It is thought that both these structures could represent an intermediate structure between the a and b forms. It is true that these numbers are still so small that it is impossible to prove anything, and this may well just be a statistical 'blip'. However, as this form is insoluble it will be very difficult to solve its structure by standard techniques. The amino acid sequence of the C-terminal part of the structure, which undergoes the transition, must be very unusual. Three types of approach can be taken to predicting secondary structure.
Some approaches look at the chemical structures of the individual amino acids.
Others rely on the fact that homologous proteins have very similar secondary structures, and can only be used when the structure of at least one homologous protein is known. Yet others examine the statistical probability of finding a particular amino acid sequence in a given structure type. Most often, now, prediction methods such as Jpred2 build up a consensus prediction using several different techniques. Unusually for a protein with a very high helical content, the prion protein contains a high proportion of residues with a strong propensity to form b structure. The results for the C-terminal domain are inconclusive, with different methods predicting different structures for the same segments.