Protein conformational stability in the hydrofluoroalkane propellants tetrafluoroethane and heptafluoropropane analysed by Fourier transform Raman spectroscopy

Due to the inherent structural instability of proteins, development of chlo rofluorocarbon (CFC) free metered dose inhalers (MDIs) containing these bio molecules is beset with numerous challenges. In assessing the conformation of proteins in any medium, both secondary and tertiary structures need to b e elucidated. This study uses Fourier transform (FT-) Raman spectroscopy to probe protein conformational stability in hydrofluoroalkane (HFA) propella nts. Assignments of molecular modes for lysozyme as a solid and in aqueous solution, and for the first time, HFAs tetrafluoroethane (HFA 134a) and hep tafluoropropane (HFA 227) are given. The Raman spectra provided molecular s tructural information on the peptide backbone, disulfide bonds and C-C stre tching vibrations of hen egg lysozyme, enabling the secondary conformation of protein in HFA propellants to be determined; structural integrity of thi s robust model protein was maintained in both propellants. These results de monstrate that FT-Raman may be a useful tool for the optimisation of protei n MDI formulations. (C) 1999 Elsevier Science B.V. All rights reserved.