UV-Vis Spectrophotometric Quantitation of Protein Concentration & Molar Extinction Coefficient (ε₂₈₀)
Absorption of near-ultraviolet light by proteins at $lambda = 280text{ nm}$ arises almost exclusively from electronic transitions within the conjugated aromatic side chains of Tryptophan ($text{Trp}, text{W}$) and Tyrosine ($text{Tyr}, text{Y}$) residues, with a minor contribution from disulfide bonds (cystine, $text{C-C}$). Phenylalanine exhibits absorption maxima at shorter wavelengths ($257text{ nm}$) with negligible molar absorptivity at $280text{ nm}$.
According to the Beer-Lambert Law, light absorbance ($A$) is directly proportional to the molar absorption coefficient ($epsilon$, $text{M}^{-1}text{cm}^{-1}$), the analyte molar concentration ($c$, $text{M}$), and the optical pathlength ($l$, $text{cm}$):
$$A_{280} = -log_{10}left(frac{I}{I_0}right) = epsilon_{280} cdot c cdot l$$
For any folded or denatured protein of known primary sequence, the theoretical molar extinction coefficient at $280text{ nm}$ is accurately predicted using the Pace / Edelhoch equation in $6.0text{ M Guanidine Hydrochloride}$:
$$epsilon_{280} = (N_{text{Trp}} times 5500) + (N_{text{Tyr}} times 1490) + (N_{text{Cystine}} times 125) quad (text{M}^{-1}text{cm}^{-1})$$
Nucleic Acid Purity Evaluation: Purine and pyrimidine bases in DNA and RNA absorb maximally at $260text{ nm}$ ($pi to pi^*$ transitions) with an extinction coefficient nearly 50-fold higher per unit mass than proteins. A pure protein solution exhibits an $A_{280}/A_{260}$ ratio of $1.75text{ to }1.85$. An $A_{280}/A_{260}$ ratio below $1.5$ indicates significant nucleotide or nucleic acid contamination, requiring baseline correction formulas (e.g., Warburg-Christian equation: $text{Protein (mg/mL)} = 1.55 cdot A_{280} – 0.76 cdot A_{260}$).
- Spectrophotometer: Shimadzu UV-2600 or Thermo Scientific Evolution 300 dual-beam UV-Vis spectrophotometer with deuterium ($190–350text{ nm}$) and tungsten-halogen lamps.
- Cuvettes: Precision matched synthetic suprasil quartz cuvettes ($1.000text{ cm} pm 0.005text{ cm}$ pathlength, $1.5text{ mL}$ semi-micro volume).
- Protein Standards: Highly purified lyophilized Bovine Serum Albumin (BSA, $ge 98%$, Fraction V, Sigma-Aldrich A7906, $M_r = 66,430text{ Da}$).
- Assay Buffer: $50text{ mM Sodium Phosphate}$, $150text{ mM NaCl}$, $text{pH } 7.4$ (PBS), vacuum-filtered through $0.22 mutext{m}$ PES membrane.
- Denaturing Buffer (for Edelhoch validation): $6.0text{ M Guanidinium Hydrochloride}$ in $20text{ mM Sodium Phosphate}$, $text{pH } 6.5$.
Power on the spectrophotometer and allow deuterium lamp to warm up for 30 minutes to achieve photometric baseline stability (≤ 0.0004 Abs/h drift). Load both reference and sample cuvette holders with matched quartz cells containing pure PBS buffer. Perform baseline scan over $220–340text{ nm}$ with a $1.0text{ nm}$ spectral bandwidth and medium scan speed ($240text{ nm/min}$). Auto-zero at $280text{ nm}$.
Accurately weigh $10.0text{ mg}$ of BSA on an analytical balance ($pm 0.1text{ mg}$) and dissolve in $10.0text{ mL}$ PBS to generate a $1.00text{ mg/mL}$ primary stock solution ($15.05 mutext{M}$). Serially dilute the stock in PBS to produce 6 working standards: $0.10$, $0.25$, $0.50$, $0.75$, $1.00$, and $1.50text{ mg/mL}$. Prepare an unknown recombinant protein sample aliquot.
Transfer $1.0text{ mL}$ of each standard into the sample quartz cuvette. Ensure outside optical windows are wiped meticulously with lens paper. Record full spectrum from $220text{ nm}$ to $340text{ nm}$. Record discrete absorbances at $A_{260}$, $A_{280}$, and $A_{340}$ (light-scattering turbidity baseline correction). Subtract $A_{340}$ from both $A_{280}$ and $A_{260}$ to yield true absorption: $A_{280,text{corr}} = A_{280} – A_{340}$.
Dilute $100 mutext{L}$ of protein stock into $900 mutext{L}$ of $6.0text{ M Guanidine-HCl}$ buffer to fully unfold tertiary structure, exposing all Trp and Tyr residues to solvent. Equilibrate for 15 minutes at $25^circtext{C}$ and measure $A_{280}$. Calculate observed experimental $epsilon_{280}$ and compare against the Pace theoretical sequence calculation.
| Standard Sample | Nominal Conc (mg/mL) | Molar Conc (μM) | A₂₈₀ (Gross) | A₃₄₀ (Scatter) | A₂₈₀ (Corr) | A₂₈₀ / A₂₆₀ |
|---|---|---|---|---|---|---|
| Blank (PBS) | 0.00 | 0.00 | 0.000 | 0.000 | 0.000 | — |
| BSA Std 1 | 0.10 | 1.51 | 0.068 | 0.001 | 0.067 | 1.81 |
| BSA Std 2 | 0.25 | 3.76 | 0.169 | 0.002 | 0.167 | 1.82 |
| BSA Std 3 | 0.50 | 7.53 | 0.336 | 0.002 | 0.334 | 1.80 |
| BSA Std 4 | 0.75 | 11.29 | 0.503 | 0.003 | 0.500 | 1.81 |
| BSA Std 5 | 1.00 | 15.05 | 0.672 | 0.004 | 0.668 | 1.82 |
| Unknown Sample | — | 8.18 μM | 0.366 | 0.003 | 0.363 | 1.82 (Pure) |
Regression Analysis: Linear regression of $A_{280,text{corr}}$ vs Molar Concentration yields:
$$A_{280} = (44,380text{ M}^{-1}text{cm}^{-1}) cdot c + 0.0003 quad (R^2 = 0.9998)$$
Extinction Coefficient Verification: For BSA ($2text{ Trp}, 20text{ Tyr}, 17text{ Disulfides}$), theoretical Pace $epsilon_{280} = (2 times 5500) + (20 times 1490) + (17 times 125) = 11,000 + 29,800 + 2,125 = mathbf{42,925text{ M}^{-1}text{cm}^{-1}}$ (or $E^{1%}_{1text{cm}} = 6.46$). The observed native $epsilon_{280} = 44,380text{ M}^{-1}text{cm}^{-1}$ reflects local hydrophobic burial and microenvironment dipole enhancement of Trp residues in native folded BSA.
