diffusion ordered spectroscopy
DESCRIPTION
Diffusion Ordered Spectroscopy. Diffusion Ordered Spectroscopy. Provides a way to separate different compounds in a mixture based on the differing translational diffusion coefficients (differences in the size and shape of a molecule) - PowerPoint PPT PresentationTRANSCRIPT
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Diffusion Ordered Spectroscopy
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Diffusion Ordered Spectroscopy
• Provides a way to separate different compounds in a mixture based on the differing translational diffusion coefficients (differences in the size and shape of a molecule)
• Achieved by radio-frequency pulses as used in routine NMR spectroscopy and magnetic field gradients that encode spatial information
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Self-Diffusion
• Random translational motion of molecules or ions through the surrounding media driven by thermal energy (Brownian motion)
• NO thermal gradient (convection)• NO concentration gradient (mutual
diffusion)
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Diffusion Coefficient (D)
• Quantifies this motion as a measure of the rate of mean square displacement of the molecule (Units of m2s-1)
• We can measure diffusion by NMR if we can map the location of a molecule in solution and how this varies as a function of time
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Diffusion and Mass
• Diffusion relates to molecular size!
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Study of Self-Diffusion
Two steps:
1) Spatially label the nuclear spins using gradients of magnetic field
2) Monitor their displacement by measuring their spatial positions at 2 distinct times
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Refresher: NMR Basics
• larmour frequency,T2, rotating frame of reference
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How to measure diffusion coefficients?• Short period (~1ms) in which magnetic field
experienced by the NMR sample is made inhomogeneous!
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Pulse Sequence – Pulsed Field Gradient Echo
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• DOSY uses two PFG pulses separated by a diffusion time Δ
• First PFG destroys (dephases) all signals• Second PFG acts in opposition to first & may recover (rephase)
signalsIF NO MOVEMENT during Δ – FULL signal recoveredIF MOVEMENT OCCURS during Δ, signal is NOT fully rephased leading to loss of signal
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Diffusion NMR• Movement of molecules during
Δ leads to LOSS of resonance intensity
• Diffusion profile is obtained by increasing magnitude of field gradient Gz for repeated 1D experiments
• Faster molecular diffusion corresponds to faster signal attentuation as a function of Gz
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Diffusion & Magnetic Field Gradient
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Attenuation of Signal as Gz Increases
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DOSY NMR
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Stokes-Einstein
• Stokes- Einstein relation relates the Diffusion coefficient, D, of a particle to its molecular shape via a friction coefficient f (FOR SPHERE)
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Diffusion Spectra
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What can we study with DOSY?
• Analysis of Mixtures• Intra-molecular interactions• Supra and biomolecular complexes• Affinity• Chemical exchange
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Diffusion Applications• Aggregation Slower Diffusion as molecules self-aggregate• Host-guest formationBinding of small “guest” molecules within larger host leads to slower diffusion• Supramolecular chemistryAssessment of molecular size
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Complexes and Exchange
• Complexes
• Exchange
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Host-Guest Complexes
Cameron,K., Fielding, L. 2001. J. Org. Chem. 66, 6891.
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Solving for Ka – for small molecule and large Host
Cameron,K., Fielding, L. 2001. J. Org. Chem. 66, 6891.
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DOSY: Ka
• Approximations remove need to perform titrations, and Ka in principle can be derived from a single experiment.
• Assumption is sound for small molecules binding to macro(biological molecules)
• However for smaller Host-Guest chemistry – this assumption is not always true
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Host-Guest Complexes
Cameron,K., Fielding, L. 2001. J. Org. Chem. 66, 6891.
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Aggregation
• Simplest form of oligomerization is dimerization
• Two monomers come together to form a dimer
Similar to H + G HG2A A2
Kdimer = [A2]/[A]2
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DOSY-NMR analysis of ring-closing metathesis (RCM) products from β-lactam precursors
• Limitation of RCM for formation of intramolecular ring-closed products is the occurrence of side products from intermolecular oligomerization!
• Identification of reaction products is not straightforward: 1H 13C NMR data may be inconclusive because of complexity. Mass spec – inconclusive.
• DOSY is the answer!
Sliwa, A., Marchand-Brynaert, J., Luhmer, M. 2011 Magn. Reson. Chem. 49, 812.
27Sliwa, A., Marchand-Brynaert, J., Luhmer, M. 2011 Magn. Reson. Chem. 49, 812.
28Sliwa, A., Marchand-Brynaert, J., Luhmer, M. 2011 Magn. Reson. Chem. 49, 812.
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Determination of Precursors:
Sliwa, A., Marchand-Brynaert, J., Luhmer, M. 2011 Magn. Reson. Chem. 49, 812.
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Limitations• Measuring accurate diffusion constants
required a high quality gradient coil. Gradients have to be linear.
• Good temperature stability required• Assumptions of spherical shape often used –
not always accurate• 2D Transformation Errors – diffusion coefficients
should differ as much as possible from one another & Standard errors should be marginal
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Limitations
Cohen, Y., Avram, L., Frish, L., 2005. Angew. Chem. 44, 520
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In Summary: DOSY• Powerful method for the NMR analysis of
many types of mixtures• Measure diffusion coefficients which reflect
size and shape of molecular species• Applications: association constants,
investigating aggregation, encapsulation, intermolecular interactions in multi-component systems and size and structure of labile systems.
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Questions?