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, [Ile(2), Ser(8,28), Ala(15), Leu(27), Hse(30)] bGRF(1-30)-NH-E thyl, acetate salt (U-90699F) was studied in solution by Fourier trans form infrared and Raman spectroscopies. Spectroscopic studies revealed that concentrated aqueous solutions of U-90699F (100 mg ml(-1)) under go a secondary structure transition from disordered coil/alpha-helix t o intermolecular beta-sheet. Disordered coil and alpha-helical structu re were grouped together in the infrared and Raman studies since the a mide I vibrations are close in frequency and overlap in assignments wa s possible. Before the conformational transition, the facile exchange of the peptide's amide hydrogens for deuterium indicated that the majo rity of amide hydrogens were readily accessible to solvent. The kineti cs of the conformational transition coincided with an increase in solu tion viscosity and turbidity. An initiation phase preceded the conform ational transition during which only minor spectral changes were obser ved by infrared spectroscopy. The initiation phase and reaction kineti cs were consistent with a highly cooperative nucleation ultimately lea ding to a network of intermolecular beta-sheet structure and gel forma tion. 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. (C) 1997 Elsevier Science B.V.