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harvey dent 2 - ionization constants of aqueous amino acids lyrics

ionization constants of aqueous amino acids at temperatures up to 250°c using hydrothermal ph indicators and uv+visible spectroscopy: glycine, +alanine, and proline
rodney g. f. clarke, christopher m. collins, jenene c. roberts, liliana n. trevani, richard j. bartholomew
and peter r. tremaine
department of chemistry, memorial university of newfoundland, st. john’s, nl, canada a1b 3×7 2
guelph+waterloo centre for graduate work in chemistry, university of guelph, guelph, on, canada n1g 2w1 3
university of ontario institute of technology, oshawa, on, canada l1h 7l7
(received january 20, 2004; accepted in revised form november 2, 2004)
abstract—ionization constants for several simple amino acids have been measurеd for the first time under
hydrothеrmal conditions, using visible spectroscopy with a high+temperature, high+pressure flow cell and
thermally stable colorimetric ph indicators. this method minimizes amino acid decomposition at high
temperatures because the data can be collected rapidly with short equilibration times. the first ionization
constant for proline and +alanine, ka,cooh, and the first and second ionization constants for glycine, ka,cooh
and ka,nh4, have been determined at temperatures as high as 250°c. values for the standard partial molar
heat capacity of ionization, r
cp
o
,cooh and r
cp
o
,nh4, have been determined from the temperature dependence of ln (ka,cooh) and ln (ka,nh4). the methodology has been validated by measuring the ionization
constant of acetic acid up to 250°c, with results that agree with literature values obtained by potentiometric
measurements to within the combined experimental uncertainty
we dedicate this paper to the memory of dr. donald irish (1932–2002) of the university of waterloo—
friend and former supervisor of two of the authors (r.j.b. and p.r.t.). copyright © 2005 elsevier ltd
1. introduction
the properties of amino acids in hydrothermal solutions are
of interest to biochemists and geochemists studying metabolic
processes of thermophilic organisms and possible mechanisms
for the origin of life at deep ocean vents. thermophilic bacteria
and archea have been found to exist at temperatures up to
121°c. much more extreme conditions are encountered at deep
ocean hydrothermal vents, where reduced aqueous solutions
from deep under the sea floor are ejected into cold, oxidizing
ocean water at temperatures and pressures that may reach
near+critical and even super+critical conditions. the resulting
solutions are rich in organic molecules, including amino acids
and it has been postulated that similar hydrothermal vents on
primitive earth may have been sites for the origin of life
(baross and demming, 1983; trent et al., 1984; yanagawa and
kojima, 1985; miller and bada, 1988; shock, 1990, 1992;
bada et al., 1995; crabtree, 1997). this hypothesis is not
without controversy, and it is an active topic of study through
both field and laboratory investigations
to investigate mechanisms for the abiogenic synthesis of
polypeptide molecules, reliable experimental thermodynamic
data must be obtained, under hydrothermal conditions, for the
speciation of amino acids, the formation of peptide bonds, and
the complexation of amino acids with metal ions, both in the
aqueous phase and at mineral interfaces
the challenges in measuring thermodynamic constants for
the amino acids under these conditions are formidable, and only
a few quantitative studies at elevated temperatures have been
reported. heat+of+mixing flow calorimetry has been used to
determine ionization constants and enthalpies of reaction for
the protonation of the amino and carboxylate groups of several
amino acids at temperatures of up to 125°c (izatt et al., 1992;
gillespie et al., 1995; w+ng et al., 1996). vibrating+tube densimeters and a picker+type flow microcalorimeter have recently
been used to determine experimental values for the standard
partial molar volumes and heat capacities of glycine, +alanine
+alanine, proline, and the dipeptide glycyl+gylcine, at temperatures as high as 275°c (hakin et al., 1995, 1998; clarke and
tremaine, 1999; clarke et al., 2000). we are aware of no other
experimental studies on the thermodynamic properties of aqueous amino acids or peptides above 100°c
amino acids exist in aqueous solutions at room temperature
as zwitterions, ha(aq). the carboxylate and ammonium ionic
groups can ionize to yield protonated and deprotonated forms
of the amino acid, according to the following equilibria:
ha(aq) h2o(l) ` h2a(aq) oh(aq) (1)
ha(aq) ` a(aq) h(aq) (2)
the equilibrium concentration of the nonzwitterionic form
hao
(aq) is negligible at room temperature (cohn and edsall
1943)
the most widely reported methods for determining acid+base
dissociation constants under hydrothermal conditions are potentiometric titrimetry and conductivity (mesmer et al., 1970
1997; ho et al., 2000). conductivity measurements of amino
acid ionization constants are impractical because, according to
reactions 1 and 2, an excess of acid or base is required to drive
the ionization. the use of ph titrations in the usual stirred
hydrogen concentration cells (mesmer et al., 1970) to study
amino acids is limited to relatively low temperatures because
amino acids have limited stability under hydrothermal conditions (povoledo and vallentyne, 1964; vallentyne, 1964, 1968;
bada and miller, 1970). the length of time required to achieve thermal equilibrium and to conduct the titration causes the
amino acids to decompose. although flow cells exist for potentiometric titrimetry (sweeton et al., 1973; lvov et al., 1999)
they are complex to operate and have been used in only a few
studies (e.g., patterson et al., 1982)
recent work at the university of texas (austin) has identified five thermally stable colorimetric ph indicators, which
have been used with success in uv+visible spectrophotometric
flow cells to determine ionization constants of simple acids
can i stop?

now i’d like to read some acknowledgements:
there are many people that deserve credit for helping me to accomplish this
educational landmark:
my parents, george and christine, who have given me confidence and faith in myself
through their constant love, support, and encouragement. my brother, adam, who is just
beginning his journey and helps me to see each day as a new adventure. my sister, nancy
who helps me to realise that many new challenges are waiting for me to explore. my
grandmother, blanche francis, who taught me through her wisdom and vitality that every
second ofthis life should be celebrated. my grandmother and aunt, blanche and joan clarke
for their kindness and many carpentry projects that provided some weekend relief from my
research
my supervisor, dr. peter tremaine, and the hydrothermal chemistry group (past and
present) for their help and support in the realm of chemistry. the staff of the machine shop
(especially randy th+rn) and the electronics shop (especially carl mulcahy) at memorial
university of newfoundland for helping to keep my equipment running and my project on
schedule. the amino acid +n+lysis facility, also at memorial university of newfoundland
for their +n+lysis of my solutions. dr.vladimir majer for the use ofhis high temperature and
pressure differential flow calorimeter at universite blaise pascal in clermont+ferrand
france. this work was supported financially by the natural sciences and engineering
v
council of canada, the international association for the properties of water and steam, and
memorial university ofnewfoundland
finally, my fiance and best friend, karen leonard, for her patience and support over
the past four years. you have helped me through many difficult times and shared in all of my
accomplishments both large and small. without you, my life and this degree would not have
been as good. and this is just the beginning of a wonderful life together
vl

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