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Exciton supplemental info

Exciton supplemental info - S pectroscopy Lecture 10 E x c...

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1 Spectroscopy Lecture 10: Exciton Coupling Karen Fleming 10 Apr 2008 Typical far-UV CD spectra of proteins a. Myoglobin (all α ) b. Prealbumin (all β ) c. Acid denatured Staph nuclease
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2 Nucleic acids have an increased extinction coefficient when melted Temperature Relative Absorbance at 260 nm Polymeric Nucleic acids have an increased extinction coefficient when they are converted to smaller units.
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3 Exciton Coupling: Interactions between chromophores Exiton Coupling : Band splitting (usually too small to see directly but can increase the linewidth Induced optical activity (e.g. circular dichroism) Band intensification (hyperchromism) Band weakening (hypochromism) Exciton : an electronically neutral excited state, often regarded as a bound state of an electron and a “hole”. Two Chromophores Monomer Dimer
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4 Interactions between two chromophores: a “monomer” and a “dimer” E o E a v 0 a = ( E a " E 0 )/ h Transition frequency: Wavefunctions for the monomeric chromophore E o E a " a " 0 " a " 0 = 0 The two states are orthogonal: " a " a = " 0 " 0 = 1 Each state is normalized: " a " 0 dx # = 0
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5 The absorption intensity of the monomer is given by the dipole strength E o E a " a " 0 D 0 a = " 0 ˆ μ " a 2 The dipole strength is the square modulus of the transition dipole moment integral: D 0 a = " 0 ˆ μ " a We write the transition dipole moment integral as: If chromophores are interacting there can be two absorbance bands or the single absorbance band can be enhanced: Frequency v 0 a v 0 a v 0 a Monomer Or
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