Raman spectroscopy of homologous plant toxins: crambin and .alpha.1- and .beta.-purothionin secondary structures, disulfide conformation, and tyrosine environment

Abstract

The Raman spectrum of crambin crystals is different from the spectrum of crambin in solution. The amide I spectrum of crambin in solution is not different from the solution spectra of proteins homologous (> 40%) with crambin, .alpha.1- and .beta.-purothionins. We have two interpretations of these results. One is that helical segments in crambin and the purothionins in solution are more irregular than those in crystalline crambin. Comparative analyses of amide I and amide III spectra, and of the conformational preferences of the amino acid sequences of these proteins, are consistent with this interpretation. The other is that, due to the way helical segments in crambin are stacked end on end along the same axis in the crystal, transition dipole coupling along the axis of these extended helixes enhances the amide I intensity of helical residues. On the basis of a combined Raman and sequence conformational analysis, the structure of the purothionins is proposed to be the same as that of crambin in solution and residues 7-12 in crystalline crambin are somewhat more regular and ordered than they are in solutions.