Infrared spectroscopy of human amyloid fibrils and immunogolbulin proteins

The presence of the antiparallel‐β‐pleated sheet coformation io isolated human amyloid protein fibrils has been confirmed by infrared spectroscopy. In most amyloid samples, this conformation was enhanced by acidic solution conditions. Infrared spectroscopy (Amide I and Amide V absorption bands) and x‐ray diffraction methods were also used to examine the immunoglobulin molecule for solid state‐β‐structure. It was found that both heavy chains and Bence Jones proteins exhibited some β‐pleated sheet content upon acid and/or heat treatment. Furthermore, pepsin digests comprising either the variable‐rich region (Fd′) of the immunloglobulin heavy chain or in particular, filamentous variable segments of κ and λ Bence Jones proteins were, as isolated, very similar to amyloid in β‐structure content. Data from other immunoglobulin‐derived sample did not exhibit extensive β‐pleated sheet content. On the other hand, most amyliod and immunoglobulin‐derived samples did display some β‐structure when cast from 50% HCOOH solution. Under these conditions, however, filamentous light chain‐variable segments exhibited well‐defined infrared patterns rich in antiparallel‐β‐pleated sheet structure and gave a “cross‐β” x‐ray diffraction pattern.