Preliminary studies of the physical stability of a glucagon-like peptide-1 derivate in the presence of metal ions

The physical stability and the secondary structure of a glucagon-like peptide-1 derivative were investigated in the presence of the metal ions Al3+, Zn2+, Mg2+, and K+, known as possible leachables from container-closure systems. Metal ions were investigated in concentrations of 0–50 ppm. Test solutions of the peptide were exposed to elevated temperature (25 °C) and rotation (37 °C) for up to 4 weeks. The samples were examined by nephelometry, thioflavine T fluorescence, and Fourier-transform infrared spectroscopy. Readily prepared test solutions were examined by tryptophan fluorescence. The stability profiles were unchanged after addition of Mg2+ and K+ in 0–50 ppm concentrations. However, a concentration-dependent increase in thioflavine intensities was observed after addition of Al3+ and Zn2+. The destabilising effect of Al3+ and Zn2+ was furthermore confirmed by FTIR as the secondary structure of the peptide changed from predominantly α-helix to a higher β-sheet content. Additionally Al3+ changed the secondary structure of the peptide using Trp fluorescence.