Re: Tanning theory of dichromated colloids

From: Ryuji Suzuki <rs@silvergrain.org>
Subject: Re: Tanning theory of dichromated colloids (was gelatin
Date: Sat, 08 May 2004 11:10:02 -0400 (EDT)

> > You say, so dismissively, "consider the building blocks." Well, I've
> > considered them, and I don't get it. In the one case carboxyl
> > groups, in the other, what?
>
> Are you saying there is no carboxyl group available for reaction in gum?

Ok, someone pointed out to me that you already mentioned this. So here
it goes.

From: Katharine Thayer <kthayer@pacifier.com>
Subject: Re: Glutaraldehyde: a different kind of cautionary tale
Date: Thu, 29 Apr 2004 18:47:27 +0000

> I meant to say that while it's quite unlikely IMO that amine groups
> are involved in gum crosslinking, it's also somewhat unlikely that
> carboxyl groups are highly involved either, since only one of the
> four sugars that make up the gum (glucuronic acid) contains a
> carboxyl group, and it's not one of the more prevalent sugars (18%
> of total sugar content) in the gum.

This is actually a fraction of the carboxyl groups you get. In
dichromated colloid process, chromium in dichromate (+6 oxidation
state) must be reduced by the colloid molecules to +3 oxidation state
before engaging in crosslinking reactions. Mannivannan et al (1993)
studied this with PVA, and there are other dichromated gellatin
studies as well. Dichromated colloid material also has significant
rate of dark reaction involving dichromate as well. So, sugars in gum
should be viewd with this in mind.

Gum arabic, according to my food chemistry reference, typically
consists of 70% polysaccharide chains with little nitrogenous
material, 30% polysaccharide chains attached to protein
molecules. Both of the polysaccharide chains are highly branched,
branch-on-branch structure. Like in pectin, end of the chains tend to
be uronic acids where terminal carbon is likely to occur in oxidized
form (that is, carboxylic acid). That's what the D-glucuronic acid is.

Other non-terminal sugars (galactose, arabinose, rhamnose) are all
aldoses, sugars containing aldehyde group. These sugars and
ketone-containing sugars (ketoses) are called reducing sugars because
the aldehyde (and ketone if in alkaline condition) can be oxidized to
make carboxylic acid. Dichromate is definitely a powerful enough
oxidizing agents. What this means is that virtually any sugar in gum
can provide carboxyl group.

Note however that crosslinking at very small fraction of these
molecules may suffice to render the gum insoluble in cold water. Note
also that I'm merely suggesting that hardening of gum by chromium
bridge between carboxyl groups is very possible, and this does not
preclude other possible reactions co-occuring.

Incidentally, primary alcohols, alkenes and alkynes can also be
oxidized to carboxylic acid with strong oxidizing agents like
permanganate or chromium (VI).

So one possible test might be to oxidize gum with suitable oxidizing
agents and see if chrome alum can harden it better. But the amount of
oxidizing agent must be suitably chosen, because you don't want gum
molecules to break down at once before they get hardened.

Another test might be to "deactivate" or esterify the carboxyl groups
by acidic methyl compounds. I don't remember what was used for the
analogous gelatin study, but probably dimethyl sulfate. This stuff
makes good ester with carboxyl groups in protein and amino acid as
well, but the stuff is pretty nasty because you don't have acute
irritation, pain, or smell during exposure but the toxicity may be
severe. Mixture of methanol and mineral acid would also work, but it
also would have the same problem. Methyl ester is weak and it
hydrolyzes at neutral or alkaline pH, so the hardening step with
chrome alum should be done in pH of 5 to 5.5 range. If carboxyl
groups are involved in hardening, this treatment would significantly
weaken the hardening effect.

From: Katharine Thayer <kthayer@pacifier.com>
Subject: Re: Glutaraldehyde: a different kind of cautionary tale
Date: Mon, 03 May 2004 09:26:08 +0000

> Since there are very few amino groups, (and of those little and none
> is lysine and hydroxylysine which are said to be the two amino acids
> that link in glutaraldehyde hardening of gelatin)-- if
> glutaraldehyde works for gum, it must work by an entirely different
> mechanism, or...... something.

I'm not sure about what happens here. It might be hemiacetal
formation. As you say it's hard to believe it's due to epsilon-amino
groups as in gelatin. But if you want to test it, it's fairly
straightforward. You can "phthalate" gum by mixing phthalate anhydride
and NaOH and cook for a couple of minutes at 40C. After this, amino
groups are inactivated. If the crosslinking is due to amino groups,
the gum should harden with chromium (III) to significantly less extent
after this treatment.

In both chromium and aldehyde cases, the degrees of crosslinking
measured at several different pH would add another piece of
information as well.

Another thing to note is that, in general, coated and dried material
responds to smaller quantity of hardener better. Thick colloid
dispersion (solution) is next best. Dilute ones are poor. In
dichromated colloid process, hardening takes place in dried
layer. When you do hardening on a dish before coating (with gum or
gelatin), especially with aldehydes, the result can't be compared with
that from dried material.

--
Ryuji Suzuki
"You have to realize that junk is not the problem in and of itself.
Junk is the symptom, not the problem."
(Bob Dylan 1971; source: No Direction Home by Robert Shelton)