I am talking about proteins. I saw in my biochemistry book that there
were three proteins in a test tube. They were label protein A, protein
B, and protein C.The band on protein A were more separate from the
other protein bands. The book said it was due to "diffusional
spreading" and other things. Please explain what diffusional spreading
is all about.

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Request for Question Clarification by pafalafa-ga on 06 Jan 2005 16:33 PST

tedmccall-ga,

You have a penchant for asking interesting questions, I see.

At a microscopic scale, diffusion is one of the most basic, important
-- and unerappreciated -- physical processes that move materials and
energy through systems.

Your probably familiar with diffusion in its everyday sense:  put a
drop of food coloring in a glass of water and  -- even without
stirring -- the coloring diffuses through the entire volume of water
until it is evenly distributed.

This happens with energy or with very small particles, such as the
molecules of food coloring.  It also happens with proteins.

Different proteins have different diffusion rates due to a variety or
factors -- size, shape, mass, charge, affinity for other substances,
etc.  The variable diffusion rates are part of the key to protein
separation techniques such as chromatography.  Different proteins
migrate up the chromatography gels at different rates, and separate in
the process.

From what I remember from my own days as a laboratory biologist, 
diffusional spreading" refers to two things:  (1) the basic spreading
and separation of proteins that occurs due to the process I just
described, and (2) the fact that some proteins produce narrow,
well-defined bands in the chromatographic process, while others
produce wider bands with ragged edges...the latter effect sometimes
attributed to "diffusional spreading".

Now...having said all that, I'm not sure how to answer your question. 
Are you looking for links to sites that explain diffusion in more
detail?  Do you want to know specifically about diffusion of proteins?
 Are you interested in mathematical treatments of the topics, or are
you happier with "plain English" sorts of explanations?

Let me know what you think.

pafalafa-ga

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Request for Question Clarification by pafalafa-ga on 06 Jan 2005 17:08 PST

Jeez...sorry about all the typos.  There appears to have been a good
deal of diffusional spreading of my fingers over the keyboard!

paf

Proteins can be either globular or fibrous type. Globular proteins are
compact in structure and can diffuse through a porous gel or matrix
easily. The movement of globular proteins depends on its size. Fibrous
proteins are typically insoluble in water and are made of repetitive
units. They are not compact and therefore the bulkiness of these
proteins increases their diffusional spreading in gels.

Globular protein example - Globulin (immunoglobulin)
Fibrous protein example - Collagen (a matrix protein that provides structure)

In addition, if a globular protein is modified with sugars or other
charged molecules, the molecule may now be spread more diffuse than
the original unmodified protein (which may otherwise move as a sharp
zone in a gel).

You can read more about it in basic biochemistry books (separation of
proteins by gel filtration or electrophoresis) as well as some
biophysics literature.