Developing luminescent probes to determine levels of environmentally relevant cations and reactive oxygen species.
1/ Environmentally relevant cations
High concentrations of Zn2+ and Hg2+ an cause severe environmental problems in our waterways. Quantification of both of these cations in environmental samples is problematic. For Zn2+, conventional testing measures the total concentration of zinc, rather than the bioavailable Zn2+. There is also no straightforward way of measuring the concentration of Hg2+ in environmental samples. Recently we have used novel, luminescent based chemosensors to detect toxic concentrations of Zn2+ in environmental samples. Our lanthanide based chemosensors have an organic chelate and an 'ntenna (Fig. 1). When the analyte of interest binds, and a time delay is introduced, background fluorescence and scattered light decay become negligible. The resulting luminescent signal is dependent only on the concentration of the luminescent probe and the target ion. Our previously synthesized sensors show good selectivity for Zn2+ over other cations,1 however, their luminescent properties and selectivity need improvement. We also wish to apply this class of sensors to the detection of Hg2+ in environmental samples.
This project will involve the synthesis and testing of this exciting new class of luminescent probes with the goal of determining the levels of Hg2+ and Zn2+ in several Melbourne wetlands.
Figure 1 The action of lanthanide based sensors, this diagram depicts the binding of Zn2+.
1. Margaret Aulsebrook, Honours thesis, Monash University, 2013.
2/ Reactive Oxygen Species
Reactive oxygen species (ROS) are present in low concentrations in seawater and it has been reported that a number of algal species can increase their production of ROS under stressful conditions. ROS include; hydrogen peroxide, singlet oxygen, superoxide, hydroxyl radicals and hypochlorite. We are looking at developing a lanthanide based sensor for ROS. This lanthanide sensor contains an organic chelate, an antenna as well as a quencher group. Once the lanthanide sensor reacts with the ROS the covalent bond between the quencher group and the antenna will cleave and a luminescent signal will result.
This project will synthesize the required probes and test them in mock systems before using the probe to detect ROS in Melbourne seawater.
BCL-XL is pro-survival protein overexpressed in many types of cancer, and it protects transformed cells from apoptosis. Natural pro-apoptotic proteins interact with BCL-XL via an α-helical peptide sequence known as the BH3 domain to trigger an apoptotic cascade, leading to cell death. We have developed facile modular synthetic approaches to pyrimidine-based scaffolds such as the phenyl-pyrimidine-oxadiazole 1 (Fig. 2b), and established that this particular scaffold has an appropriate spatial arrangement of substituents to mimic the i, i+3 and i+7 positions of an α-helix (Fig. 2c). This honours project will involve the synthesis of novel scaffolds with the key amino acid residues to mimic the BH3 domain of appropriate pro-apoptotic proteins. This strategy therefore holds great promise as a means of enabling the discovery of new non-peptidic BCL-XL antagonists as potential cancer therapeutics.
This project will synthesize novel compounds which will then be tested for activity.
Figure 2: a) polyalanine α-helix showing i, i+3 and i+7 R-group distances; b) energetically minimised triaryl scaffold showing R-group distances; c) minimised triaryl scaffold overlayed onto a polyalanine α-helix.
Figure 3 Image of the backbone of ω-conotoxin GVIA (yellow) with main amino acid side chains thought
This project involves further development of peptide mimics inspired by the cone shell toxin, ω-conotoxin GVIA (Fig. 3), a small cystine knot which binds tightly to neuronal N-type calcium channels. GVIA has non-addictive analgesic properties but is not suitable for use as a drug because of its very strong binding to the calcium channel. In addition, it suffers from the usual unfavorable characteristics associated with peptides such as low bio-availability. Peptide mimics have been designed with the aid of molecular modeling and aim to simulate the way key amino acid side chains of GVIA are presented to the receptor. First and second generation compounds have been tested and show very promising activity. We are now involved in investigating benzimidazole mimetics to improve activity.
Honours projects in this area will involve the synthesis of analogues of active compounds and to probe their binding modes to N-type calcium channels. Analogues prepared by honours students will be tested for biological activity.
For useful background information see: Andersson, A., Baell, J. B., Duggan, P. J., Graham, J. E., Lewis, R. J., Lumsden, N. G., Tranberg, C. E., Tuck, K. L., Yang, A. Bioorg. Med. Chem.2009, 17, 6659–6670 and Tranberg, C., Yang, A., Vetter, I., McArthur, J.R., Baell, J.B., Lewis, R.J., Tuck, K.L., Duggan, P., Marine Drugs2012, 10, 2349-2368.