BIO 232 Study Notes: Protein Extraction, Bradford Assay, and Spectrophotometry

Functional and Structural Properties of Proteins

• Functional Diversity of Proteins: Proteins carry out nearly all cellular functions. They are categorized into nine principal functional classes:

  • Enzymes: Catalyze specific covalent bond breakage or formation reactions. Examples include tryptophan synthetase (makes the amino acid tryptophan), pepsin (degrades dietary proteins in the stomach), ribulose bisphosphate carboxylase (Rubisco, converts carbon dioxide into sugars in plants), DNA polymerase (copies DNA), alcohol dehydrogenase, and protein kinase (adds a phosphate group to a protein molecule).

  • Structural Proteins: Provide mechanical support to cells and biological tissues. Examples include collagen and elastin (common constituents of the extracellular matrix forming fibers in tendons and ligaments), tubulin (forms long, stiff intracellular microtubules), actin (forms filaments supporting the plasma membrane), and keratin (reinforces epithelial cells and forms the major protein of hair and horn).

  • Motor Proteins: Generate movement within cells and tissues. Examples include myosin (provides motive force for muscle movement), kinesin (interacts with microtubules to move organelles around the cell), dynein (enables eukaryotic cilia and flagella to beat), and flagellin.

  • Receptor Proteins: Detect extracellular signals and transmit them to the cell's internal response machinery. Examples include rhodopsin (detects light in the retina), acetylcholine receptor (activated by acetylcholine released from nerve endings at the membrane of muscle cells), insulin receptor (allows cells to respond to insulin by taking up glucose), and adrenergic receptor (increases heartbeat rate when bound to adrenaline).

  • Storage Proteins: Store amino acids or ions for later biological use. Examples include ferritin (stores iron in the liver by binding it), ovalbumin (egg white protein used as an amino acid source for the developing bird embryo), casein (milk protein providing amino acids for baby mammals), and endosperm proteins in plant seeds.

  • Transport Proteins: Carry small molecules or ions across biological membranes or throughout the circulatory system. Examples include serum albumin (carries lipids in the bloodstream), hemoglobin (carries oxygen in blood), transferrin (carries iron in blood), bacteriorhodopsin (light-activated proton pump transporting $\text{H}^+$ out of bacterial cells), glucose carriers (shuttle glucose into and out of cells), and $\text{Ca}^{2+}$ pumps (clear $\text{Ca}^{2+}$ from muscle cytosol after contraction).

  • Gene Regulatory Proteins: Bind directly to DNA sequences to switch specific genes on or off. Examples include the lactose repressor (silences genes for enzymes degrading lactose in bacteria), homeodomain proteins (act as genetic switches controlling development in multicellular organisms, including humans), and transcriptional activators.

  • Signal Proteins: Carry extracellular signals from one cell to another to coordinate physiological functions. Examples include insulin (small protein controlling blood glucose levels), netrin (attracts growing nerve cell axons to specific locations in the developing spinal cord), nerve growth factor (NGF, stimulates axon growth in specific nerve cells), epidermal growth factor (EGF, stimulates growth and division of epithelial cells), and stress response hormones.

  • Special-Purpose Proteins: Highly variable functional proteins that confer unique selective advantages. Examples include antifreeze proteins (protect blood of Arctic and Antarctic fishes against freezing), green fluorescent protein (GFP, emits green light in jellyfish), monellin (intensely sweet protein found in an African plant), and marine mussel glue proteins (attach organisms firmly to rocks underwater).

• Levels of Structural Organization: DNA specifies the linear amino acid sequence of proteins. Polypeptide chains fold into unique conformations determined by lowest energy states, establishing distinct structural and chemical properties across four hierarchical levels:

  • Primary Structure: The unique linear sequence of amino acids linked together covalently by peptide bonds.

  • Secondary Structure: Local structural folding stabilized by hydrogen bonds along the polypeptide backbone, predominantly forming $\alpha$-helices and $\beta$-pleated sheets.

  • Tertiary Structure: The complete three-dimensional folding of a single polypeptide chain, driven by side-chain interactions (hydrophobic interactions, ionic bonds, hydrogen bonds, disulfide bridges), often forming discrete globular functional domains.

  • Quaternary Structure: The stable spatial organization and non-covalent or covalent association of two or more individual polypeptide chains (subunits) into a multi-subunit protein complex.

• Protein Denaturation and Renaturation:

  • Definition: Denaturation is the structural alteration of a protein that leads to the loss of its secondary, tertiary, and quaternary structures, while leaving the primary amino acid sequence intact.

  • Denaturing Agents: Structural loss is caused by extreme temperatures (heat or radiation), pH changes, mechanical forces (sonication, shearing, or foaming at air/liquid interfaces), organic solvents, heavy salts, or chemical detergents such as sodium dodecyl sulfate (SDS).

  • Consequences: Results in structural unfolding, permanent loss of enzymatic activity, destruction of active/binding sites, loss of substrate specificity, and reduced protein solubility.

  • Renaturation: Certain globular proteins can spontaneously refold back into their native, biologically active tertiary conformation when returned to physiological conditions.

Principles of Protein Extraction and Buffer Chemistry

• Selection and Functions of Buffer Components: Preparing crude cell lysates requires preserving the native structure and biological activity of target proteins while suppressing cellular degradation factors.

  • Base Buffers: "Good buffers" (such as Tris-HCl or Phosphate buffers) maintain stable hydrogen ion concentration without interfering side effects or high metal binding.

  • Thiol Compounds: Protect protein sulfhydryl groups from oxidation and prevent unwanted disulfide bonding. Frequently added thiols include dithiothreitol (DTT) or $2$-mercaptoethanol ($\beta$-ME).

  • Chelating Agents: Bind free heavy metal ions to prevent heavy-metal-induced enzyme inactivation, protein aggregation/precipitation, substrate inhibition, or cleavage by metalloproteases. Ethylenediaminetetraacetic acid (EDTA, commonly disodium salt) is standard.

  • Monovalent and Divalent Cations: Added to preserve ionic strength ($\text{K}^+$, $\text{Na}^+$) or provide essential catalytic/structural stabilizing interactions ($\text{Mg}^{2+}$).

  • Substrates: Occasionally included to stabilize enzyme active sites against thermal denaturation.

  • Protease Inhibitors: Suppress endogenous proteolytic degradation. Phenylmethylsulfonyl fluoride (PMSF) irreversibly inactivates serine proteases.

  • Osmotically Active Solutes: Maintain solution tonicity equal to native tissue levels to prevent osmotic lysis or plasmolysis of fragile organelles (mitochondria, plastids). Examples include sucrose, sorbitol, and glycerol (or polyols), which also increase solution viscosity to stabilize tertiary structures.

  • Detergents: Solubilize cell membrane lipids and membrane-associated proteins. Triton X-100 is a non-ionic detergent typically added at concentrations of $0.1\% - 0.5\%$.

  • Polyvinylpolypyrrolidone (PVPP): Added at $2\% - 10\%\,\text{w/w}$ to plant extracts to bind alkaloids and polyphenolic compounds (flavonoids, tannins). PVPP prevents plant tissue "browning" and covalent or hydrogen-bonding modifications of peptide oxygens and amino acid residues.

• Extraction Physical Conditions:

  • Foaming Avoidance: Foaming introduces air/liquid interfaces that cause protein denaturation and must be avoided during homogenizing.

  • Temperature Control: Extractions are routinely conducted at $4^\circ\text{C}$ (or using frozen buffer slurries) to suppress proteolysis and unwanted chemical reactions.

• Cell Lysis Procedures:

  • Bacteria: Lysed using a French press (pressurizing cell suspensions to $7{,}000 - 10{,}000\,\text{psi}$ followed by rapid pressure release, generating liquid shear) or sonication (applying focused ultrasound waves to induce cavitation).

  • Eukaryotic Cells and Tissues: Cultured cells use hand-held or motor-driven homogenizers. Small plant tissues are ground in a mortar and pestle with liquid $\text{N}_2$. Fibrous tissues require blenders or polytron homogenizers with counter-rotating blades.

Quantitative Buffer Preparation Mechanics

• The Conservation of Mass Dilution Formula: Calculations for making stock solutions and target working buffers utilize:

\nc_1 V_1 = c_2 V_2\n

where $c_1$ is stock concentration, $V_1$ is stock volume, $c_2$ is final concentration, and $V_2$ is final working volume.

• Formulation of 5 mL Plant Protein Extraction Buffer: To prepare $5\,\text{mL}$ ($V_2$) of functional plant extraction buffer, components are calculated from concentrated liquid stocks as follows:

  • Tris-HCl (pH 8.0): Stock $c_1 = 0.25\,\text{M} = 250\,\text{mM}$, target $c_2 = 50\,\text{mM}$.

$V_1 = \frac{50\,\text{mM} \times 5\,\text{mL}}{250\,\text{mM}} = 1.0\,\text{mL} = 1000\,\mu\text{L}$

*   **$\text{MgCl}_2$**: Stock $c_1 = 200\,\text{mM}$, target $c_2 = 5\,\text{mM}$.

$V_1 = \frac{5\,\text{mM} \times 5\,\text{mL}}{200\,\text{mM}} = 0.125\,\text{mL} = 125\,\mu\text{L}$

*   **EDTA**: Stock $c_1 = 200\,\text{mM}$, target $c_2 = 1\,\text{mM}$.

$V_1 = \frac{1\,\text{mM} \times 5\,\text{mL}}{200\,\text{mM}} = 0.025\,\text{mL} = 25\,\mu\text{L}$

*   **$\beta$-mercaptoethanol ($\beta$-ME)**: Stock $c_1 = 14.3\,\text{M} = 14{,}300\,\text{mM}$, target $c_2 = 10\,\text{mM}$.

$V_1 = \frac{10\,\text{mM} \times 5\,\text{mL}}{14{,}300\,\text{mM}} \approx 0.0035\,\text{mL} = 3.5\,\mu\text{L}$

*   **Deionized Water ($\text{H}_2\text{O}$)**: Liquid stocks are added directly to deionized water to bring the total volume to $5\,\text{mL}$.

$V_{\text{water}} = 5.0\,\text{mL} - (1.0 + 0.125 + 0.025 + 0.0035)\,\text{mL} = 3.8465\,\text{mL} = 3846.5\,\mu\text{L}$

Methodologies of Protein Quantification

• Comparison of Major Analytical Protein Quantification Assays:

  • UV Absorbance ($280\,\text{nm}$): Direct photometric reading based on UV absorption by aromatic side chains (tryptophan, tyrosine).

    • Advantages: Simple, fast, non-destructive, requires no secondary assay reagents.
    • Disadvantages: Highly error-prone in complex protein mixtures, lysates, or samples containing nucleic acids.

  • Biuret Methods (BCA and Lowry Assays): Protein-copper chelation in alkaline solutions followed by secondary detection of reduced copper ($\text{Cu}^+$).

    • Advantages: Compatible with most detergents/surfactants, linear response curve ($R^2 > 0.95$), lower protein-to-protein variability than dye-binding assays.
    • Disadvantages: Incompatible with copper-reducing agents (DTT, $\beta$-ME) and chelating chemicals.
    • Representative Reagents: Bicinchoninic Acid (BCA) Assay, Lowry Assay.

  • Colorimetric Dye-Based Methods (Bradford Assay): Protein-dye binding followed by direct visual/photometric detection of color changes.

    • Advantages: Rapid, simple, room temperature, compatible with salts, solvents, buffers, thiols, reducing agents, and chelators.
    • Disadvantages: Incompatible with surfactants/detergents, higher protein-to-protein response variations.
    • Representative Reagents: Bradford Assay (Coomassie Blue G-250).

  • Fluorescent Dye Methods: Detection of fluorescence increase associated with dye binding to protein.

    • Advantages: Excellent sensitivity, requires minimal protein sample, adaptable for high-throughput automation.
    • Disadvantages: Requires specialized fluorometric instrumentation.
    • Representative Reagents: EZQ Fluorescent Assay, Qubit Protein Assay.

Bradford Protein Assay Protocol and Experimental Workflow

• Chemical Principle of the Bradford Assay: The assay utilizes Coomassie Brilliant Blue G-250 dye under acidic conditions. Unbound dye exists in a reddish-brown cationic state ($A_{\max} = 465\,\text{nm}$). Upon binding to basic amino acids (primarily arginine, lysine, histidine) and aromatic residues, the dye converts to a blue anionic form ($A_{\max} = 595\,\text{nm}$). Light absorbance at $595\,\text{nm}$ ($A_{595}$) is measured and is proportional to protein concentration within a linear working range of $0.05 - 0.5\,\text{mg/mL}$ ($2 - 1500\,\mu\text{g/mL}$).

• Exercise 5.1: Plant Homogenization and Extraction Protocol:

  1. Chill mortar and pestle on ice. Prepare $5\,\text{mL}$ extraction buffer in a $15\,\text{mL}$ tube on ice.
  2. Label two $1.5\,\text{mL}$ tubes CR (crude extract) and E.
  3. Weigh $0.5\,\text{g}$ of spinach leaf tissue (veins removed) and place in chilled mortar. Add $4\,\text{mL}$ extraction buffer and grind vigorously to homogenize.
  4. Transfer 1 drop of crude extract to tube CR using a transfer pipette. Add $50\,\mu\text{L}$ of $2\text{X}$ SDS dye (blue dye in green tube), close, store on ice.
  5. Transfer remaining crude extract to tube E up to the $1\,\text{mL}$ mark.
  6. Balance centrifuge rotor and spin tube E for $5\,\text{min}$ at maximum speed ($12{,}000\,\text{rpm}$).
  7. Transfer the clear supernatant into a new tube labeled S (Supernatant) using a P1000 pipette. Keep tube S on ice.
  8. Resuspend the pellet remaining in tube E in $200\,\mu\text{L}$ extraction buffer, flick to mix, incubate $5\,\text{min}$ on ice.
  9. Transfer $50\,\mu\text{L}$ supernatant from tube S to a new tube SN. Add $50\,\mu\text{L}$ $2\text{X}$ SDS dye, store on ice.
  10. Centrifuge tube E (pellet mixture) for $3\,\text{min}$ at $12{,}000\,\text{rpm}$. Transfer $50\,\mu\text{L}$ supernatant into new tube P (Pellet). Add $50\,\mu\text{L}$ $2\text{X}$ SDS dye.
  11. Heat tubes CR, SN, and P in a heating block at $90^\circ\text{C}$ for $5\,\text{min}$. Store at $-20^\circ\text{C}$ for future gel electrophoresis.

• Exercise 5.2: Bradford Microplate Setup and Quantification:

  • Standard Curve Preparation: Prepare Bovine Serum Albumin (BSA, stock $1\,\text{mg/mL}$) standards and extract dilutions (PE-1, PE-2, PE-3) in $1.5\,\text{mL}$ microfuge tubes to a final volume of $100\,\mu\text{L}$:

    • Tube 1 (Blank): $0\,\text{mg/mL}$ BSA ($0\,\mu\text{L}$ BSA + $100\,\mu\text{L}\,\text{H}_2\text{O}$)
    • Tube 2 (Standard 1): $0.05\,\text{mg/mL}$ BSA ($5\,\mu\text{L}$ BSA + $95\,\mu\text{L}\,\text{H}_2\text{O}$)
    • Tube 3 (Standard 2): $0.1\,\text{mg/mL}$ BSA ($10\,\mu\text{L}$ BSA + $90\,\mu\text{L}\,\text{H}_2\text{O}$)
    • Tube 4 (Standard 3): $0.2\,\text{mg/mL}$ BSA ($20\,\mu\text{L}$ BSA + $80\,\mu\text{L}\,\text{H}_2\text{O}$)
    • Tube 5 (Standard 4): $0.3\,\text{mg/mL}$ BSA ($30\,\mu\text{L}$ BSA + $70\,\mu\text{L}\,\text{H}_2\text{O}$)
    • Tube 6 (Standard 5): $0.4\,\text{mg/mL}$ BSA ($40\,\mu\text{L}$ BSA + $60\,\mu\text{L}\,\text{H}_2\text{O}$)
    • Tube 7 (PE-1): $2$-fold dilution ($50\,\mu\text{L}$ extract S + $50\,\mu\text{L}\,\text{H}_2\text{O}$)
    • Tube 8 (PE-2): $10$-fold dilution ($10\,\mu\text{L}$ extract S + $90\,\mu\text{L}\,\text{H}_2\text{O}$)
    • Tube 9 (PE-3): $20$-fold dilution ($5\,\mu\text{L}$ extract S + $95\,\mu\text{L}\,\text{H}_2\text{O}$)

  • Microplate Loading: Pipette samples into a 96-well microplate:

    • Well A1: $10\,\mu\text{L}$ Blank ($\text{H}_2\text{O}$)
    • Wells 2A–2C through 6A–6C: $10\,\mu\text{L}$ of Standards 1–5 in triplicates
    • Wells 7A–7C: $10\,\mu\text{L}$ of PE-1 ($2$-fold dilution) in triplicates
    • Wells 8A–8C: $10\,\mu\text{L}$ of PE-2 ($10$-fold dilution) in triplicates
    • Wells 9A–9C: $10\,\mu\text{L}$ of PE-3 ($20$-fold dilution) in triplicates
    • Add $190\,\mu\text{L}$ Bradford assay dye reagent to each occupied well using a P200 pipette.
    • Read absorbance at $595\,\text{nm}$ ($A_{595}$) on a microplate reader within $5 - 60\,\text{min}$.

• Data Analysis & Calculations:

  • Subtract Blank absorbance reading from all sample readings.

  • Calculate average $A_{595}$ for each triplicate set.

  • Plot Standard Curve ($X$ = BSA conc in $\text{mg/mL}$, $Y$ = average $A_{595}$).

  • Determine linear regression equation:

\ny = m x + b\n

*   Evaluate $R^2$ value (acceptable range: $0.90 - 1.00$).

*   Calculate unknown protein concentration ($x$) for each dilution:

\nx = \frac{y - b}{m}\n

*   Multiply calculated concentration $x$ by the corresponding dilution factor ($DF = 2$, $10$, or $20$) to determine the absolute protein concentration of the original extract.

Principles of Spectrophotometry and Optical Instrumentation

• Fundamental Principles: Spectrophotometry measures the quantitative absorption or transmission of light by a chemical substance across specific wavelengths. It comprises two functional systems: a spectrometer (produces, disperses, and isolates specific wavelengths of light) and a photometer (photoelectric detector measuring light intensity).

  • UV-Visible Spectrophotometry: Uses light across the ultraviolet range ($185 - 400\,\text{nm}$) and visible spectrum ($400 - 700\,\text{nm}$).

  • Infrared (IR) Spectrophotometry: Uses light across the infrared spectrum ($700 - 15{,}000\,\text{nm}$).

• Color Theory and Light Transmission: The visual appearance of a solution sample corresponds directly to transmitted light wavelengths:

  • A solution absorbing all visible wavelengths appears black.

  • A solution transmitting all visible wavelengths appears white/clear.

  • A solution absorbing light at $\sim 700\,\text{nm}$ (red) appears green because green is the complementary color of red.

• Component Architecture of a Spectrophotometer:

  • Light Source: Provides continuous polychromatic light radiation.

  • Collimator (Lens): Focuses light into parallel beams.

  • Monochromator (Prism or Grating): Disperses polychromatic light into its constituent wavelength spectrum.

  • Wavelength Selector (Slit): Isolates and transmits a narrow band of desired monochromatic light.

  • Cuvette / Sample Compartment: Holds liquid samples in a precise, standard light path length ($l = 1\,\text{cm}$).

  • Photoelectric Detector (Photometer): Converts transmitted photons ($I_t$) into an electrical current signal.

  • Digital Display / Meter: Outputs calculated transmittance or absorbance values.

• Absorbance Spectra and Isosbestic Points:

  • Para-nitrophenol (pNP): The acidic form of p-nitrophenol exhibits maximum absorption at $\sim 320\,\text{nm}$. The basic form (p-nitrophenolate anion) exhibits maximum absorption at $400\,\text{nm}$.

  • Isosbestic Point: A specific wavelength at which two or more chemical species in equilibrium exhibit identical molar extinction coefficients ($\varepsilon$). The sample's total absorbance remains constant at this wavelength regardless of reaction progress or pH shifts. The presence of a sharp isosbestic point indicates a simple two-component interconversion without intermediate accumulation.

Quantitative Photometric Laws and Calculations

• Transmittance Equation: Transmittance ($T$) represents the fraction of incident light transmitted through a sample:

\nT = \frac{I_t}{I_o}\n

where $I_o$ is incident light intensity before passing through the sample and $I_t$ is transmitted light intensity after passing through the sample.

• Absorbance Equation: Absorbance ($A$) represents the logarithm of the inverse of transmittance:

\nA = -\log_{10}(T) = -\log_{10}\left(\frac{I_t}{I_o}\right) = \log_{10}\left(\frac{I_o}{I_t}\right)\n

• The Beer-Lambert Law: Describes the linear relationship between absorbance and sample concentration:

\nA = \varepsilon c l\n

where $A$ is absorbance (dimensionless), $\varepsilon$ is molar extinction coefficient ($\text{L}\cdot\text{mol}^{-1}\cdot\text{cm}^{-1}$ or $\text{M}^{-1}\cdot\text{cm}^{-1}$), $c$ is molar concentration ($\text{mol/L}$ or $\text{M}$), and $l$ is optical path length ($\text{cm}$, standard cuvette $= 1\,\text{cm}$).

• Worked Calculation Example: Guanosine has a maximum absorption at $275\,\text{nm}$ with $\varepsilon_{275} = 8400\,\text{M}^{-1}\text{cm}^{-1}$. Measured absorbance $A_{275} = 0.70$ in a $1\,\text{cm}$ path length cuvette. Calculate molar concentration ($c$):

\nA = \varepsilon c l\n

\n0.70 = (8400\,\text{M}^{-1}\text{cm}^{-1}) \times c \times (1\,\text{cm})\n

\nc = \frac{0.70}{8400\,\text{M}^{-1}\text{cm}^{-1} \times 1\,\text{cm}} = 8.33 \times 10^{-5}\,\text{mol/L}\n

• Limitations of the Beer-Lambert Law:

  1. Concentration Limits: Strictly valid only at low analyte concentrations ($< 10\,\text{mM}$). High concentrations cause solute-solute interactions that modify individual absorptivity and alter solution refractive index.

  2. Monochromaticity Requirements: Assumes strictly monochromatic incident radiation. Polychromatic light leads to non-linear deviations.

  3. Stray Radiation: Light reaching the photodetector without passing through the sample (due to scattering or internal instrument reflection) introduces structural measurement errors.

Spectrophotometric Characterization Experiments

• Exercise 6.1: Determination of Optimum Wavelength for pNP:

  • Instrumentation: UNICO 1000 Spectrophotometer (equipped with sample compartment, wavelength knob, digital display, mode switch, and 100%T/0A zeroing button).

  • Procedure:

    1. Power on instrument $15\,\text{min}$ prior to use. Set Mode switch to Absorbance (Abs).
    2. Prepare Blank tube B containing $3\,\text{mL}$ Tris-HCl buffer using a P1000 pipette.
    3. Prepare sample tube 5A containing $3\,\text{mL}$ of $0.05\,\text{mM}$ p-nitrophenol (pNP) solution.
    4. Set wavelength selector knob to target test wavelength ($390\,\text{nm}$ to $500\,\text{nm}$).
    5. Insert Blank tube B, close cover, press red zeroing button (100%T/0A) until display reads BL and then 0.00.
    6. Insert sample tube 5A and record absorbance value.
    7. Repeat zeroing and reading at incremented wavelengths ($390, 400, 410, 420, 430, 450, 460, 470, 480, 490, 500\,\text{nm}$).
    8. Plot Absorbance vs Wavelength to determine maximum absorption wavelength ($\lambda_{\max} \approx 400 - 410\,\text{nm}$).

• Exercise 6.2: Extinction Coefficient ($\varepsilon$) Calculation:

  • Prepare duplicate dilution sets (Set A: 1A–5A, Set B: 1B–5B) in $3\,\text{mL}$ final volumes using $1\,\text{mM}$ pNP stock and Tris-HCl buffer:
    • Blank (BL): $0\,\text{mM}$ pNP ($0\,\mu\text{L}$ pNP + $3000\,\mu\text{L}$ Tris buffer)
    • Tube 1 (0.01 mM): $30\,\mu\text{L}$ pNP + $2970\,\mu\text{L}$ Tris buffer
    • Tube 2 (0.02 mM): $60\,\mu\text{L}$ pNP + $2940\,\mu\text{L}$ Tris buffer
    • Tube 3 (0.03 mM): $90\,\mu\text{L}$ pNP + $2910\,\mu\text{L}$ Tris buffer
    • Tube 4 (0.04 mM): $120\,\mu\text{L}$ pNP + $2880\,\mu\text{L}$ Tris buffer
    • Tube 5 (0.05 mM): $150\,\mu\text{L}$ pNP + $2850\,\mu\text{L}$ Tris buffer
  • Set instrument wavelength to $\lambda_{\max}$ ($410\,\text{nm}$).
  • Zero instrument using Blank tube BL.
  • Measure absorbance $A_{410}$ for Set A and Set B, then compute Average A_{410} = \frac{A_A + A_B}{2}$.\n * Calculate extinction coefficient \varepsilon for each concentration:\n\n \varepsilon = \frac{\text{Average } A_{410}}{c \times l} \n\n* **Laboratory Waste Management and Clean-Up Protocols**:\n\n * Turn off spectrophotometer upon completion.\n\n * Dispose of concentrated 1\,\text{mM} pNP stock in biohazard waste bin.\n\n * Pour diluted pNP solutions into designated chemical hood liquid waste containers.\n\n * Rinse glass cuvettes thoroughly with water, remove marker labels using ethanol, and place inverted in chemical hood drying trays.\n\n * Wipe down bench surfaces and digital equipment (iPads) with 70\%$$ ethanol and Kimwipes. Wash hands thoroughly before leaving.