Department of Chemistry, University of San Francisco, San Francisco, CA 94118
Matthew Derfus and Dr. Osasere Evbuomwan.
Towards the Development of Redox-Responsive Eu(III) Complexes for
Cancer Imaging
Figure 1. An example of the difference in
contrast between Eu(II) and Eu(III)-based
contrast agents.
3
Methods and Materials
Figure 2. Structures of the compounds used in this study
(glycine, aspartate, lysine and tyrosine).
Ligand Synthesis: All reagents and solvents were purchased from
commercial vendors. Cyclen was tetra-alkylated with the corresponding
protected chlorinated amino acid in the presence of base. The protected
ligands were purified by flash chromatography before being deprotected by
acid or base hydrolysis. All ligand identities were verified by
1
H NMR.
Metal Complexation: All ligands were complexed with an equimolar
amount of EuCl
3
solution at pH 5.5–6.0. Excess metal presence was
determined by Xylenol Orange test.
Cyclic Voltammetry Studies: Cyclic voltammograms were acquired with a
Pine Research WaveNow Potentiostat using a glassy carbon working
electrode, platinum counter electrode and Ag/AgCl reference electrode. The
supporting electrolyte for all samples was 100 mM KCl. The pH was
maintained using MES (5.5 & 6.5) and TRIS (7.5 & 8.5) buffers.
Results and Discussion
-2.0 -1.5 -1.0 -0.5 0.0 0.5
-0.00010
-0.00005
0.00000
0.00005
0.00010
Eu(Gly)
4
pH 7.5
Potential (V) vs Ag/AgCl
Current (A)
Figure 3. Cyclic voltammograms of the Eu(III) complexes with respective sidearms. Taken at a scan rate of 100 mv s
-1
.
E
1/2
Data
▪ Complexes with the glycine and lysine sidearms demonstrated a constant E
1/2
over the pH range 5.5 – 8.5.
▪ Eu(Gly)
4
was found to have an average midpoint of -1.00 ± 0.01 V.
▪ Eu(Lys)
4
was found to have an average midpoint of -1.03 ± 0.005 V.
▪ The E
1/2
of the aspartate and tyrosine complexes demonstrated pH dependence.
▪ Eu(Asp)
4
demonstrated a 250 mV increase in Eu(II) stability at pH 5.5 relative to 8.5.
▪ Eu(Tyr)
4
demonstrated a 70 mV increase in Eu(II) stability at pH 5.5 relative to 8.5.
Discussion
▪ The pH dependent redox properties found in Eu(Asp)
4
and Eu(Tyr)
4
suggest that changes in the protonation state of their
R-groups affect the ability of the europium ion to accept or donate electrons.
▪ Conversely, the pH independent redox properties found in Eu(Lys)
4
and Eu(Gly)
4
suggest these amino acid R-groups do
not significantly affect the redox properties of the coordinated metal. In Eu(Gly)
4
this is likely because there is no R
group protonation occurring, and in Eu(Lys)
4
we hypothesize there may be too large a distance between the amine and
metal for any R-group protonation to affect the redox properties of the metal ion.
-2.0 -1.5 -1.0 -0.5 0.0
-0.00008
-0.00006
-0.00004
-0.00002
0.00000
0.00002
Eu(Lys)
4
pH 7.5
Potential (V) vs Ag/AgCl
Current (A)
-2.0 -1.5 -1.0 -0.5 0.0
-0.00008
-0.00006
-0.00004
-0.00002
0.00000
0.00002
0.00004
Eu(Asp)
4
pH 7.5
Potential (V) vs Ag/AgCl
Current (A)
-1.5 -1.0 -0.5 0.0 0.5
-0.000006
-0.000004
-0.000002
0.000000
0.000002
Eu(Tyr)
4
pH 7.5
Potential (V) vs Ag/AgCl
Current (A)
Table 1. Electrochemical data over pH 5.5 – 8.5 range.
Conclusion
▪ R-groups in Eu(Asp)
4
and Eu(Tyr)
4
appear to induce a
dependence of the redox potentials on pH. This is
probably due to closer proximity of these groups to the
metal, and their possession of pK
a
values within the pH
range studied.
▪ Eu(Asp)
4
displayed the most redox sensitivity to pH over
the pH 5.5 – 8.5 range.
Acknowledgements
1. Schafer, F. Q., & Buettner, G. R. (2001). Redox environment of
the cell as viewed through the redox state of the glutathione
disulfide/glutathione couple. Free Radical Biology and Medicine.
2. Hegedűs, C., Kovács, K., Polgár, Z., Regdon, Z., Szabó, É.,
Robaszkiewicz, A., Virág, L. (2018). Redox control of cancer cell
destruction. Redox Biology.
3. Funk, A. M., Clavijo Jordan, V., Sherry, A. D., Ratnakar, S. J., &
Kovacs, Z. (2016). Oxidative Conversion of a Europium(II)-
Based T
1
Agent into a Europium(III)-Based paraCEST Agent that
can be Detected in Vivo by Magnetic Resonance Imaging.
Angewandte Chemie - International Edition.
▪ Perform electrochemical investigations over an expanded
pH range to encompass pK
a
s of all amino acids.
▪ Synthesize additional complexes with different amino
acids and evaluate their redox properties.
▪ We would like to acknowledge funding from the USF
Faculty Development Fund.
▪ We would like to thank Dr. West for use of his lab and
help with electrochemical analyses.
▪ We would also like to thank the Faculty and Staff of the
Chemistry Department at USF for their support.
Future Work
▪ Redox conditions within the cell are the result of homeostatic maintenance of numerous redox couples including GSSG/2GSH,
NAD
+
/NADH, and NADP
+
/NADPH, all together to sustain a redox potential around -200 mV.
1
▪ Cancer cells display redox dysregulation and lowered cytosolic pH as a means to maintain their over-proliferation.
2
▪ The development of an approach to probe tissue redox state could potentially allow enhanced discrimination between healthy and
cancerous tissues.
▪ MRI yields high-quality images without the use of ionizing radiation making it an attractive option for the diagnosis of numerous cancers.
▪ The quality of an MR image can be enhanced with the aid of contrast agents, the majority of which are Gd(III) complexes that provide a
brightened image via T
1
weighted imaging.
▪ The reduction potential (E
1/2
) of the Eu(II)/Eu(III) redox couple (-600 mV) has drawn
attention for its relative proximity to biological redox conditions (-200 mV) and
applications in MRI contrast agent design.
▪ The Eu(II) oxidation state has an identical electron configuration to Gd(III) and
provides similar image brightening T
1
MRI properties.
▪ Eu(III) complexes provide darkened images or negative MRI contrast through a
different MR imaging mechanism called PARACEST.
▪ However, most ligands used for Eu(III)-based PARACEST agents strongly stabilize the
Eu(III) and as a result Eu(II) isn’t present in substantial concentrations when these
complexes are in an aqueous environment.
Project Goals
▪ The goal of this project is to synthesize a library of Eu(III) complexes with ligands
containing various amino acid R groups and investigate the effects of amino acid
charge, electron density, and pK
a
on Eu(II)/Eu(III) redox potentials and stabilities by
cyclic voltammetry.
References
▪ We will also investigate the response to pH of these properties over a pH range of 5.5-8.5.
Introduction and Project Goal
E = E
1/2
