lab 10 Comprehensive Study Notes: Chromophore Properties, Protein Concentration, and Acid-Base Lab Analysis

Overview of Chromophore Properties and Lab Objectives

• This lecture covers the experimental material required for the chromophore properties lab, focusing on the absorption and fluorescence of the chromophore, as well as its acid-base properties.

• The lab investigates the absorption and fluorescence spectra of the chromophore under two primary conditions: with a histidine tag (His-tag) and without, depending on the success of the chymotrypsin cleavage process.

• Absorption in this system arises from two distinct sources:

  • The chromophore itself.

  • The protein structure surrounding the chromophore.

• Key objectives include:

  • Determining protein concentration via absorbance at $280\,nm$.

  • Comparing protein concentration to chromophore concentration to determine the percentage of molecules containing a functional chromophore.

  • Observing denaturation effects under basic and neutral conditions.

  • Determining the $pKa$ of the chromophore by placing the protein in a range of buffered solutions.

Fundamental Principles of Absorbance and the Beer-Lambert Law

• Absorbance ($A$) is defined as the negative logarithm of transmittance ($T$). Transmittance is the ratio of light exiting the sample ($I$) to the light entering the sample ($I_0$).

  • Formula: $A = -\log(T) = -\log\left(\frac{I}{I_0}\right)$

• Absorbance is proportional to the concentration of the absorbing molecule in the solution. This relationship is quantified by the Beer-Lambert Law:

  • Formula: $A = \epsilon \times c \times l$

  • Where:

    • $\epsilon$ is the extinction coefficient (the proportionality constant).

    • $c$ is the concentration.

    • $l$ is the path length through which the light travels.

• The units for the extinction coefficient ($\epsilon$) are typically designed to cancel out concentration and path length units to render absorbance unitless. For a molar extinction coefficient:

  • Concentration: Molarity ($M$)

  • Path length: Centimeters ($cm$)

  • Units of $\epsilon$: $\text{M}^{-1}\,\text{cm}^{-1}$

• Absorbance must be unitless because it is the logarithm of a ratio of intensities ($I/I_0$), and the intensity units cancel out.

Protein Absorbance at 280 nm and Calculation Parameters

• The predominant contributors to protein absorbance at $280\,nm$ are the three aromatic amino acids:

  • Tryptophan ($Trp$)

  • Tyrosine ($Tyr$)

  • Phenylalanine ($Phe$)

• Minor contributors include the histidine residues and the carbonyl groups in the peptide bonds. While peptide bonds primarily absorb near $220\,nm$, they maintain residual absorbance at $280\,nm$.

• The specific protein used in this lab contains:

  • $1$ Tryptophan residue.

  • $11$ Tyrosine residues (Note: one of these is involved in forming the chromophore, effectively leaving $10$ standard tyrosine residues).

  • $12$ Phenylalanine residues.

• The estimated extinction coefficient for this protein composition is approximately $20,000\,\text{M}^{-1}\,\text{cm}^{-1}$.

• Students are required to calculate the specific extinction coefficient and other parameters (molecular weight, isoelectric point/$pI$) using the ExPASy ProtParam tool.

  • Procedure: Input the protein sequence into the tool.

  • Accuracy Correction: To get an accurate reading for the protein (excluding the chromophore's specific light absorption), the tyrosine residue used in the chromophore should be manually changed to a Serine ($Ser$) or Threonine ($Thr$) residue in the sequence. Doing so significantly improves the representation of the protein's extinction coefficient, as it accounts for the chemical modification of that residue. Comparing a sequence with $11$ tyrosines to one with $10$ tyrosines and a serine replacement will show a small drop in the extinction coefficient value.

The Chemical Structure and Formation of the Chromophore

• The chromophore is composed primarily of a tyrosine residue and the peptide backbone. Specifically:

  • The nitrogen atom from the tyrosine residue.

  • The nitrogen atom from a glycine residue.

  • Modified portions of the peptide backbone.

  • R-groups from either serine or threonine (contributing a hydroxyl group and amide nitrogen).

  • The carboxylic acid carbon from the original structure forms a carbon-nitrogen double bond via dehydration.

• Chromophore formation is an internal, non-enzymatic, three-step process requiring specific protein folding:

  1. Folding and Cyclization: A rotation occurs around a specific bond, flipping the tyrosine residue into position. This allows the nitrogen to perform a nucleophilic attack on a carbonyl carbon, forming the first covalent bond of the chromophore.

  2. Dehydration: A water molecule is lost (a hydroxyl group and a hydrogen atom) to form a carbon-nitrogen double bond.

  3. Oxidation: Molecular oxygen ($O_2$) acts as an oxidizing agent to form a new carbon-carbon double bond, completing the pi ($\pi$) conjugation system. Oxygen is reduced to hydrogen peroxide ($H_2O_2$) during this step.

• In clinical/lab observations, chromophore formation is rarely $100\%$. Typically, fewer than $75\%$ of the protein molecules successfully form the chromophore due to the complexity of the internal folding and chemical steps required.

Denaturation Studies and Chromophore States

• The chromophore is normally protected inside the beta-barrel of the protein. The local environment (contacts with amino acid residues and a water molecule) maintains the chromophore in an anionic (deprotonated) state.

• The anionic state is responsible for the intense greenish-yellow fluorescence and peak absorbance around $490\,nm$.

• Denaturation Mechanism: Using chaotropic denaturants like Urea or Guanidine Hydrochloride ($GUHCl$/"guacol").

  • These agents cause solvation of hydrocarbons by providing water with hydrogen-bonding alternatives. This leads to a loss of the hydrophobic effect, causing the protein to lose its secondary and tertiary structures.

• Denaturation Outcomes:

  • Basic Conditions (pH > 12): The chromophore is removed from the protective beta-barrel but remains in its deprotonated/anionic state due to the high pH. It maintains an absorption maximum near $490\,nm$.

  • Neutral Conditions: Removing the chromophore from the beta-barrel environment at neutral pH allows it to become protonated. Protonation removes two electrons from the pi ($\pi$) system, shifting the absorption to a shorter wavelength (higher energy) and significantly reducing the extinction coefficient.

Assessment of Acid-Base Properties and pKa Determination

• The $pKa$ of the chromophore is determined by measuring absorbance across a wide range of pH values using buffers.

• States of the Chromophore:

  • Deprotonated State: Measured by absorbance at $489\,nm$ ($A_{489}$).

  • Protonated State: Represented as the difference between maximum possible absorbance and current absorbance ($A_{max} - A_{489}$).

• The Henderson-Hasselbalch equation ($pH = pKa + \log\left(\frac{\text{base}}{\text{acid}}\right)$) is rearranged to solve for $pKa$:

  • Formula: $pKa = pH - \log\left(\frac{A_{489}}{A_{max} - A_{489}}\right)$

• Spectral Observations:

  • At high pH (> 7.0): The chromophore is fully deprotonated; absorbance at $489\,nm$ is at its maximum.

  • At low pH (e.g., $pH\,4.73$): The peak at $489\,nm$ virtually disappears, and a new peak appears below $400\,nm$ (approximately a $90\,nm$ shift). This shorter-wavelength peak corresponds to the protonated chromophore which has fewer electrons in its pi system.

  • The transition region (where absorbance at $489\,nm$ drops significantly, around $pH\,5$ to $6$) identifies the $pKa$.