Microscopic FT-IR/DSC system used to simultaneously investigate the conversion process of protein structure in porcine stratum corneum after pretreatment with skin penetration enhancers.

A newly developed microscopic Fourier transform infrared (FT-IR) spectrometry combined with differential scanning calorimetry (DSC) was used to investigate the thermal response and IR spectral changes of protein structure in porcine stratum corneum (SC) after pretreatment with the skin penetration enhancers propylene glycol (PG), azone/PG, oleic acid (OA)/PG, vitamin C, and vitamin C+ OA/PG. The amide I and II bands of protein were used as probe to determine the structural transformation of protein with temperature. A reheating process was also performed. Dual effects of enhancer and temperature on the protein conformational changes of porcine SC were studied. The results indicate that the new FT-IR/DSC system can continuously determine the thermoresponsive conversion process from alpha-helix to beta-sheet in keratin structure of porcine SC pretreated with different enhancers. The thermally induced keratin conversion in protein structure of porcine SC, independent of pretreatment with skin penetration enhancers, was irreversible. The process of conformational transition in protein was found to be partially from alpha-helix to random coil structure or partially from alpha-helix to beta-sheet structure during heating. The kinetics of this conversion between first-heating and second-heating processes were significantly different; the process of conversion for all first-heated porcine SC samples during second-heating process were slower than that of porcine SC samples during first-heating process. Moreover, it was also found that the skin penetration enhancers, when present during the heating process, were able to synergistically and promotively alter the keratin conversion in protein structure of porcine SC, with the PG, OA/PG and azone/PG enhancers being the most effective.