XV 74. Flouorescence-Polarization-Circular-Dichroism- Jablonski diagram Where does the energy go?

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1 XV 74 Flouorescence-Polarization-Circular-Dichroism- Jablonski diagram Where does the energy go? 1) Excite system through A Absorbance S 0 S n Excite from ground excited singlet S = 0 could be any of them must change dipole 2) If S n > S 1 often rapidly decay/relax down S n S 1 IC internal conversion ( E ~ 0 fastest) 3) Options for release of energy in υ = 0 next step is a) non-radiative decay to S 0 less probable implies IC to another singlet, S 0, but big separation Process: Vibrational relaxation (VR) through collision vibrational energy taken to solvent (surround) relax to lowest vibrational state υ = 0 (relatively fast process < ns) b) Fluorescence emit photon to S 0 most probable for S 1 trap Fluorescence lifetime / quantum yield reflects probability of this process ns µs typical c) ISC cross over to triplet -- intersystem crossing In triplet VR to υ = 0 again trap energy Phosphorescence much slower ( S 0) aided by heavy atoms (spin-orbit)

2 XV 75 Fluorescence intensity depends on how molecules are excited and on the probablity of transition to ground state. It is a kinetic process compete between pathways in Jablonski diagram, typically excite by absorbance Compete between fluorescence, non-radiative decay, and intersystem crossing. First order process: -d[m*]/dt = k d [M*] Lifetime: τ = 1/k d if only fluorescence: τ 0 = 1/k f Other processes take away excitation, lead to shorter observed lifetimes -d[m*]/dt = k f [M*]+ k nr [M*]+ k Q [M*][Q] = k d [M*] where k nr [non-radiative decay], k Q [quenching] observed lifetime: τ = 1/(k d + k nr + k Q [Q]) Quantum yield ratio: photons fluoresce / photon absorb φ f = k f [M*]/ k d [M*] = τ/τ 0 Quantum Mechanics role 1) Quantum mechanics used to describe excited states much less accurate than for vibrations requires a surface not just single geometry calculations need configuration interaction states become mix of configuration: (σ) n (π) m idea need to change orbitals of electron states mix different orbital configuration impact calculations large and less accurate 2) Quantum mechanics used to describe which vibronic excitation are allowed S = 0 Electric field cannot change spin (Phosphor mix spins with spin-orbit coupling) Dipole must change: A ~ ψ ex * µ el ψ g dτ 2 integral zero if ψ ex and ψ g same dipole υ = 0, ±1, ±2 no restriction on υ - symmetric A: Absorb: most transition start υ g = 0 (most population) F: Fluorescence is same but υ ex = 0 by relaxation (VR) 3) Transitions seen usually determined by symmetry group theory tool for organizing symmetry useful in small molecules (Chem 444) Biomolecules less use no symmetry use correlation to small molecule components

3 XV 76 Polarization Since µ is vector fixed on molecule, E-field can interact with molecule differently if change E-orientation gas or solution no impact / average out solid can orient molecule crystal used for small molecule Alternative dissolve in oriented material a) liquid xtal Net orientation long axis of inserted molecule favor orientation b) lipid membrane composed of charged head groups and alkyl tails, bilayer form: hydrophobic interior favor orientation, e.g. Helices surface, alkyl tails eg trans membrane protein / peptide hydrophilic surface could bind charges c) Flow long molecules orient to flow Works well for DNA, fibers, etc. Useful if chromophore absorbing species has different absorbance with one polarization called dichroism (linear) can use for analysis of orientation in fluorescence if excite with one polarization can observe emission in and orientation fluorescence anisotropy degree of motion / flexibility

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6 XV 79 DMPG α helix β sheet DOPG α helix β sheet DSPG α helix β sheet Wavelength/cm -1 Changes in fluorescence anisotropy 0.08 A ph6.8 DPH TMA-DPH DM PG B ph4.6 DOPG DSPG DMPG DOPG DSPG

7 XV 80 Circular Polarization if 2 waves displaced by λ/4 along z combine get rotation of E as propagate (helix in space) Circular Polarization Right or Left Now molecule sees both linear polarizations (x + y) but due to the rotation between them at ν has different selection rules Trick measure difference: A = A L A R Circular Dichroism Theoretically this ~ R = I m [( ψ ex m ψ g ) ( ψ ex µ ψ g )] m electronic dipole operator µ magnetic dipole operator µ m 0 only for chiral molecules

8 XV 81 eg asymmetry l-atom / no plane or center of symmetry Perfect for biology all bio-molecular, chiral i.e. proteins L AA DNA chiral ribose sugars several centers lipids well Measurement of CD is most widely used for protein secondary structure most intense α-helix 222&207nm weaker β-sheet, neg 215, pos 200- DNA typical band patterns vary Big success: B Z differ: right - left Sugars problem, absorbances in VUV Lipids -- similar issues In IR can also do CD, called Vibrational Circular Dichroism Signals smaller (need more concentration) but differentiation between states/conformations is higher Also can use isotopes to localize structural information

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