Circular Dichroism (CD) Overview

This set of flashcards covers key vocabulary and concepts related to Circular Dichroism (CD) in protein science, including definitions and key principles.

What is Circular Dichroism (CD) spectroscopy?

CD spectroscopy is a technique that measures the differential absorption of left- and right-circularly polarized light by chiral molecules. It's particularly useful for studying the conformation of macromolecules in solution.

What types of molecules exhibit Circular Dichroism?

Molecules that are optically active (chiral) exhibit CD. This includes many biological molecules like proteins, nucleic acids, and carbohydrates.

What information can CD spectroscopy provide about proteins?

CD spectroscopy can provide information about a protein's secondary structure (e.g., alpha-helices, beta-sheets, random coils), tertiary structure changes, and protein folding/unfolding transitions.

What are the common wavelength regions used in CD spectroscopy for proteins?

• Far-UV Region (190-250 nm): Primarily reflects the protein's secondary structure due to peptide bond absorption.

• Near-UV Region (250-320 nm): Reflects the environment and chirality of aromatic amino acid side chains (tryptophan, tyrosine, phenylalanine) and disulfide bonds, providing insights into tertiary structure.

How does Circular Dichroism work?

When plane-polarized light passes through a birefringent medium, it becomes elliptically polarized. Chiral molecules differentially absorb left- and right-circularly polarized light. CD measures this difference in absorption ( \Delta A = AL - AR ), which is non-zero only for chiral molecules at specific wavelengths corresponding to electronic transitions.

What is Circular Dichroism (CD) spectroscopy?

CD spectroscopy is a technique that measures the differential absorption of left- and right-circularly polarized light by chiral molecules. It's particularly useful for studying the conformation of macromolecules in solution.

What types of molecules exhibit Circular Dichroism?

Molecules that are optically active (chiral) exhibit CD. This includes many biological molecules like proteins, nucleic acids, and carbohydrates.

What information can CD spectroscopy provide about proteins?

CD spectroscopy can provide information about a protein's secondary structure (e.g., alpha-helices, beta-sheets, random coils), tertiary structure changes, and protein folding/unfolding transitions.

What are the common wavelength regions used in CD spectroscopy for proteins?

1. - Far-UV Region (190-250 nm): Primarily reflects the protein's secondary structure due to peptide bond absorption.

2. - Near-UV Region (250-320 nm): Reflects the environment and chirality of aromatic amino acid side chains (tryptophan, tyrosine, phenylalanine) and disulfide bonds, providing insights into tertiary structure.

How does Circular Dichroism work?

When plane-polarized light passes through a birefringent medium, it becomes elliptically polarized. Chiral molecules differentially absorb left- and right-circularly polarized light. CD measures this difference in absorption ( \Delta A = AL - AR ), which is non-zero only for chiral molecules at specific wavelengths corresponding to electronic transitions.

What makes a molecule chiral or optically active?

A molecule is chiral if its mirror image is non-superimposable on the original molecule. This molecular asymmetry is fundamental for a molecule to exhibit Circular Dichroism.

What are the common units used to express Circular Dichroism data?

CD data can be expressed in terms of differential absorbance ( \Delta A ) or, more commonly, molar ellipticity ( [\Theta] ), which is expressed in degrees cm^2 dmol^{-1} . Molar ellipticity normalizes the signal for concentration and path length.

How do common protein secondary structures appear in Far-UV CD spectra?

• Alpha-helices: Typically show two negative minima at approximately 208 nm and 222 nm, and a positive maximum at 190 nm.
• Beta-sheets: Usually have a negative minimum near 217 nm and a positive maximum around 195 nm.
• Random coils: Characterized by a strong negative minimum around 195-198 nm and very low ellipticity at higher wavelengths.