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From: chiaravi@copper.ucs.indiana.edu (Lucius Chiaraviglio)
Newsgroups: sci.chem,sci.bio
Subject: Thermodynamics of reduction of phosphate to phosphine
Keywords: marsh gas, wierd organisms
Message-ID: <60883@iuvax.cs.indiana.edu>
Date: 29 Sep 90 01:31:13 GMT
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Organization: Department of Biology at Indiana University, Bloomington
Lines: 41

_

	Something that has intrigued me (and some microbiologists) for a while
is the process, so far as I know almost completely uncharacterized, of
phosphine production in marshes.  Most of marsh gas is methane and carbon
dioxide, but it also contains hydrogen sulfide and can contain small amounts
of phosphine (PH[3]), and probably also traces of P[2]H[4], since marsh gas
can auto-ignite upon contact with air.  The biogenesis of carbon dioxide,
methane, and hydrogen sulfide has been extensively studied (although by no
means to completion at least in the case of the latter two), and many
organisms have been found which produce these gases (-: just about everybody
makes carbon dioxide, although numerous organisms consume more than they
produce :-).  However, no organisms have yet been found which produce
phosphine, or P[2]H[4] for that matter, and we remain completely ignorant of
the biochemistry of this process.

	Phosphate seems to be the most likely source of the phosphorus in
phosphine, given that it is by far the most common phosphorus-containing
compound (for that matter, has anyone heard of any other naturally-occurring
phosphorus-containing compounds found in nature, except for the phosphine and
P[2]H[4]?).  What I am wondering is whether the reduction of phosphate to
phosphine would be thermodynamically favorable for microorganisms.  To make it
simpler, assume that they use hydrogen gas as the reductant -- many organisms
actually do use hydrogen gas at partial pressures of <0.01 atm, and common
internally-used reductants such as NADH are thermodynamically not too far from
equilibrium with the corresponding oxidized compound (such as NAD+) and
hydrogen gas at low partial pressure.  I would go ahead and figure this out
myself and post it, except that the table of standard free energies of
formation of various compounds in the excellent review article I have on
energy conservation in anaerobic bacteria (unfortunately, no mention of
phosphine generation) lacks phosphorus-containing compounds, and a table of
bond energies just won't cut it (the one in my organic chemistry textbook
doesn't have phosphorus-containing bonds anyway, even though a later part of
the book discusses phosphorus chemistry).  Could someone point me to a
suitable table of standard free energies of formation, and/or would someone
like to take a crack at this problem themselves?

|   Lucius Chiaraviglio    |    Internet:  chiaravi@copper.ucs.indiana.edu
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