A retroprotein has exactly the same sequence as a naturally-occurring protein, except that the direction in which the polypeptide backbone runs through the protein is exactly the opposite of what it was in the first place. In this paper, I re-explain the concepts already laid out in a paper in an earlier paper in FEBS Letters, and elucidate the dihedral angle transformations involved. I point out that phi would become –phi in an inverso transformation. Phi would become – psi in a retro transformation. And, finally, phi would become psi in a combined retro-inverso transformation. With an alpha helix, a retro-transformation would place the dihedral angles across the origin of the Ramachandran diagram, and I argued that this would be unlikely for a sequence of D-amino acids. However, with a beta sheet, the phi-psi transformation from phi to –psi, and psi to –phi, would retain the dihedral angles within the region defining the beta sheet structure in the left-top quadrant of the Ramachandran diagram, allowing a beta sheet to remain a beta sheet even after a retro-transformation. Thus, a retroprotein of a beta sheet-based protein would retain the tendency to form a beta sheet. It would need to be examined whether this beta sheet would be a mirror-image or just a new structural format.

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