Octreotide used for probing the type-II' β-turn CD and Raman markers

Octreotide, a potent somatostatin (SST) analogue, is used as an antiproliferative drug in numerous endocrine tumors. Previous NMR investigations, basically performed in DMSO, had evidenced a type-II' β-turn structure for this cyclic peptide. However, apart a few incomplete studies by circular dichroism, a systematic analysis of the structural behavior of octreotide in aqueous solution as a function of concentration and ionic strength was still lacking. Here, we report the chemical synthesis and purification of octreotide for optical spectroscopic purposes accompanied by its structural analysis. Furthermore, we have used octreotide as a short size, well-defined model compound for analyzing the CD and Raman markers of a type-II' β-turn. CD data collected in the 25-250 μM range revealed the general trend of octreotide to undergo a disordered toward ordered structural transition upon increasing concentration. Especially, the β-turn CD markers could be characterized above 50 μM by a negative band at ~202 nm flanked by a shoulder at ~218 nm. On the basis of Raman spectra recorded as a function of concentration (1-20 mM), we could assign the markers at ~1678 and ~1650 cm(-1) in the amide I region, and at ~1303, ~1288, and ~1251 cm(-1) in the amide III region, to the type-II' β-turn structure. The stability of the intermolecular antiparallel β-sheet formed in octreotide could be confirmed by the rigidity of the disulfide bridge which adopts a preferential gauche-gauche-gauche rotamer along the -Cβ-S-S-Cβ- moiety of the linked cysteines. The present analysis permits a better understanding of the differences between the structural features of SST-14 and its routinely used analogue, octreotide.