rewwth
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gelatin as buffer
I'm about to do an indigo dyeing experiment with a range of fabrics, both cellulosic and protein.
In a few recipes (and they may all be copied from an ur-source) gelatin or hide glue is to be added to the vat for dyeing the protein fibers. (This
vat is to be held under pH 9, unlike the vat for cellulosic fibers at pH 11.)
The author of an authoritative book on natural dyes (with whom I have been in touch) says that the gelatin or hide glue (added to the vat at 3% --
whether that's to the weight of the fiber or the solution is unclear) will be acting as a BUFFER.
I simply cannot find information about gelatin as a buffer. Type A gelatin has an isoionic point from pH 7- pH 9; Type B gelatin around pH 4.5.
I've found numberless papers in which gelatin is a component of a buffer solution (with Tris, HEPES, etc.) in biochemical experiments.
Is this an old wives' tale? Is there some rationale for gelatin as a buffer on its own? By the way, ammonia, lye, lime are all candidates for raising
the pH in the vat.
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chemoleo
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Well it is odd that one would add gelatin (protein) to act as a buffer in a dye solution that is meant to stain protein (silk?) just as well.
On the other hand, I guess that indigo is reactive towards amines (??? couldn't find info on that), and gelatin (which is largely composed of
collagen) does not contain lysines (free amines).
Anyway, of course proteins work well as a buffer, particularly proteins that have a regular composition such as collagen/gelatin.
Collagen/gelatin is/was of course a cheap buffer source, but today of course phosphates, Tris, Hepes, Mes, and you name it, are synthetic, clean and
the preferred choice for any buffering system. So use gelatin if no other buffer is available, or use Tris, phosphate etc if that's available as it is
cleaner, and less likely to undergo any strange side effects/reactions.
Never Stop to Begin, and Never Begin to Stop...
Tolerance is good. But not with the intolerant! (Wilhelm Busch)
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Xenoid
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I think perhaps in this case (dyeing) the term "buffer" is being used in a more general sense compared to the more specific chemical meaning of
opposing a sudden change in composition (usually pH). The gelatin is perhaps added to slow down the absorption of the dye by the material so as to
allow more subtle nuances of colour to be achieved in the dyeing process!
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rewwth
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gelatin & indigo dyeing of protein fibers
Indigo doesn't actually have any affinity whatsoever for any fibers. Getting it reduced/dissolved (e.g., in an alkaline environment with sodium
hydrosulfite) enables one to move fibers through the solution to get the leuco-indigo to intercalate within the fibers. When the fiber is removed from
solution, the indigo oxidizes and is physically stuck in place. (By the way, this looks pretty cool, as the fiber turns yellow-green to blue in front
of your eyes.)
The problem protein fibers have with this is their degradation in the alkali, and thus it would be helpful to have a buffering system which would keep
the pH somewhat closer to neutral than the wildly alkaline conditions in which the cellulosic fibers are content.
Chemoleo, you've given me an indication that proteins have been used as buffers in the past. This is something of which I'd never heard; can you
please give me a reference? (The vagueness of statements such as "add gelatin to protect the fibers" or "add gelatin to prevent the indigo from coming
out of solution" has been driving me nuts.)
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chemoleo
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Proteins are by their very nature buffering systems, due to the presence of both acidic and basic groups. Different proteins have different pI's
(isoelectric points) depending on the quantity of acidic vs basic groups, which are solvent exposed. I don't have a reference, but you'll probably
find it mentioned in biochem books, it's nothing special and a widely known effect of most protiens. Just as an example, in blood: http://chimge.unil.ch/En/ph/1ph75.htm. The amino acid constituents of proteins are also used as buffer, i.e. glycine (amino acetic acid)
Anyway, there may be more to the gelatin than just the buffering effect. For instance, in hydrazine synthesis gelatin is included, but not as a buffer
but as an effective chelator of undesired transition metal ions that catalyse unwanted side reactions. EDTA can be used instead, but it is said that
gelatin is still superior. Perhaps this is in part what is required here.
Never Stop to Begin, and Never Begin to Stop...
Tolerance is good. But not with the intolerant! (Wilhelm Busch)
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rewwth
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Thank you for the reference. It's been about 25 years since I studied any biochemistry in the classroom, and the reading the names
"Henderson-Hasselbach" & "Bronsted-Lowry" are like, well, coming across anything else I haven't thought about in a million years. Having those
names related to proteins "cum" buffers is extremely helpful. Thanks so much.
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chemrox
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please Google and read some of the easily obtained articles. I love the color and the molecule is really cool too. It seems that urea has been used
to bind it.
You can't really ask for a ref on the buffering capacity of proteins. Proteins are made up of amino acids and any text that covers those two will
discuss the "zwitterion".
[Edited on 16-2-2008 by chemrox]
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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rewwth
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I've read books on indigo dyeing in vats: the leuco dyes are really cool. The molecule doesn't bind to the fiber; rather, in it's dissolved/reduced
form it migrates into more amorphous regions of the fibers. Upon oxidation (which causes the precipitation of the indigo), the indigo is trapped.
Ammonia is one source of base, which is the basis of the urine vat.
There are numerous recipes but I had never read one containing gelatin or any other buffering system until recently. And my age is such that I was
unfamiliar with proteins as buffers. (Nor did I study zoological systems to even consider what's happening in blood.) I'm relieved to fill in some
gaps in my understanding.of what's going on ionically.
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