Hydration of human nails investigated by NIR-FT-Raman spectroscopy

Abstract The human nail, although it is usually stable against outer influences, becomes soft and flexible after soaking in water. Frequent washing increases brittleness of nails. Hydration of nails is thought to be the most important factor influencing the physical properties of nails and possibly acts through changes in keratin structure. Here NIR-FT-Raman has been used to examine molecular structural changes of intact moisten nails. Raman spectra were obtained both in vitro from nail samples and in vivo before and after soaking in water. The water uptake of normal nail samples during the first 15 min was reflected in the increasing intensity ratio of the ν (OH)/ ν (CH 2 ) bands. A saturating effect appeared soon after 10 min which is explained by a defined water holding capacity. R( ν ) representation of the low frequency range of the Raman spectra showed that mainly bound water is found both in dry and in wet nails. This implies water–protein interaction. Protein backbone vibration at 932 cm −1 indicating α-helical proteins increased in intensity in the wet nails. The ν (S–S) which is sensitive to changes in conformation of proteins showed a 4% higher intensity. Additional protein–water interactions could lead to a slight change of the dihedral angle of the C–S–S–C bonds and to geometric changes in coiling behavior of the α-helical protein. Suggesting a separation between matrix proteins and fiber proteins giving them a greater freedom of flexibility. The in vivo spectra detected from the distal part of the nail resembled spectra in vitro. Raman spectra of the proximal part of the nail showed that it was fully saturated with water. The proximal part of the nail did not show changes in water content and protein structure during nail moisturizing in the Raman spectra. Our results suggest that the softening of the nail following hydration may be due to changed matrix protein molecular structure induced by water.