Discovering Small Peptides with High Affinity to Montmorillonite Using Phage Display Library Technology.

The latest terrestrial N cycle paradigm considers soluble organic nitrogen (SON) as a rate limiting step for regulating the overall N availability in the ecosystem. Soluble organic nitrogen is comprised of free amino acids and amino-N compounds such as peptides and small proteins accounting for  up to  30-91% of the pool. However, little is known about the relative contribution of the peptides and proteins to SON as well as the specific components involved, even though it is suspected that there is a vast number of different species in this pool. The organo-mineral interactions play an important role affecting the N flux between the SON and insoluble organic N pools thus impacting  bioavailability of N to microorganisms and plants. However, the exact nature of these interactions remains unclear. We hypothesized that specific small peptides in the SON pools have high affinity for certain soil minerals, that in turn, affects the stability and bioavailability of the adsorbed peptides. To test our hypothesis, we use the phage library display technique which allows the screening of billions of random peptide sequences with a selective affinity for the mineral surface following  an hour of incubation. We successfully screened and identified 21 peptides (12-mer) which exhibit affinity to montmorillionite. Initial examination of the properties of these peptides indicated that ~ 50% of peptide residues are small amino acids ( e.g. threonine, serine, prolline). Non-polar and polar residues were noted to be 30-40% more abundant compared to acidic and basic residues. Uncharged polar residues were mostly enriched on peptides which are implicated to have highest affinity with the mineral. We speculate that  presence of polar amino acids may have formed hydrogen bonds between the peptides and mineral surfaces while nonpolar amino acids may have packed  against the inorganic surface reducing exposure to water. These mechanisms may have  established high affinity between the peptides and the mineral. The results  suggest considerable discrimination of particular peptide sequences to soil minerals and thus clues to the mechanisms of organic matter sequestration in soils. Research on the various soil minerals, with and with organic matter and different pH’s will be studied to further identify the nature of mineral-binding properties of this important pool of organic matter and N in soil..