Deciphering solid state photostability of polymyxin B using deconvolution based FTIR insights

Polymyxin B, a cyclic nonapeptide antibiotic, showing high susceptibility towards photodegradation with questionable stability at room temperature, thus necessitating a full-scale investigation of its solid-state photostability. In the present work, deconvolution-based Fourier Transform Infrared (FTIR) spectroscopy has been used to explain the structural changes of polymyxin B under room temperature (RT), UV, and visible light illumination. These measurements in the spectral values are categorized into five regions (3771-3119, 3125-2485, 1845-1419, 1419-1203, and 1205-890 cm^-1), where the calibration curve of predefined target (deconvoluted) peaks showed good linearity for chosen analytical responses. The photostability investigations were done on 1st, 2nd, 10th, 20th and 30th days. The findings depicted that the cyclo-heptapeptide fragmented at a higher rate compared to the other areas, and the most degradation happened during the UV light exposure. Overall, Degradation of Polymyxin B proceeded in second order kinetics at RT, whereas the photo-mediated degradation was relatively higher compared to RT. A Principal component analysis (PCA) was conducted to trace the nonlinear regression between the correlation of individual functional group degradation in regard of stress conditions to understand the trends and dynamics. Hydrolysis of the peptide bonds and oxidation of the remains of amino acids occurred mainly within the tri peptide and cyclo-heptapeptide zones. The fatty acyl tail, because of its hydrophobic nature, remained less vulnerable to hydrolytic reactions and more stable. Nonetheless, visible light degradation was primarily focused on the oxidation of hydroxyl groups, UV degradation reflected unique degradation pattern for peptide bonds. At room temperature, zero-order breakdown takes place in the cyclo-heptapeptide area (3048 cm^-1), and exposure to visible and UV radiation greatly enhances sensitivity. These studies highlighted the differential degradation behaviors of polymyxin B's functional regions and provided insights into its stability and environmental susceptibility under varying storage conditions.