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Linear dichroism for the study of nucleci acids, fibrous and membrane proteins. Electrons bonds structure. UV/visible light: l ~ 180 nm – 800 nm, energy h n = hc/ l causes electrons to go to higher energy levels. l required depends on electron rearrangement needed.
UV/visible light:l ~ 180 nm – 800 nm, energy hn = hc/l
causes electrons to go
to higher energy levels.
l required depends on electron
In solution: broad bands due to
diff. vibrational levels
in excited state &
molecules having slightly
different energy levels.
UV: –350 nm
Vis: 400 nm –
Excited electronic state
Ground electronic state
Ground vibn’l level
Far UV (250 – 180 nm) dominated by peptide group
Many buffers absorb here — so beware!
First n* 210 – 220 nm, weak (e~100 cm1dm3mol1)
First * 180 nm, stronger (e ~ 7000 cm1dm3mol1)
In -helix only has a component at 208 nm
?? ns* transition 175 nm
Side chains absorb in this region:
aromatics, Asp, Glu, Asn, Gln, Arg & His. Usually small.
Directionelectrons move during a transition
Oriented samples and polarised light:only light whose
electric field pushes the electrons along the polarisation
direction causes a transition
Nucleic acids: DNA bases: all transitions perpendicular
to helix axis
Poly proline II
LD = A//- A^w.r.t. orientation axis
stretched film or flow most common
LD = A// A
= absorbance // orientation direction absorbance to it
annular gap LD cell
with thermal control
50 m annular gap LD cell
with CaF2 optics,
222 nm, n-p*
20 mM tropomyosin
10 mM SAF-p1/02
100 mM matured
% Secondary structure content
% Aromatic amino acids
Collagen type I
Protein FibresTim Dafforn, Dave Halsall. Louise Serpell
206 nm //
the reduced LD, LDr = LD/isotropic absorbance, is:
where is the angle the transition moment of interest makes with the normal to the cylinder surface (i.e. to the lipid long axis), S is the orientation factor that denotes the fraction of the liposome that is oriented as a cylinder perfectly parallel to the flow direction.
220 nm n-* polarised helix axis
208 nm // helix axis
Helix // lipids at 208 nm LD<0, 220 nm >0
Abs of SWNT
In SDS < CMC
LD of SWNT
LD of anthracene
LD of SWNT
1. Intercalation between base pairs:
DNA lengthens and stiffens
2. In major groove:
more options for selectivity
3. In minor groove:
planar aromatics with AT rich DNA
4. External binding:
DNA binding & structure control by transition metal supramolecular helicates
Alison Rodger and Mike Hannon
Isabelle Meistermann, Karen Sanders, Chris Isaac, Jemma Peberdy, Laura Childs, Syma Khalid, Mark Rodger
University of Warwick
Virtudes Moreno, Barcelona (AFM)
Einar Sletten, Norway (NMR)
Simple imines can be used for supramolecular assembly
M.J. Hannon, C.L. Painting, A. Jackson, J. Hamblin, and W. Errington, Chem. Commun., 1997, 1807.
Absorbance spectra of ct-DNA with 20 mM [Fe2L3]Cl4
LD of spectra of [Fe2L3]Cl4 in a stretched polyvinylalcohol film
The flow LD experiment not only reveals a binding angle of 60°(±10 °) but also reveals changes in the DNA region.
The decrease in the magnitude of the LD signal in the DNA region with respect to free DNA means a decrease in DNA orientation.
This indicates that the [Fe2L3]Cl4 helicate is kinking the DNA.
LD spectra of free and bound ct-DNA
DNA:M = 10:3
DNA:P = 10:3
DNA:rac = 10:3
AFM images of a fragment of 200 pb linear DNA
incubated at 37ºC for 5 h with the iron cylinders.