Hi all - a puzzler for you. We're using the protocol detailed at https://www.nature.com/articles/nprot.2011.379 to extract and isolate silk fibroin from silkworm cocoons. In brief, you boil 5 g of cocoons in 0.02 M NaCO3, wash the matted silk fibers in H2O, and then dissolve in 9.3 M LiBr.
Some people in our lab find that after dissolution in LiBr, they get a pale gold solution (roughly the color of a lager). But some people get a rose gold/pink solution. If you move on to the dialysis steps later, you eventually see the pink fade to the normal pale gold. Anecdotal evidence suggests that if you do a better job washing, you are more likely to get the pink, and pink solutions tend to be slightly more acidic.
Any ideas why this is happening? In my naive understanding, colors tend to mean pi bonds...there are a large number of tyrosine residues in silk (288/5,525), some tryptophans (13/5,525), and some phenylalanine (37/5,525). Silk is also especially prone to assembling into beta sheets--though the point of dissolution is supposed to be disrupting those.
Possibly also relevant: these solutions are normally made in house DI water, not 18 MOhm water.
Hi, did you get the answer? I also found some of my silk fibroin products turned pink under low pH. Weird.
Both the pink or the amber (maybe dark) colour could possibly be caused by (ferric) iron or manganese(III impurities, perhaps even if just at trace level.
You may possibly be able to eliminate, concentrate, and possibly better recognize such coloured impurities by washing (liquid-liquid extraction) with 2,2,4-trimethylpentane (iso-octane); possibly repeatedly. You could alternatively consider a suitable adsorbent, such as activated alumina or activated charcoal ― followed by filtering. Transmission visible light spectroscopy may possibly help to identify colouring ion(s).
For those interested at spider silk ― It may be interesting to check another discussion at this forum, where its (possible) future commercial viability was debated: https://www.researchgate.net/post/Wanted_Killer_Application_for_Spider_Silk
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