8
Units of CD
Measurement
Units of CD Measurement
CD is reported in units of absorbance or ellipticity.
Each of these can be normalized for molar
concentration of the sample. The most direct
data from the Olis CD instrument is absorbance
(Abs(L)- Abs(R)). This value is often reported in
milliabsorbance units (mA), which are a thousandth
of an absorbance unit.
CD data are commonly reported as ellipticity (θ),
which is related to absorbance by a factor of 32.98
(θ = 32.98 ΔAbs). Ellipticity is usually reported in
millidegrees (mdeg or m
ο
), which are a thousandth
of a degree.
Molar ellipticity ([θ]) is CD corrected for
concentration. The units of molar ellipticity are
historical (deg×cm
2
/dmol). Conversion from molar
extinction (absorbance corrected for concentration)
to molar ellipticity uses a factor of 3298 ([θ] =
3298Δε). To calculate molar ellipticity, the sample
concentration (g/L), cell pathlength (cm), and the
molecular weight (g/mol) must be known.
If the sample is a protein, the mean residual
weight (average molecular weight of the amino
acids it contains) is used in place of the molecular
weight, essentially treating the protein as a
solution of amino acids.
From
To
Absorbance1Milliabsorbance2Molar Extinction3Degrees4Millidegrees5Molar Ellipticity6
(A)
A
A*1000
A*M/(C*L)
A*32.98
A*32980
A*M*3298/(L*C)
(mA)
mA/1000
mA
A*M/(C*L*1000)
mA*0.03298
mA*32.98
mA*M*3.298/
(L*C)
(
)
*C*L/M
*C*L*1000/M
*C*L*32.98/M
*C*L*32980/
M
*3298
(
°
)
°
/32.98
°
/0.03298
°
*M/(C*L*32.98)
°
°
*1000
°
*M*100/(L*C)
(m
°
)
m
°
/32980
m
°
/32.98
m
°
*M/(C*L*32980)
m
°
/1000
m
°
m
°
*M/(10*L*C)
[
Θ
]
[Θ]
*C*L/(3298*M)
[Θ]
*C*L/(3.298*M)
[Θ]
/3298
[Θ]
*C*L/(100*M)
[Θ]
*C*L*10/M
[Θ]
C is concentration in g/L
M is average molecular weight (g/mol)
L is path length of cell (cm)
1
Units are Absorbance (Abs)
2
Units are milliabsorbance (mAbs)
3
Units are A*L/mol*cm
4
Units are degrees (
º
)
5
Units are millidegrees (m
º
)
6
Units are deg*cm2/dmol
