Objective

To demonstrate that the tryptophan (Trp) fluorescence of natural white hair is much weaker than other unpigmented keratin fibres such as wool, cashmere, rabbit hair and mink fur, and to explore possible reasons for this behaviour. The origin of the blue visible fluorescence (~450 nm) excited by UVA radiation in the range 360—380 nm, often associated with Trp degradation products, is also discussed and compared to other fibrous and globular proteins.

Methods

As the fluorescence spectrum of keratin fibres usually contains at least two major features, a visual comparison is more effectively demonstrated by creating a 3D contour plot of excitation versus emission wavelength, which is sometimes referred to as an excitation emission matrix (EEM).

Results

The Trp fluorescence from white hair is very much weaker than for wool, cashmere, rabbit hair and mink fur, but its visible fluorescence emission is stronger. Oxidation and reduction have little effect on the Trp intensity, which suggests quenching by cystine is not a major factor. Decuticulation of hair fibres had no effect on the Trp intensity showing that the increased number of cuticle scales surrounding the fibre cortex is not responsible. Trp fluorescence is very sensitive to exposure to UVB wavelengths, so possibly its low intensity in hair is due to greater levels of environmental exposure to sunlight than the other fibres examined.

Conclusion

Trp fluorescence from natural white hair is either extremely weak or completely absent, in contrast to the four other keratin fibres examined. It is possible that environmental exposure to UV wavelengths present in sunlight contributes to a reduction in the Trp fluorescence intensity of white hair. However another explanation is that Trp is quenched, by either an unknown substance introduced into hair during keratinisation, or as a result of regular exposure to personal care products, which may interact with Trp or tyrosine residues and disrupt the energy transfer process involved in keratin fluorescence. Further studies will be required to definitively determine the cause.