click image to enlarge (212K) FIGURE 9

Visible spectra of both dyes were also obtained through the Raman system.  This was accomplished by removing the notch filter from the instrument and scanning across the visible region of the electromagnetic spectrum.  Sending the white light from the microscope back through the system to the detector resulted in a composite spectrum.  A background was then collected and subtracted from the spectrum.  Both samples absorbed over the 500 to 700 nm range (Figure 10).  The minor inconsistencies in the spectra are more likely due to the mathematical operations performed rather than actual variations between the samples.  Literature data only reports visible spectra results in various solvents and not in the solid state therefore making a direct comparison irrelevant.

click image to enlarge (149K) FIGURE 10

Comparison of the organically synthesized dye with the commercial dye showed matching spectra in the infrared, Raman and visible, which also matched with published spectra of Tyrian purple.  This illustrates not only the possibility of achieving nature’s molecules in the laboratory, but also obtaining dyes of analytical quality. 

Further in-depth analysis of Tyrian purple requires a good method to fully facilitate the analysis of the minor components of the compound.  The results may differ depending on whether the dye was synthesized or acquired from the mollusk.  Likewise, variations in the chemical pathway may determine how prevalent the di- and mono- bromonated indigos are and the isomers, if any, present.  Furthermore, there may be differences depending on the species of mollusk or where and how the dye was processed.  Work with the HPLC has been done (6), but it focused more on the synthesis of the di- and mono- bromonated indigos rather than the analysis of the components of Tyrian purple.  Until extensive studies on the relationship of structure to peak in the spectra are accomplished, it is difficult to know if the IR and Raman are sensitive enough to detect the minor components.  Solubility problems prevent the use of the liquid phase nuclear magnetic resonance (nmr), which is sensitive to substitution patterns.  Due to the complex nature of Tyrian purple, it is likely that standards would have to be run, on any of these instruments, or the original sample separated into its components.  A study of the visible spectra of the various bromoindigos could provide a correlation between bromination pattern and absorption spectra.  The solvent chosen may also have an effect on this relationship. 

The author would like to thank the following people without whom this project would not have been possible.  First and foremost, thanks to Kimberly Lawler-Sagarin, PhD of Elmhurst College who helped with the brainstorming of the project and the laying down the of ground work.  Her patience and enthusiasm are much appreciated.  The author is also grateful to Eugene N. Losey, PhD, also of Elmhurst College, for acting as tour guide and mentor through the sometimes frustrating, but intriguing world of organic chemistry.  This project would never have made it this far were it not for the encouragement of Anna Teetsov and Gretchen Shearer, PhD of McCrone Associates.  It is with much gratitude that the author thanks them for their expertise in microscopy and for their guidance through the writing and publishing process.  Additionally, the author would like to thank Ken Smith, PhD of McCrone Associates for all his help with the spectroscopy instrumentation.

REFERENCES

1.      Cooksey, Chris.  “Tyrian purple: history, chemistry and sources.” http://www.chriscooksey.demon.co.uk/tyrian.  Accessed March 2003

2.      Hoffmann, Roald.  Marginalia, Blue as the Sea.  American Scientist, 78 (4)308-309, (1990)

3.      Clark, Robin J.H.; Cooksey, Christopher J.; Daniels, Marcus A.M.; Withnall, Robert.  Indigo, woad and Tyrian Purple: important vat dyes from antiquity to the present.  Endeavour 17 (4) 191-199 (1993)

4.      Cooksey, Chris.  Tyrian Purple: 6-6’-dibromoindigo and Related Compounds. 
Molecules 6 736-769 (2001)

5.      Stieglitz, Robert R.  The Minoan Origin of Tyrian Purple.  Biblical Archaeologist, 57:1, 46-54 (1994)

6.      Clark, J.H.; Cooksey, Christopher J.  Monobromoindigos: a new general synthesis, the characterization of all four isomers and an investigation into the purple colour of 6-6’-dibromoindigo.  New Journal of Chemistry.  323-328 (1999)

7.      Imming, Peter; Imhof, Ingo; Zentgraf, Matthias.  An improved synthetic procedure for 6-6’-dibromoindigo (Tyrian Purple).  Synthetic Communications, 31 (23), 3721-3727 (2001)

8.      Gettens, Rutherford J. and Stout, George L.  “Tyrian purple.”  http://www.kremer-pigmente.ed/intl.catalog/36010e.htm  Accessed May 2003

9.      Nelson, Phil.  “Ancient Dyes.”  http://www.crwflags.com/fotw/flags-dye.html.  Accessed March 2003

10.  Schatz, Paul F. Indigo and Tyrian Purple – In Nature and in the Lab.  Journal of Chemical Education, 78 (11), 1442 (2001)