Spectroscopic studies on the conformational transitions of a bovine growth hormone releasing factor analog

The secondary structure of the bovine growth hormone releasing factor analog, [Ile2, Ser8.28, Ala15, Leu27, Hse30] bGRF(1-30)-NH-Ethyl, acetate salt (U-90699F) was studied in solution by Fourier transform infrared and Raman spectroscopies. Spectroscopic studies revealed that concentrated aqueous solutions of U-90699F (100 mg ml-1) undergo a secondary structure transition from disordered coil/alpha-helix to intermolecular beta-sheet. Disordered coil and alpha-helical structure were grouped together in the infrared and Raman studies since the amide I vibrations are close in frequency and overlap in assignments was possible. Before the conformational transition, the facile exchange of the peptide's amide hydrogens for deuterium indicated that the majority of amide hydrogens were readily accessible to solvent. The kinetics of the conformational transition coincided with an increase in solution viscosity and turbidity. An initiation phase preceded the conformational transition during which only minor spectral changes were observed by infrared spectroscopy. The initiation phase and reaction kinetics were consistent with a highly cooperative nucleation ultimately leading to a network of intermolecular beta-sheet structure and gel formation. Increased temperature accelerated the conformational transition. The conformational transition was thermally irreversible but the beta-sheet structure of aggregated or gelled peptide could be disrupted by dilution and agitation.