Astrobiology Science Conference 2017 (LPI Contrib. No. 1965)
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POLYPHOSPHATES FROM THE OXIDATION OF PHOSPHITE AND PHOSPHIDE: DRIVING
PREBIOTIC CHEMISTRY? M. A. Pasek, 1University of South Florida, 4202 E Fowler Ave, NES 204, Tampa
FL 33620, [email protected].
Introduction: Biochemical energy is stored in
polyphosphates, notably ATP. The formation of polyphosphates by prebiotic reactions has historically been
driven by dehydration processes, such as by heating
ammonium phosphates [1]. Such processes tend to
produce polyphosphates at concentrations that are
linked to equilibrium chemistry. While a change in
environment may result in driving the system away
from equilibrium, for instance, by a wet-dry cycle, the
energy difference by these environmental factors from
equilibrium is typically not large, especially in contrast
to modern biochemical systems [2].
Our prior work has focused on the possible role of
reduced oxidation state phosphorus compounds in
prebiotic chemistry [3, 4]. These compounds include
the anions phosphite—HPO32-—and hypophosphite—
H2PO2-—as well as phosphide minerals including
schreibersite, (Fe,Ni)3P. Such compounds, while not
widespread on the earth today, were more abundant in
early earth environments due to lower redox conditions
and higher meteoritic flux [5]. These compounds, produced by natural abiotic geochemical processes, include phosphorus in a far-from-equilibrium state.
Hence the oxidation of these compounds, which is inevitable under aqueous conditions, may couple to production of energetic intermediates.
Reduced P oxidation. The oxidation of schreibersite produces a mixed valence sequence of phosphorus
oxyanions, including phosphate, phosphite, hypophosphate (P2O64-) and pyrophosphate (P2O74-). In addition,
other compounds such as hypophosphite and triphosphate are also produced, albeit at lower concentration
[6]. These compounds are believed to be produced by
the evolution of the phosphite (PO32-) radical produced
during reaction of schreibersite with water.
Additionally, the fenton-chemistry drivin oxidation
of phosphite and hypophosphite, which reacts these
ions with OH radicals, produces principally phosphate,
pyrophosphate, triphosphate, and trimetaphosphate [7].
These reactions occur at room temperature and pressure, and are independent of the oxidation state of the
atmosphere. Intriguingly, the amount of polyphosphates formed is typically 10-30% of the amount of
phosphate.
Disequilibrium calculations. We revisited our
prior results of reduced P oxidation [6, 7], and compared the ratios of pyrophosphate to phosphate from
the experiments ([PPi]/[Pi]2) with those expected at
equilibrium (K). The ratio of polyphosphates to phos-
phate in all cases was out of equilibrium, with the minimum ratio 103.5 and the maximum 107. These are
equivalent to about 20-40 kJ/mol of free energy from
this displacement. These values, while less than modern biological ATP to ADP and Pi ratios (~5 × 10 8, 50
kJ/mol), still imply that some geochemical processes
can produce out-of-equilibrium polyphosphate.
Prebiotic environments. In order for reduced P to
be applicable to prebiotic systems, it must be widely
available on the early earth. We have found reduced P
in lightning strikes and metoerites [6], and recently in
Archean rocks [5]. Additionally, a new route being
explored is phosphite by low-temperature diagenesis of
ferruginous sediments, which may have been widely
active on early earth environments. As a consequence
of these multiple routes, there are several environments
where disequilibrium polyphosphate production may
be plausible, and hence free energy in the form of polyphosphates may have been avialble to drive other
prebiotic chemical reactions.
References:
[1] Osterberg R. and Orgel L. E. (1972) J Molec
Evol, 1, 241-248. [2] Branscomb E. and Russell M. J.
(2013) Biochim Biophys Ac, 1827, 62-78. [3] La Cruz
N. L., Qasim D., Abbott-Lyon H., Pirim C., McKee A.
D., Orlando T., Gull M., Lindsay D., and Pasek, M. A.
(2016). Phys Chem Chem Phys, 18, 20160-20167. [4]
Gull M., Mojica M. A., Fernández F. M., Gaul D. A.,
Orlando T. M., Liotta C. L., and Pasek M. A.
(2015) Scientific reports, 5, 17198. [5] Pasek M. A.,
Harnmeijer J. P., Buick R., Gull M. and Atlas Z.
(2013) Proc Natl Acad Sci USA, 110, 10089-10094.
[6] Pasek M., Herschy B. and Kee T. P. (2015) Origins
Life Evol B, 45, 207-218. [7] Pasek M. A., Kee T. P.,
Bryant D. E., Pavlov A. A. and Lunine J. I. (2008)
Ange Chem Int Ed, 47, 7918-7920.