Green fluorescence from the hair of Lindow Man.

Smith, Gerald J.

'Lindow Man II' was discovered in a peat bog in Cheshire, England and his well preserved body has been carbon-dated at |approximately~ 2000 years BP (Stead et al. 1986). Bright green fluorescence has been observed from the hair of Lindow Man II and also from the animal fur arm-band he was wearing (Priston 1986). Trace elemental analysis of skin from Lindow Man III recovered from the same peat bog as that in which Lindow Man II was found revealed the presence of very high levels of copper (Pyatt et al. 1991). This observation, together with contemporary Roman records of Celtic customs, prompted Pyatt et al. (1991) to propose that the copper found in the skin of Lindow Man III was associated with pigments used by the Celts to paint their bodies. Further, it was suggested that the copper in such pigments may have been responsible for the green fluorescence observed from the fur arm-band of Lindow Man II. However, X-ray energy dispersive analysis of the elements contained in the hair of Lindow Man II shows no signs of copper (Connelly et al. 1986), and despite this the hair exhibits green fluorescence, which therefore must originate from some other species. From the work reported here, it is concluded that the green fluorescence observed from the hair of Lindow Man II probably originates from species derived from the hair keratin by an anaerobic, chemical reaction with acid present in the peat bog.

Fluorescence is the emission of light which follows and results from absorption of radiation by some molecules. The intensity of fluorescence depends on the amount of light absorbed by the fluorescing molecule. This in turn depends on the wavelength of the light in a way which is characteristic of the molecule. Fluorescence is always at somewhat longer wavelengths than the light which is absorbed, and thereby excites the fluorescence. The spectrum of the fluorescence is characteristic of the molecule from which it originates. Because of this and its dependence on the wavelength of the exciting light, fluorescence provides a useful, non-destructive analytical probe for some materials. For example, fluorescence spectroscopy has already helped to identify fluorescent component species in wool and hair including various protein degradation products (Leaver 1978; Smith & Melhuish 1985; Collins et al. 1988).

Animal fibres such as fur, hair and wool are largely composed of the protein, keratin. Indeed, in scoured wool the fibre is almost entirely protein or protein degradation products. Proteins are composed of a variety of amino acids. As such, the species responsible for the fluorescence observed from keratin are most likely to be the aromatic amino acids, viz. tryptophan, tyrosine and phenylalanine or their degradation products. Tryptophan is responsible for the fluorescence from keratin excited at short wavelengths (|is less than~300 nanometre). The spectrum of this fluorescence peaks at a wavelength of 340 nm and extends to about 450 nm (Smith et al. 1980). In addition to the fluorescence from tryptophan, Smith and Melhuish (1985) reported observing blue fluorescence when keratin is exposed to light at wavelengths between 320 nm and 400 nm and green fluorescence when excited in the region of 450 nm. Collins et al. (1988) have also observed fluorescence from wool in the visible region of the spectrum and noted that the green fluorescence is most intense at the weathered tip region of the fibre. While the blue fluorescence could originate from oxidation products of tryptophan and tyrosine (Smith & Melhuish 1985; Collins et al. 1988), these materials do not produce strong green fluorescence.

In the absence of oxygen, proteinaceous materials have been found to undergo chemical reactions with acid to produce chemicals known as beta-carbolines (Tschesche et al. 1958; Dillon et al. 1976; Dillon 1981). Some members of the beta-carboline chemical family exhibit blue fluorescence while other beta-carbolines display green/yellow fluorescence (Dillon et al. 1976). Since anaerobic, acid conditions exist in the peat bog where Lindow Man was discovered, prolonged burial of keratin fibres in such an environment may well have resulted in the formation of the green fluorescent beta-carbolines in the fibres.

To test this possibility, keratin in the form of solvent-cleaned Merino wool was examined.

The tips of the fibres were cut off to remove the green fluorescence species found in this weathered region of the fibre and the wool was then treated (hydrolysed) with dilute hydrochloric acid in the absence of oxygen. Following hydrolysis for 30 days, the spectrum of the fluorescence resulting from excitation of the treated keratin at 440 nm was recorded. The maximum fluorescence intensity appeared at |approximately~500 nm (green) which is about the same position as the fluorescence maximum of one of the products resulting from hydrolysis of human lens protein and which was identified as a beta-carboline (Dillon et al. 1976). Green fluorescence has also been observed from an acid hydrolysate of the protein casein, and was also attributed to the same beta-carboline compounds (Tschesche et al. 1958).

The spectrum of the fluorescence resulting from excitation of hair fibres from Lindow Man at wavelengths between 420 nm and 450 nm also displays a maximum at |approximately~500 nm as shown in FIGURE 1. The similarity of the positions of the fluorescence spectra observed from Lindow Man hair and from acid treated wool with fluorescence from beta-carboline in acid hydrolysates of other proteinaceous material (Dillon 1981; Tschesche et al. 1958; Dillon et al. 1976) suggests that the species responsible for the green fluorescence in Lindow Man hair is a beta-carboline.

Exposure of hair from Lindow Man to radiation with short wavelengths at |approximately~360 nm results in a rather different fluorescence spectrum which reflects contributions from other fluorescent species present in the fibre. Although the fluorescence appears green/yellow to the eye, spectral measurements reveal a broad spectrum with some blue fluorescence apparent between 430 nm and 450 nm, of comparable intensity to the fluorescence in the green region of the spectrum at |approximately~500 nm. In this respect the fluorescent behaviour of the Lindow Man hair sample was quite different from that of recently formed (undegraded) keratin. Excitation at 360 nm of modern human hair containing a similar pheomelanin (red) pigmentation as that present in Lindow Man hair results in a strong blue fluorescence with a prominent maximum at 425 nm and a shoulder at 410 nm. Untreated wool exhibits similar fluorescence when excited at 360 nm (Smith & Melhuish 1985). In these modern keratins, the green fluorescence is only just discernible and much less intense than the blue fluorescence. It is clear that the species responsible for the blue fluorescence is unstable under the anaerobic, acid conditions of the peat bog, whereas the green fluorescent material (possibly beta-carboline) is either remarkably stable or was produced by a reaction with acid in the bog.

Conclusion

The green fluorescence observed from some proteinaceous materials originates from some species produced as a result of 'weathering' and/or by exposure of keratin protein to acid in the absence of oxygen. Protein diagenesis is a problem in the dating of protein-containing archaeological specimens such as bone, teeth, bog bodies, hair and shell (van Klinken 1991; Hedges & van Klinken in press). The green fluorescence from keratin described in this paper may provide a guide to the extent of diagenesis in other proteinaceous materials.

Acknowledgements. The author thanks the Australian Wool Research and Development Committee for financial support of this work, Dr I.M. Stead (British Museum) for providing a sample of hair from Lindow Man II and Mr D. Madill (Royal Institution of Great Britain) for assistance with early observations of green fluorescence from wool keratin.

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