Coatings World - July 2012

10-10 M, or in the parts per trillion, depending on the specific metal-binding domain employed [ 8].

Two common classes of metal binding peptides are metallothioneins and phy-tochelatins [ 8-10]. Both of these are cys-teine-rich peptides that bind divalent metal ions through sulfhydryl groups (-SH). Metallothioneins are typically formed in mammals, plants and microorganisms in response to the presence of cadmium [ 11]. Related proteins that are structurally similar can be associated with metal ions like zinc, copper and cadmium [ 10]. Phy-tochelatin peptides are produced by enzymatic synthesis in fungi, algae, and some prokaryotic organisms, worms, and plants to bind heavy metals such as Ag+, As3+, Cd2+, Cu+/2+, Hg2+ Ni2+, or Zn2+.

Peptides with unique metal binding properties can either be pulled from nature, designed de novo or selected by screening libraries. His6 is an example of a peptide designed from a template in nature, and which can be used to functionalize metal binding coatings. (Fig 2) Nickel generally provides good binding efficiency to His6, but also tends to bind nonspecifically to endogenous proteins that contain histidine clusters. Cobalt ions, and even more so copper ions, exhibit a more specific interaction with histidine tags, resulting in less nonspecific interaction. The research presented here demonstrates that it is possible to selectively and reversibly bind metals to a coating containing metal binding peptides, thus granting the coating anti-fouling and anti-microbial properties.

Fig 4.

Fig 3. Correlation of stability and activity with metal.

Functionalized Coatings with
Metal Chelated Protein
It has previously been demonstrated that

proteins and short chain peptides can be used to functionalize coatings [14-

19]. In those studies, we used bio-
based molecules that were themselves
inherently antimicrobial, typically im-
pacting the cell wall or membrane of
the target microorganisms. The series
of studies presented here demonstrate
the use of biobased additives which
themselves are not inherently antimi-
crobial to create re-chargeable coat-
ings. The enzyme organophosphorus
hydrolase (OPH, E.C. 3.1.8.1) is a metal-
loenzyme in which four histidine residues
coordinate two divalent cations. Although
the identity of the divalent metal ions in
the active-center influences the activity
and stability of the enzyme (Zn2+, Co2+,
Cd2+, Ni2+ or Mn2+) all support the cat-
alytic activity of the enzyme. A two Zn2+
center has the greatest effect on stability,
while a two Co2+ center increases the ac-
tivity of the enzyme at the expense of sta-
bility. If the enzyme loses either of the

Fig 4. OPDtox hydrolysis of paraoxon shows hydrolysis rate is affected by bulk volume (film thickness). Control films had no activity, while progressive increases in film thickness of material containing the OPH enzyme demonstrated a clear dose response.