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Spectrally Undiscerned Isomers Might Lead to Erroneous Determination of Water Exchange Rates of paraCEST Eu(III) Agents
journal contribution
posted on 2017-06-22, 22:04 authored by Nevenka Cakić, Ben Tickner, Moritz Zaiss, David Esteban-Gómez, Carlos Platas-Iglesias, Goran AngelovskiWe
report a detailed study of the solution structure and water
exchange rate of a Eu(III) complex with the cyclen-based ligand L1, containing (S)-2-(2-acetamido)-3-(4-(trifluoromethyl)phenyl)propanoate pendant
arms at positions 1 and 7 of the cyclen ring and acetylglycinate pendants
at positions 4 and 10. The EuL1 complex was characterized by a combination of NMR and luminescence
spectroscopy and density functional theory (DFT) calculations. The
chemical exchange saturation transfer (CEST) spectra obtained at different
temperatures and saturation powers present a CEST signal attributed
to the coordinated water molecule. However, the spectra recorded at
low temperatures (10 °C) and low saturation powers revealed the
presence of two different species with coordinated water molecules
having very similar chemical shifts. Determination of the water exchange
rates of the coordinated water molecules was carried out by using
the Bloch four-pool model that accounts for the presence of these
isomers, and this model was compared to conventional methods for CEST
quantification, namely the Omega plot and QUESP (quantification of
exchange rate as a function of saturation power), which assume the
presence of a single CEST active species. The results indicated that
only the four-pool Bloch equations provide reasonable water exchange
rates and activation parameters. Solution NMR studies and DFT calculations
indicated that the two isomers present in solution correspond to the SS-Δ(λλλλ) and SS-Λ(δδδδ) isomers, which present capped
square-antiprismatic (SAP) coordination environments. Additional NMR
studies on the EuL2 and EuL3 complexes, which present four
(S)-2-(2-acetamido)-3-(4-(trifluoromethyl)phenyl)propanoate
or acetylglycinate pendant arms, respectively, confirm the results
obtained for EuL1.
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Bloch four-pool modelSAPAdditional NMR studiesfour-pool Bloch equationsSSacetylglycinate pendant armsEuL 3 complexesSpectrally Undiscerned Isomerssolution NMR studieswater moleculesQUESPEuL 1cyclen-based ligand L 1presencewater exchange ratessaturation powersDFTWater Exchange RatesCESTisomerschemical exchange saturation transferwater exchange rate
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