Internal structural changes in age-related curved hair due to cyclical extension: An IR spectroscopy study.

Abstract

Objective: This study aimed to identify structural changes in age-related curved hair (referred to as "YUGAMI" hair in Japanese) induced by cyclical extension using infrared (IR) spectroscopy coupled with chemometrics, such as multivariate curve resolution (MCR) and two-dimensional correlation spectroscopy (2DCOS).

Methods: The hair fibres were stretched at a strain level of 0.3-N, and this operation was counted as one cycle and was repeated 500 cycles. Every 25 cycles, IR spectra were obtained using an attenuated total reflection IR (ATR-IR) microscope. The spectra were analysed by MCR to investigate the structural changes of keratin protein inside of the hair. 2DCOS was conducted to investigate the genesis pathway of SO₃H in the 1000-1150 cm^-1 range and the change in the secondary structure of keratin protein of hair in the 1600-1700 cm^-1 range.

Results: For age-related curved hair, 0.3-N strain levels increased the CH₂ and C=O signals after 125 cycles and the SO₃H signal after 225 cycles, but no architectural modifications were observed in straight hair. Two results were obtained from 2DCOS. One was cyclical extension increased the S-SO₃H signal, followed by SO₃H signal. The other in the 1600-1700 cm^-1 range indicated an increase in the secondary structural motif and β-structure after the formation of SO₃H.

Conclusion: This study shows that the process of internal transformations can be split into three steps in age-related curved hair caused by cyclical extension. The first step is the surface transfer of internal lipids, and the second step is two distinct patterns of disulfide bond cleavage: C-S and S-S fission. The final step is further transformation of the unravelled α-helices into β-structures which occurs above roughly 225 stretched. Our findings can be an effective factor in care the increase of curvature by cyclical extension such as everyday grooming.