CD Full Form: Principle, Application and Advantages, Download PDF

By Renuka Miglani|Updated : December 25th, 2021

CD Full Form: Optical activity is a common feature of most of the biological compounds. Аn орtiсаlly асtive соmроund simрly meаns thаt the рlаne оf роlаrizаtiоn оf а lineаrly роlаrized light rоtаtes аs it раsses thrоugh the соmроund. Biophysical methods are usually employed for measurement of the optical rotation and Circular Dichroism (CD) Spectroscopy is a prime example of one such biophysical method used for studying optically active compounds, particularly the conformation of proteins and nucleic acid.

We hаve соme uр with аn аrtiсle tо knоw everything аbоut the Circular Dichroism (CD) inсluding its full fоrm, Principle, Advantages, and Disadvantages. Sсrоll dоwn the соmрlete аrtiсle tо get the full infоrmаtiоn оn Circular Dichroism (CD).

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CD Full Form: Working Principle 

The physical basis of CD is the utilization of circularly polarized light; a CD instrument records the unequal absorption of left-handed and right-handed circularly polarized light.

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Fig: A CD instrument

CD Full Form: Polarisation of light

  1. Unpolarized light - The directions of oscillations randomly change in the same plane with time.
  2. Plane polarized light - The magnetic and electric fields components oscillate in a definite plane being perpendicular to each other.
  3. Circularly polarized light - The magnetic field keeps oscillating but the electric field vector changes direction in a rotary motion.

CD Full Form: Origin of Circularly Polarized Light:

  • It is obtained by superimposing two plane-polarized light of the same wavelength and amplitude which are polarized in two perpendicular planes, but there is a phase difference of 90° between them.
  • The wavelength range of 190nm-250nm (far-uv region) is used.

 

Fig: Linearly and Circularly Polarized Light

CD Full Form: Mode of Working

  • The right circularly (R) and left circularly polarized (L) light is incident on a molecule/sample.
  • Circularly polarized light when passed through a dichroic sample will be elliptically polarized.
  • This becomes possible since the circularly polarized components of the original linear polarized light will now not be of equal magnitudes due to differential absorbance (i.e., circular dichroism).
  • Different deviation (in degrees) occurs in each as the molecule is chiral; giving 2 different color illuminations; hence the name circular dichroism.
  • It is measured ellipticity (millidegrees or degrees) or the difference in absorption of left and right circularly polarized light.

         [Left circularly polarized light - Anti-clockwise rotation of the electric vector

          Right circularly polarized light- Clockwise rotation of electric vector]

  • Thus, an elliptically polarized light is obtained by superimposing two plane-polarized light vibrating at the right angle to each other (phase difference of 90° between them) having the same wavelength but unequal amplitude.

Fig: Mode of working of Circular Dichroism

So, CD = AL-AR

            = ΔA = (eL-eR) cl

            = Δecl

[Δe - Molar circular dichroism]

Δe is typically <10 M-1cm-1

So, the CD signal is a very small difference between 2 large originals

The ellipticity is proportional to the difference in absorbance of 2 components (left and right circularly polarized light). So, the CD is equivalent to ellipticity.

q = 2.303 (AL – AR) 180/4p

q = 33(AL- AR) = 32.98 DA = 33DA

Unit of q = Degree cm2 dmol-1

Hence, CD measures the ellipticity of the transmitted light (i.e the light that is not absorbed)

Ellipticity value can be both positive and negative [AL > AR: Positive and AL < AR: Negative]

CD Full Form: Typical Initial Concentrations

  • Protein Concentration: Around 0.5mg/ml (adjustments Made to Produce the Best Data)
  • Cell path length: For any problem arising in the absorption, at such times cells with a shorter path (0.1mm) with a correspondingly increased protein concentration and a longer scanning time can be utilized.
  • Buffer concentration: It should be as low as possible around 5mM or even lower while maintaining protein stability. Generally, 10mM phosphate buffer is used in CD spectra, although low concentrations of Tris, perchlorate, or borate are also acceptable.

CD Full Form: Sample Preparation and Measurement

  • Additives, Buffers and Stabilizing Compounds: Those Compounds Which Absorbs in The Region of Interest Should Be Avoided.
  • Solvent selectivity: A large number of organic solvents like THF, CHCl3, CH2Cl2 cannot be used
  • Protein solution: The protein solution should only contain those chemicals necessary to maintain protein stability/solubility and at the lowest concentration possible. The protein should be pure devoid of any sort of contamination.
  • Lamp selectivity: In place of traditional Xe-arc lamps, high-pressure short Xe lamps are used for performing low UV-CD spectroscopy.
  • Contaminants: Any particulate matter (scattering particles) that adds significant noise to the CD spectra should be avoided. Solutions must be filtered to improve the signal-to-noise ratio.

CD Full Form: Some Standard CD Patterns for Secondary Structure

1. For a-helical proteins:

  • The negative peak at 222nm and 208nm
  • The positive peak at 193nm

2. For b-sheet proteins:

  • The positive peak at 195nm
  • The negative peak at 218nm

3. For random-coil:

  • The positive peak at 215nm
  • The negative peak at 195nm

 

Fig: Standard CD graphs of secondary structures

CD Full Form: Enantiomers and CD Spectra:

Two enantiomers (mirror image of each other) are in equal amounts in a sample; then the resultant CD-signal will be zero. Both the signals will cancel out each other in this racemic mixture.

CD Full Form: Advantages of Circular Dichroism

  • Relatively low concentrations/amount of sample is required for analyzing
  • Microsecond time resolution
  • Timescale is much shorter thus allowing to study of dynamic system and kinetics

CD Full Form: Limitations

  • Certain buffer compounds in the sample strongly absorb in the Far-UV region and cause interference.
  • Carbohydrates cannot be easily studied through CD.
  • Oxygen must be completely absent from the system to experiment 200nm
  • It does not provide atomic-level structural analysis.
  • It can be used only for qualitative analysis of data.
  • Not able to provide a piece of detailed residue-specific information as in NMR and X-Ray Crystallography

CD Full Form: Applications of CD-Spectra

  • Determine the protein’s secondary structure (at far-UV region: 180-240nm) and the protein’s tertiary structure (at near-UV region: 280-380nm)
  • It is the best method for monitoring structural alterations due to pH, temperature, and ionic strength
  • Structural, kinetic, and thermodynamic information about macromolecules can be derived from CD spectra,
  • It can be used to estimate a-helix, b- sheet, and random coil configuration.
  • It is used to determine the conformational changes due to protein-protein interactions, protein-DNA, and protein-ligand interactions.
  • It is used to measure the folding and unfolding state of proteins due to temperature changes.

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