Peptides are short chains of amino acids with a generic chemical formula H2NCHRCOOH which has the chemical structure shown below. The R is an organic “side chain” which distinguishes one amino acid from another. There are 20 naturally occurring amino acids.
Peptides are organic molecules but they interact in many ways with inorganic molecules. There is a large amount of research on using peptides in templates for the growth of nano-size semiconductor devices. This requires peptides that bind specifically to semiconductor materials, some of which are sulfides (e.g., zinc sulfide).
If there are peptides that can bind to zinc sulfide (or sphalerite), then perhaps there are peptides that can bind to other minerals of economic interest. If such “binders” exist, then perhaps they could afford a method of mineral identification or a highly selective method for mineral separation.
A collaboration between Dr. Scott Dunbar of Mining Engineering and Drs. Ross MacGillivray and Susan Curtis of the Department of Biochemistry at UBC was formed. Here they are. (Scott does not usually wear a tie.)
The purpose of the collaboration is to investigate the possibility that particular amino acids would bind specifically to minerals of economic interest and to find ways in which such binding could be used in mineral separation or identification.
A cartoon picture of binding peptides on a small mineral crystal is shown below.
We have identified a peptide with 7 amino acids that binds specifically to sphalerite. (Curtis et al, 2009) We tagged this peptide with a red fluorescent molecule and then introduced a solution containing the peptide to a mixture of silica and sphalerite particles mostly 45 microns in size. Shown below is a view through a microscope (200 x magnification) of the particle mixture after the peptide mixture was added. The fluorescence is clearly visible on the black sphalerite particles, but not the clear silica particles.
We also identified an 8 amino acid peptide binder for chalcopyrite which was tagged with a red fluorescent molecule. A solution containing the tagged peptide was introduced to a mixture of chalcopyrite particles. Uniform fluorescence is clearly visible on the chalcopyrite particles.
The peptide binds to the chalcopyrite particle and, being polymers, the peptides bind to each other. This causes agglomeration of the chalcopyrite particles as shown below.
Control – no binder With 1012 binders/ml
However, the binder does not cause agglomeration of silica particles as shown below. This demonstrates its specificity.
Control – no binder With 1012 binders/ml
Delivering peptides to mineral particles in slurry
Having identified binders, the problem is how to scale up the process to beaker size or greater and find a way to use the specificity of the peptides to separate minerals. There is a number of possibilities.
The peptides could be displayed on a bacteriophage particle. Bacteriophage are viruses that infect bacteria (and nothing else). Shown below is a phage particle with the binders displayed on its tail protein coating. We have found that a coating of phage makes the mineral particles more hydrophobic, likely because some parts of the phage particle are naturally hydrophobic. (See Curtis et al, 2011) This might be used as a means of separation. The idea would be to introduce a solution of such phage to a slurry and use the increase in hydrophobicity to separate the minerals of interest by a flotation method.
Such a scheme has been tested in at bench scale by Liao (2010) to separate the mineral francolite from dolomite (a contaminant) in phosphate ore. A francolite specific binder was displayed on a phage particle. As shown below, the binder attached to the francolite particles (F) and the hydrophobic domains of the phage particle attached to an air bubble. The recovery of francolite from the slurry of francolite and dolomite particles was about 60%.
Something similar could be done by changing the genetic machinery of a bacterium such as E coli to grow the binder on its outer protein coat as shown below. Once binding occurs, the coating of bacteria might change the surface properties of the mineral enough to allow separation. One problem with this is that, although the peptides would be quickly produced as the bacteria reproduce, the bacteria would have to be kept alive.
Yet another scheme is to attach the binders to a polymer scaffold – perhaps along with reagents to decompose the mineral of interest. Once binding occurs, the reagents would be released. This idea is a direct “steal” from targeted drug delivery technology. We are not sure if it would work.
Probably the most promising scheme is to display the binders on magnetic nanoparticles (aka “functionalize” the nanoparticles) as shown below. Once the binders attach to the mineral particles of interest, a magnet would be used to separate them.
We are working on this now and will get back with any results.
Curtis SB, Hewitt J, MacGillivray RTA, and Dunbar WS, 2009. Biomining with bacteriophage: Selectivity of displayed peptides for naturally occurring sphalerite and chalcopyrite. Biotechnology and Bioengineering 102:644–650.
Curtis SB, MacGillivray RTA, and Dunbar WS, 2011. Effects of bacteriophage on the surface properties of chalcopyrite (CuFeS2), and phage-induced flocculation of chalcopyrite, glacial till, and oil sands tailings. Biotechnology and Bioengineering, 108(7):1579-1590
Liao, C-H. 2010. Inorganic Binding Peptides Designed by Phage Display Techniques for Biotechnology Applications. Unpublished PhD thesis, University of Florida, Gainesville, FL.