Washington, DC, 14 April 2011- How amino acids attach to mineral surfaces is important for understanding bioadhesion, biomineralization, solar cell development and the origin of life. A study by Geophysical Laboratory scientists sheds new light on this important interaction.

Bioadhesion can occur through proteins that contain a particular type of amino acid called dihydroxyphenylalanine (DOPA). Closely related molecules are of considerable interest in the development of solar cells involving titanium dioxide and for biomedical imaging of nanoparticles. The extent and mechanisms of DOPA adsorption on oxides in salt solutions have not been measured. Nor was the dependence on environmental conditions such as pH and salt concentrations understood.

Salima Bahri, an intern from Barnard College, performed the experiments at the Geophysical Laboratory with Dimitri Sverjensky, Robert Hazen, Caroline Jonsson, Christopher Jonsson, and David Azzolini of Johns Hopkins University. They measured DOPA adsorption on well-characterized rutile (α-TiO2) particles over a range of pH, salt concentration and amounts of DOPA.  An electric double-layer model enabled them to propose the chemical reactions responsible for the attachment of the DOPA. This attachment of mussels to rocks is of great interest in the development of new artificial adhesives and anti-fouling materials. The theoretical model also permits predictions for environmental conditions that are difficult to achieve experimentally and for more complex systems not yet measured.

The authors found that DOPA attaches in two different ways on rutile, the relative amounts of each varying with changes in environmental conditions such as pH.  At pH < 4.5 a species involving four attachment points ("lying down") is important, whereas at pH > 4.5 a species involving only two attachment points via phenolic oxygen's ("standing up") predominates.
 
The authors used evidence for strong attachment of DOPA to titanium dioxide from single molecule atomic force microscopy and previous work on the adsorption of related molecules, they then suggested that one or more of the DOPA attachments for the two surface species involves a strong linkage of the O from the DOPA directly to the Ti of the rutile (inner-sphere type), whereas the others are likely to be weaker attachments (H-bond type).  The existence of the two surface species being tested at the Geophysical Laboratory using surface enhanced Raman spectroscopy with Andrew Steele and Namhey Lee.

The work is important for bioadhesion because the “standing up" species is expected to be important when the DOPA is part of a protein.  The new study has demonstrated the specific environmental conditions under which this species might be important.  Also, because of the suggested way that the DOPA is attached (i.e. one strong bond and one weak one), it may be able to "walk" across the rutile surface in water in a way analogous to that recently suggested in the literature for the simpler molecule catechol in vacuum and this might help the development of reversible adhesives in water further study see Lee H. et al., Proc. Natl. Acad. Sci. 103, 12999-12003 (2006).

This study can be found at: S. Bahri et al., Environ. Sci. Technol.10,1021, (2011).

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