |
ε = 0.75 / (5×10-4 × 1) = 1500 M-1 cm-1. This rounds exactly, confirming the answer.
Common Pitfalls and Options Explained
- Option-like checks: If ε were 1490 (literature exact), expected A = 1490 × 5×10-4 × 1 ≈ 0.745 (close to 0.75). 1280 gives A ≈ 0.64; 1450 gives 0.725—1500 fits best.
- Units: M-1 cm-1 standard; forget l=1 cm and ε=150000 (error).
- CSIR NET tip: Verify with protein formulas ε = (nY × 1490) + …, but here it’s pure tyrosine.
Applications in Life Sciences
Used in protein assays (A280), enzyme kinetics, and biotech. For CSIR NET, master alongside NADH ε=6220 at 340 nm. Accurate ε ensures reliable quantification without dyes.
