68
Circular Dichroism in Protein Analysis
UV CD spectrum: two negative bands with
separate minima of similar magnitude at
222 nm (the n
π
transition) and 208 nm
(the parallel
ππ
exciton band) and a
positive band, the NV
1
band, at 190 nm
due to the antiparallel
ππ
exciton band.
The ratio [
θ
]
208
/
[
θ
]
222
is sensitive to the
backbone dihedral angles.
The average fractional helicity,
f
H
,ofan
N
-residue peptide may be related to the
observed mean residue molar ellipticity at
222 nm by
f
H
=
[
θ
]
222
[
θ
H
]
222
(
1–
k
/
N
)
(
8
)
where [
θ
H
]
222
is the ellipticity of an in-
Fnite, completely helical peptide,
k
is an
end-effect correction, which is
3, and
N
is the number of residues. Estimated
values for [
θ
]
222
of an inFnite, com-
pletely helical peptide range from
37 000
to
44 000 deg cm
2
dmol
1
.
However,
anomalously intense bands have been ob-
served in some peptide models under
certain experimental conditions.
6.2
3
10
-helix
The 3
10
-helix has (
i
,
i
+
3) main-chain
hydrogen bonds. There are three represen-
tative dihedral angles (
ϕ, ψ
)for3
10
-helices,
type I (
60
,
30
); type II (
54
,
28
),
and type III (
44
,
33
). The three types
of 3
10
-helices are anticipated to have dif-
ferent CD spectra based on Frst-principles
calculations. The CD spectrum for a type I
3
10
-helix is similar to that for an
α
-helix,
but a little weaker band at 222 nm relative
to the 208 nm band, and the [
θ
]
222
/
[
θ
]
208
ratio ranges from about 0.3 to 0.85; this ra-
tio is greater than 1 for an
α
-helix. Type II
has the strongest experimental foundation;
type I and III have weaker and stronger
n
π
transitions than type II, respectively.
It may be possible to distinguish reliably
between 3
10
-helix and
α
-helix by CD, but
it is still a matter of debate.
6.3
β
-sheet
β
-sheets are composed of
β
-strands, which
can be thought of as helices with two
residues per turn. Typical backbone dihe-
dral angles are
φ
=−
120
and
ψ
=+
120
producing a translation of 3.2 to 3.4
˚
Aper
residue for residues in antiparallel and
parallel strands respectively. An antiparal-
lel arrangement is shown in ±ig. 5. The CD
spectra for
β
-sheet proteins are less intense
and have fewer features than those of
α
-
helical proteins, with a negative band near
217 nm, a positive band near 195 nm, and
another negative band near 180 nm. How-
ever, the CD spectra for
β
-sheet proteins
have much greater variability than those
for
α
-helices: for example, the intensities
Fig. 5
The main chain of a polypeptide in an antiparallel two-stranded
β
-sheet, with hydrogen bonds
depicted by dashed lines.
previous page 1388 Encyclopedia of Molecular Cell Biology and Molecular Medicine read online next page 1390 Encyclopedia of Molecular Cell Biology and Molecular Medicine read online Home Toggle text on/off