What is gluten? First are the gliadins which are soluble in alcohol/water solutions. The second group are the glutenins which are not soluble in alcohol/water solutions, but are soluble in some salt solutions.

The gliadin proteins are separated into Alpha, Beta and Gamma gliadins which contain intramolecular disulfide bonds. These bonds link one part of a polypeptide chain to another. The Omega proteins do not contain disulfide bonds which means that they do not contain cysteine (or probably methionine) in their primary structure. (see pp 638-639, Skerritt, Devery and Hill, "Gluten Intolerance: Chemistry, Celiac Toxicity, and Detection of Prolamins in Food," The American Association of Cereal Chemists, July 1990). Thus, the Alpha, Beta, Gamma and Omega gliadins are defined as toxic to celiacs.

There are amino acid sequences that have been found to be toxic in the Alpha, Beta, Gamma and Omega gliadins: Pro-Ser-Gin-Gin and Gin-Gin-Gin-Pro. [Pro-proline, Gin-glutamine, Ser-serine]. See the following table as referencing these details in an outline format.

Note: Rice does not contain any similar amino acid sequences.
[Pro=proline, Gin=glutamine, Ser=serine, Val=valine, Cys=cysteine.]

Sources:

John Skerritt, Jannine Devery, and Amanda Hill, "Gluten Intolerance: Chemistry, Celiac Toxicity, and Detection of Prolamins in Food," The American Association of Cereal Chemists, July 1990, 35:638-639.

G.L. Mantzais, et al, "Cellular Hypersensitivity to a Synthetic Dodecapeptide Derived From Human Adenovirus 12 which Resembles a Sequence of A-gliadin in Patients with Coeliac Disease," Scandinavian Journal of Gastroenterology, 1991, 26:393-398.

Michael N. Marsh, "Celiac Sprue: Gluten, MHC, and Intestinal Immunopathy," New England Journal of Medicine, Dec 12, 1991, 102:333.

Donald Kasarda, "Toxic Cereal Grains in Coeliac Disease," unpublished paper, 6.