ChemicalExchangediffusion2010

ChemicalExchangediffusion2010 - Effects of Chemical...

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ffects of Chemical Exchange on NMR Spectra Effects of Chemical Exchange on NMR Spectra •Chemical exchange refers to any process in which a nucleus exchanges between two or more environments in which its NMR parameters (e.g. chemical shift, scalar coupling, or relaxation) differ. •DNMR deals with the effects in a broad sense of chemical exchange processes on NMR spectra; and conversely with the information about the changes in the environment of magnetic nuclei that can be derived from observation of NMR spectra. Conformational quilibrium K ex equilibrium hemical Chemical equilibrium K B
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ypes of Chemical Exchange Types of Chemical Exchange Intramolecular exchange otions of sidechains in proteins – Motions of sidechains in proteins – Helix-coil transitions of nucleic acids – Unfolding of proteins A B – Conformational equilibria Intermolecular exchange – Binding of small molecules to macromolecules – Protonation/deprotonation equilibria – Isotope exchange processes – Enzyme catalyzed reactions M+L ML ecause NMR detects the molecular motion itself rather the numbers Because NMR detects the molecular motion itself, rather the numbers of molecules in different states, NMR is able to detect chemical exchange even when the system is in equilibrium
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2-state First Order Exchange B k1 g A k -1 Lifetime of state A: τ A = 1/k +1 Lifetime of state B: τ = 1/k - B 1 Use a single lifetime 1/ τ =1/ τ A + 1/ τ B = k 1 + k - +1 1
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ationale for Chemical Exchange Rationale for Chemical Exchange For slow exchange T g For fast exchange FT FT Bloch equation approach: dM AX /dt = -( Δ ω A )M AY -M AX / τ A + M BX / τ B dM BX /dt = -( Δ ω B )M BY BX / τ B + M AX / τ A
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- ate 2nd Order Exchange +L ML k +1 2 state 2nd Order Exchange [M] [L]/[ML] = k M+L k -1 K d = [M] [L]/[ML] = k -1 /k +1 10 - 0 - K d =10 3 – 10 9 M k on = k +1 ~ 10 8 M -1 s -1 (diffusion-limited) 10 - 0 - - k -1 ~ 10 1 – 10 5 s 1 Lifetime 1/ τ =1/ τ ML + 1/ τ l = k -1 (1+f ML /f L ) f L and f are the mole fractions of bound and free ligand, ML L respectively
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Typical Motion Time Scale for Physical Processes SLOW FAST sm s μ sn sp s f s very slow slow fast very fast ultrafast MACROSCOPIC DIFFUSION, FLOW CHEMICAL EXCHANGE MOLECULAR ROTATIONS MOLECULAR VIBRATIONS
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ChemicalExchangediffusion2010 - Effects of Chemical...

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