Comparative study on keratin structural changes in onychomycosis and normal human finger nail specimens by Raman spectroscopy

For the purpose of investigating the secondary structural changes of keratins in onychomycosis patient nail, Raman spectra of nail clippings from onychomycosis patients were obtained and used to describe the molecular structure differences from normal nail specimens. Raman spectroscopy revealed that nail from onychomycosis patients had a significant decrease in the content of the sulfur-containing amino acids in comparison to normal nail. And the spectral analysis also demonstrated that the disulfide bonds had the trend to the energetically less favored gauche–gauche–trans (g–g–t) forms in onychomycosis patient nail samples. These results showed that the increased amounts of less stable disulfide conformers were contributed to or associated with the nail brittleness observed in onychomycosis patients. We also found that the positions of the S–S band (500–550 cm−1) in onychomycosis patients were different, which gave us an indication that the disulfide groups might be related to the histopathological level and morphological characteristics. In the region of amide I, the α-helical (1652 cm−1) and β-sheet (1671 cm−1) content changed, these ratios (peak area of 1652 cm−1 divided by peak area of the 1671 cm−1) were between 0.67 and 0.75, while drastically altered ratios for the two conformers were changed to 0.25–0.63 in onychomycosis patient nail, the α-helix form seriously decreased in comparison to the normal nail. This result suggested unfolding of proteins and a more amorphous structure in the onychomycosis nail. On the other hand, the amide III (unordered) band intensity, assigned to the random coil form in normal nail clippings, at 1250 cm−1 is much higher than that for the amide III (α) content (1272 cm−1), and the peak area ratios of random coil band and α-helix band were about 10.00, while in the onychomycosis nail, these ratios were close to 2.00, the random coil form remarkably decreased. All the results of the present work implied that fungal erosion led to the secondary structure changes of proteins and Raman spectroscopy can be further used as a tool biomarker to diagnose this widespread disease.