Re: Glutaraldehyde: a different kind of cautionary tale

Katharine Thayer wrote:
> MartinM wrote:

> > That's very interesting. This seems to confirm what is stated in a paper
> > dealing with dichromated gelatin: there is a fundamental difference between
> > chrome hardening and aldehyde hardening. "When the commonest organic
> > hardener (formaldehyde) is used, the gelatin molecules bind to their amino
> > groups at relatively high pH values and the carboxylic groups are considered
> > absolutely free."(Mazakova et al, Dichromated gelatin for volume holographic
> > recording with high sensitivity. Part II, Optical and Quantum Electronic 14,
> > 1982, p.318)
> > "The results from the experiments show that if for bias hardening of
> > dichromated layers a hardening agent is used which binds not the carboxylic
> > but, say, the amino groups of the gelatin molecules, the light energy is
> > used more thoroughly, i.e. the sensitivity improves." ( p.319)
> > I ignore how this relates to gum crosslinking though.

> This is interesting, thanks. How it relates to gum crosslinking is
> anyone's guess, since there is little or no information extant on gum
> crosslinking per se and what is thought about it is mostly extrapolated
> from other processes. But I think it's unlikely that amine groups are
> involved. Amino acids form only about 1% of the total gum (N is then a
> fraction of 1%) and furthermore, most of that is locked so deep in the
> molecule (which is thought to be somewhat spherical in shape) that it
> can't be attacked by proteolytic enzymes; it seems reasonable (to me
> anyway) to assume that entities that are unavailable to be attacked by
> enymes are also unavailable for crosslinking.
>

> 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.

I was thinking this might explain why chrome alum didn't work very well
when I was doing my gum-hardening experiments for painting, because of
the relatively low proportion of carboxyl groups in the gum (relative to
gelatin). But how to explain why glyoxal worked very well to harden gum,
when there are even way fewer amine groups available in gum than
carboxyl groups? Either glyoxal links to something other than amine
groups in crosslinking gum, or..... it makes a great lot out of a very
few amine groups, or.......something.

I didn't (and probably won't, given my history with this substance) try
hardening gum with glutaraldehyde. It would be interesting to see
whether it works as well as glyoxal. 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 think all this shows is that the chemistry of gum crosslinking is
probably quite different from the chemistry of gelatin crosslinking,
which I already knew, and I wish to goodness people would stop telling
me that if I would only read this or that about gelatin hardening, I
would know all I need to know about the chemistry of the gum process.
Sorry, I don't THINK so!

Katharine Thayer
Received on Mon May 3 10:32:21 2004