Re: gloy for tricolor on yupo?

On Apr 2, 2006, at 5:10 PM, Ryuji Suzuki wrote:

> From: Katharine Thayer <kthayer@pacifier.com>
> Subject: Re: gloy for tricolor on yupo?
> Date: Sun, 02 Apr 2006 09:01:18 -0700
>
>
>> [...] gloy is PVA, a very simple and well-understood polymer, mixed
>> with some other stuff,
>>
>
> Since you called gum arabic "extremely complex" and "not entirely
> understood and somewhat indefinite structure," I would add that PVA is
> also complex and somewhat indefinite structure.

Point taken; I was speaking relatively, not absolutely. While PVA
may be complex and somewhat indefinite, it is remarkably simple and
definite relative to gum arabic, which for starters has three
components each comprised of different material and each with a
different approximate molecular weight and a different structure.
The below description of the structure of gum arabic is taken from my
website page on the chemistry ot gum printing, which is still waiting
to be uploaded because I still keep hoping to discover something
definite I can say about it:

"The first component, which comprises around 90% of the gum, has a
molecular weight of around 250,000 and contains virtually no amino
acid; its structure is globular and highly branched. The second,
comprising around 10%, has a molecular weight of 1,500,000 or so,
contains about 10% protein, and is thought to have a structure of
five globular lobes of carbohydrate, about 250,000 mw each, which are
attached to a common polypeptide chain. The predominant amino acids
in this portion are hydroxyproline and serine. The third, comprising
less than 1% of the gum, contains 20-50% protein but is not degraded
by proteolytic enzymes, suggesting that the protein is located deep
within the "molecule", available neither to be attacked by enzymes
nor to participate in crosslinking. The molecular weight of this
component is about 200, 000, and is highly compact. The predominant
amino acids in this fraction are aspartine, serine, leucine, and
glycine."

The above description is my summary from several sources, including
the following. I commend those interested to read the original
sources instead of arguing with me about whether the above
description seems "right" to you.

(1) P.A. Williams, O.H.M. Idris, and G.O. Phillips. "Structural
analysis of Gum from Acacia Senegal (Gum Arabic)". In Cell and
Developmental Biology of Arabinogalactan-Proteins. Edited by Eugene
A. Nothnagel, Antony Bacic, and Adrienne E. Clarke. NY: Kluwer
Academics, 2000.

P.A. Williams amd G.O Phillips. "Gum Arabic." In Handbook of
Hydrocolloids, edited by G.O. Phillips and P.A. Williams. Cambridge
(UK): Woodhead Publishing, 2000.

(11) A. Bacic, G. Currie, P. Gilson, S.I. Mau, etc. "Structural
Classes of Arabinogalactin-Proteins" In. Cell and Developmental
Biology of Arabinogalactan-Proteins." Edited by Eugene A. Nothnagel,
Antony Bacie and Adrienne Clarke. NY: Kluwer Academic, 2000.

>
>> while gum arabic is an extremely complex arabinogalactan protein,
>> with an enormous, not entirely understood and somewhat indefinite
>> structure.
>>
>
> Gum arabic is not a protein.

Now there's a categorical statement. I would tend to agree that it
seems odd to classify gum arabic as a protein; I would be more likely
to classify it as a complex polysaccharide, since 98-99% of it is
comprised of monosaccharides, (the bulk of these galactose,
arabinose, rhamnose, and glucuronic acid) and only 1-2% is amino
acids. But you and I don't get to decide how to classify it, and it
is in fact classified as an arabinogalactan protein. See:

  P.A. Williams, O.H.M. Idris, and G.O. Phillips. "Structural
analysis of Gum from Acacia Senegal (Gum Arabic)". In Cell and
Developmental Biology of Arabinogalactan-Proteins. Edited by Eugene
A. Nothnagel, Antony Bacic, and Adrienne E. Clarke. NY: Kluwer
Academics, 2000.

> Gloy and gum arabic may behave the same, and the crosslinking
> mechanism may well be the same (although this is only a hypothesis)
>

When the crosslinking of PVA is now reported to vary if moisture is
present, how can you make such a statement?

Sorry, I'm having trouble keeping up; I'm not sure I'm seeing all
your posts. Okay, so new information about PVA crosslinking makes
it unlikely that PVA crosslinking and gum arabic crosslinking are
similar, so you would no longer cite literature on PVA crosslinking
to answer questions about gum crosslinking? In which case, we don't
even have a clue about how crosslinking occurs in gum.

(Note to Terry: when I refer to gum, I always mean the specific
substance known as gum arabic, or gum senegal, unless I refer
specifically to another gum like xantham or tragacanth). To me it is
not useful to construct a vague class of substances that "work" in a
dichromated colloid process and call everything in this class "gum"
whether it is gum or not; the only possible result would be
confusion. If it floats your boat to say that you are engaged in gum
printing when you're using something other than gum, I have no
problem with that; you can call the process anything you like. Call
it a gum print, call it a Terry-print. But to call it a gum print
and then extend that argument to say that because it is a gum print,
then therefore PVA is gum, is illogical. PVA is PVA; gum is gum.)

Katharine
Received on Mon Apr 3 12:22:40 2006