A New Reaction for Improved Calibration of EPR Rapid-Freeze Quench Times: Kinetics of Ethylene Diamine Tetraacetate (EDTA) Transfer from Calcium(II) to Copper(II)

Abigail L. Schroeter; Hao Yang; Christopher D. James; Brian M. Hoffman; Peter E. Doan

doi:10.1007/s00723-021-01448-6

A New Reaction for Improved Calibration of EPR Rapid-Freeze Quench Times: Kinetics of Ethylene Diamine Tetraacetate (EDTA) Transfer from Calcium(II) to Copper(II)

Abigail L. Schroeter, Hao Yang, Christopher D. James, Brian M. Hoffman, Peter E. Doan^*

^*Corresponding author for this work

Chemistry

Research output: Contribution to journal › Article › peer-review

Abstract

The kinetics of the transfer of the chelate, ethylenediamine tetraacetate (EDTA), from Calcium(II) to Copper(II) in imidazole (Im) buffers near neutral pH, corresponding to the conversion, [Cu(II)Im₄]²⁺→ [Cu(II)EDTA]²⁻, are characterized with stopped-flow absorption spectroscopy and implemented as a tool for calibrating the interval between mixing and freezing, the freeze-quench time (t_Q), of a rapid freeze-quench (RFQ) apparatus. The kinetics of this reaction are characterized by monitoring changes in UV–visible spectra (300 nm) due to changes in the charge-transfer band associated with the Cu²⁺ ions upon EDTA binding. Stopped-flow measurements show that the rates of conversion of the Cu²⁺ ions exhibit exponential kinetics on millisecond time scales at pH values less than 6.8. In parallel, we have developed a simple but precise method to quantitate the speciation of frozen solution mixtures of [Cu(II)(EDTA)]²⁻ and tetraimidazole Cu(II) ([Cu(Im)₄]²⁺) in X-band EPR spectra. The results are implemented in a simple high-precision ‘recipe’ for determining t_Q. These procedures are more accurate and precise than the venerable reaction of aquometmyoglobin with azide for calibrating RFQ apparatus, with the benefit of avoiding high-concentrations of toxic azide solutions.

Original language	English (US)
Pages (from-to)	1195-1210
Number of pages	16
Journal	Applied Magnetic Resonance
Volume	53
Issue number	7-9
DOIs	https://doi.org/10.1007/s00723-021-01448-6
State	Published - Sep 2022

Funding

We dedicate this report to Klaus Moebius and Kev Salikhov on the occasion of their 85th birthdays. As inspired by the first part of a two-part toast by Kev, may they live to be 120! The authors gratefully acknowledge Professor Amy Rosenzweig (Northwestern University) and Dr. Grace Kenney (Northwestern University) for access to and training on the stopped-flow instrument. Financial support was provided by the National Institute of Health (NIH) grant GM111097 to B.M.H and National Science Foundation (NSF) grant MCB1908587 to B.M.H.

ASJC Scopus subject areas

Atomic and Molecular Physics, and Optics

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10.1007/s00723-021-01448-6

Cite this

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title = "A New Reaction for Improved Calibration of EPR Rapid-Freeze Quench Times: Kinetics of Ethylene Diamine Tetraacetate (EDTA) Transfer from Calcium(II) to Copper(II)",

abstract = "The kinetics of the transfer of the chelate, ethylenediamine tetraacetate (EDTA), from Calcium(II) to Copper(II) in imidazole (Im) buffers near neutral pH, corresponding to the conversion, [Cu(II)Im4]2+→ [Cu(II)EDTA]2−, are characterized with stopped-flow absorption spectroscopy and implemented as a tool for calibrating the interval between mixing and freezing, the freeze-quench time (tQ), of a rapid freeze-quench (RFQ) apparatus. The kinetics of this reaction are characterized by monitoring changes in UV–visible spectra (300 nm) due to changes in the charge-transfer band associated with the Cu2+ ions upon EDTA binding. Stopped-flow measurements show that the rates of conversion of the Cu2+ ions exhibit exponential kinetics on millisecond time scales at pH values less than 6.8. In parallel, we have developed a simple but precise method to quantitate the speciation of frozen solution mixtures of [Cu(II)(EDTA)]2− and tetraimidazole Cu(II) ([Cu(Im)4]2+) in X-band EPR spectra. The results are implemented in a simple high-precision {\textquoteleft}recipe{\textquoteright} for determining tQ. These procedures are more accurate and precise than the venerable reaction of aquometmyoglobin with azide for calibrating RFQ apparatus, with the benefit of avoiding high-concentrations of toxic azide solutions.",

author = "Schroeter, {Abigail L.} and Hao Yang and James, {Christopher D.} and Hoffman, {Brian M.} and Doan, {Peter E.}",

note = "Publisher Copyright: {\textcopyright} 2021, The Author(s), under exclusive licence to Springer-Verlag GmbH Austria, part of Springer Nature.",

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A New Reaction for Improved Calibration of EPR Rapid-Freeze Quench Times: Kinetics of Ethylene Diamine Tetraacetate (EDTA) Transfer from Calcium(II) to Copper(II)

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