Circular dichroism and electron microscopy studies in vitro of 33-mer gliadin peptide revealed secondary structure transition and supramolecular organization.

Gliadin, a protein present in wheat, rye, and barley, undergoes incomplete enzymatic degradation during digestion, producing an immunogenic 33-mer peptide, LQLQPF(PQPQLPY)3 PQPQPF. The special features of 33-mer that provoke a break in its tolerance leading to gliadin sensitivity and celiac disease remains elusive. Herein, it is reported that 33-mer gliadin peptide was not only able to fold into polyproline II secondary structure but also depending on concentration resulted in conformational transition and self-assembly under aqueous condition, pH 7.0. A 33-mer dimer is presented as one initial possible step in the self-assembling process obtained by partial electrostatics charge distribution calculation and molecular dynamics. In addition, electron microscopy experiments revealed supramolecular organization of 33-mer into colloidal nanospheres. In the presence of 1 mM sodium citrate, 1 mM sodium borate, 1 mM sodium phosphate buffer, 15 mM NaCl, the nanospheres were stabilized, whereas in water, a linear organization and formation of fibrils were observed. It is hypothesized that the self-assembling process could be the result of the combination of hydrophobic effect, intramolecular hydrogen bonding, and electrostatic complementarity due to 33-mer's high content of proline and glutamine amino acids and its calculated nonionic amphiphilic character. Although, performed in vitro, these experiments have revealed new features of the 33-mer gliadin peptide that could represent an important and unprecedented event in the early stage of 33-mer interaction with the gut mucosa prior to onset of inflammation. Moreover, these findings may open new perspectives for the understanding and treatment of gliadin intolerance disorders.