Single-crystal raman spectra of native insulin: Structures of insulin fibrils, glucagon fibrils, and intact calf lens

The Raman spectra of native insulin were reinvestigated employing perfectly shaped single crystals. The spectra in the 280- to 1800-cm −1 region show more detail than those of crystalline powder presented previously. Comparison of the singlecrystal and powder spectra obtained under the same instrumental conditions reveals the existence of some meaningful spectral differences. The most apparent discrepancies appear in the amide III backbone region and in the regions near 299, 514, 667, 731, 950, 1330, and 1610 cm −1. These differences can be traced to the effect of dehydration caused by the laser beam on the powder surface. We also report a single-crystal Raman spectrum of deuterated insulin from which the contribution of side-group vibrations in the amide III region was determined. The true amide III contour of native insulin was obtained by graphical subtraction. In addition, we compare the Raman spectra in the amides I and III regions of insulin fibrils, glucagon fibrils, and intact calf lens, and conclude that these biological materials may contain predominantly antiparallel β structures.