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The material here was emailed by Joe Mack, an Australian walker currently living in North Carolina, to aus.bushwalking.
I did research on these compounds for 15yrs. A quick look at google doesn't show any interesting chemistry, which is a pity since text books are full of it. The pathway which starts all of this off starts with phenylalanine (see http://www.botany.ubc.ca/douglas/phenylpropanoid.htm) and produces the C6-C3 (benzene ring - propyl side chain) phenol on the right (at the referred page). This C6-C3 unit then goes off to a whole slew (thousands, summed over all plants) of chemicals, including flower smells, flow colors (and the red, orange and yellow leaf colours seen in deciduous leaves in autumn), and most importantly lignin. Lignin, together with cellulose, makes wood - a matrix material (like reinforced concrete, fibreglass, skin and collagen). Matrix materials consist of two components - flexible but incompressible 1 dimensional rods (cellulose, steel rods) buried in soft 3 dimensional glue (lignin, cement). 25% of all the carbon in the biosphere is lignin.
When you boil up, mash or wash wood/bark/leaves, out comes the smaller molecular weight components which were originally derived from the precursor C6-C3 unit. Because of the phenols, the taste can be astringent (tea, coffee), and the water can be coloured (usually red or brown). Water run-off from pine forests after rain here (in the USA) is red. The lakes in Algonquin Park (Canada) are a deep, transparent brown, all from the C6-C3 phenols.
The phenol together with charged groups (carboxylic acid), can cross link proteins (skin) to produce a toughened form of skin called leather in a process called tanning. Some plants produce a mixture of C6-C3 molecules which are better for tanning than others and the heterogeneous mixture is called tannic acid. You used to be able to buy tannic acid powder at the chemist to paint on your feet to harden the soul [I think he means 'sole' - RNC], in the days when you had to condition your feet to make them compatible with boots. Because the C6-C3 molecules crosslink proteins, plants with lots of them will crosslink and inactivate the eater's digestive enzymes (which are proteins) making the plant indigestable, discouraging the eater from eating any more of that type of plant. If you're a plant biochemist (as I was) and you want to prepare active enzymes from plants, you first have to stop them from being crosslinked by the C6-C3 phenols when you break open the cells to extract the enzymes.
Water coloured by C6-C3 phenols from plants is going to have no more effect on you than the same amount of coloured tea (except you won't get any caffeine). It's the bugs in the water that you need to worry about.
Joseph Mack, NA3T EME(B,D), FM05lw North Carolina jmack (at) wm7d (dot) net - azimuthal equidistant map generator at http://www.wm7d.net/azproj.shtml Homepage http://www.austintek.com/ It's GNU/Linux!