Re: Glutaraldehyde: a different kind of cautionary tale

MARTINM wrote:
> " The more I hear about colloid hardening, the less I seem to know about it.
> This might have to do with the vagueness of the term "hardening".

Do you think it means such different things? In the leather industry,
the test for knowing leather is "tanned" is whether it shrinks in
boiling water. I *believe* that properly-hardened gelatin doesn't
dissolve in boiling water (am I wrong about that?), and I know for a
fact that properly-hardened gum doesn't dissolve in boiling water. So
wouldn't being impervious to boiling water be a reasonable criterion for
hardening?

>
> Working mainly with gelatin, I am not very familiar with gum (except for
> gelatin/gum mixtures). Since it is crosslinked by borates like borax,

 gum
> seems to resemble PVA, sodium alginate and the like.

 Saying gum is like PVA is somewhat more helpful than saying gum is like
gelatin, and in fact I'm using a paper on dichromated PVA to explain the
chemistry of dichromated gum, since there isn't any literature on
dichromated gum.

At least there's no question about whether PVA links by amine or
carboxyl groups, since there aren't any of those in PVA. So whether gum
is like gelatin is highly questionable, but there's no doubt whatever
that PVA is not at all like gelatin.
Katharine

>
> Martin
>
> ----- Original Message -----
> From: "Katharine Thayer" <kthayer@pacifier.com>
> To: <alt-photo-process-l@sask.usask.ca>
> Sent: Monday, May 03, 2004 11:26 AM
> Subject: 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 Wed May 5 10:56:14 2004