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Lecture notes of 10 pages for the course Techniques For Biological And Chemical Sciences at QMUL
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Circular dichroismLearning objectives:
1. Describe the physical principle of circular dichoism
2. Describe polarised and circularly polarised light
3. Describe the components of a CD instrument
4. Relate the practical considerations when obtaining a CD spectrum
5. Define the conventional units for a CD spectrum
6. Apply CD spectroscopy to determine the secondary structure elements of protein
Principle
CD is another absorption spectroscopy
CD has similarities with UV-Visible spectroscopy
CD measures the absorbance of light in the UV and Visible region due to electronic transitions from a ground state to an
excited state.
However, CD is the measurement of the difference between the absorbance of left and right circularly polarised light
DA=Abs (left) –Abs (right)
Circularly polarised light is the only difference
Difference in absorbance is small but can be measured using an instrument
What is polarised light and circularly polarised light?
Light is a wave oscilate in all different directions: totally random
Polarised liht oscilates in one direction only
CPL: uses circularly light, which is the same as polarised light but at any one point keeps turning around as travelling to
the sample.
Circular dichroism
A technique, which bombards the sample with circularly polarised light
The phase shifts between left and right handed polarised light is measured
, CD spectra is the difference in absorption
Subtle difference between left and right absorption
CD spectrum shows the difference between the absorption spectra
50 times per second the spectrometer is measuring left and right light
50Hz
The detector knows that the spectrometer is switching left to right at 50Hz
Difference in absorption of left and right CP-Light produced Ellipticity in signal
CD signal sometines called ellipticity
If absorb at the same length get a perfect circle
If one beam longer than the other because one absorbed more than the other it results in no elliptical signal
No CD signal = no elliptical appearance
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