CHAPTER 9 . Binding Ability of Inorganic Major Components of Sea Water
239
assimilation of amino acids. Investigations carried out on some organism tests showed
that uptake of glycine and a-alanine continues until salinity decreases to about 1/3 of
normal sea water, i.e. a salinity of about 12. At lower salinities, the animals survive but
can no longer acquire free amino acids from solution, probably because the osmotic
and chloride regulation processes of the body fluid are reduced (Stephens 1972). Moreover, increasing salinity determines a lowering of amino acid assimilation, probably
owing to their reduced availability due to complexation with the cation macro components of sea water. This last point further demonstrates the importance of chemical speciation studies in natural waters.
Besides their biological role, among the low molecular weight ligands present in
natural waters, amino acids are interesting because of their acid-base behaviour, which
can be considered to be intermediate between that of carboxylic acids and amines (De
Stefano et al. 1995). Of the twenty amino acids that commonly occur in proteins, about
half contain side chain donor atoms that are potentially capable of forming stable
complex species with metal ions, and several studies have been described in literature
on this subject (Silien and Martell 1964, 1971; Martell and Smith 1997). Most of these
studies concern the co-ordination chemistry of transition metal ions, and relatively
few data regarding their complexation with alkali and alkaline earth metal ions have
been published (Evans and Guevremont 1979; Casale et al. 1989; De Robertis et al. 1991;
De Stefano and Gianguzza 1991; De Stefano et al. 1995). While taking into account the
above considerations on the dependence of amino acid assimilation on sea water salinity, because of their importance in defining the chemical speciation of amino acids
in natural waters, we also thought it would be worthwhile examining the interactions
of this class of compounds with sodium, calcium and magnesium (i.e. the cation macro
constituents of natural waters).
The protonation constants of glycine, alanine, histidine, aspartic, glutamic and
iminodiacetic acids were studied in different aqueous media and at different ionic
strengths (De Stefano et al. 1995, 2000b). These studies showed that: (a) carboxylic
groups interact weakly with Na + and rather strongly with Ca 2 + and Mg2+,
(b) unprotonated aminogroups interact weakly with Ca 2 + and Mg2+ and (c) protonated
aminogroups interact weakly with cr and SO~-. Moreover, by studying the dependence
of protonation constants on ionic strength and complexing ability, amino acids were
grouped into three different types: the first includes glycine, alanine and serine; the
second lysine and histidine; and the third aspartic, glutamic and iminodiacetic acids.
Apparent protonation constants (logf3H*) for glycine, alanine, histidine, and
glutamic and aspartic acids (De Stefano et al. 1995, 2000b) and for serine, valine, leucine, threonine and methionine (Fiol et al. 1995a,b, 1998) determined in artificial sea
water are shown in Tables A9.4 and A9.5, respectively, in Sect. A9.2 of the appendix
(in Sect. A9.1 of the appendix formulas and abbreviations of all amino acids considered here are reported).
The dependence on salinity of Mog f3r values [Mogf3r = 10gf3t(NaCI) -logf3t(SSWE)]
is shown in Fig. 9.9, for the first (Fig. 9.9a) and the second (Fig. 9.9b) protonation step
of glycine, histidine and glutamic acid. As can be seen, gly and his behave in a similar
manner (in particular as regards 10gf3r\ whilst protonation constants for glutamic
acid are significantly lower. This is due to the presence of a second carboxylic group,
which binds cations (see formation constants for alkali and alkaline earth complexes
below) more strongly than aminogroups.
239
assimilation of amino acids. Investigations carried out on some organism tests showed
that uptake of glycine and a-alanine continues until salinity decreases to about 1/3 of
normal sea water, i.e. a salinity of about 12. At lower salinities, the animals survive but
can no longer acquire free amino acids from solution, probably because the osmotic
and chloride regulation processes of the body fluid are reduced (Stephens 1972). Moreover, increasing salinity determines a lowering of amino acid assimilation, probably
owing to their reduced availability due to complexation with the cation macro components of sea water. This last point further demonstrates the importance of chemical speciation studies in natural waters.
Besides their biological role, among the low molecular weight ligands present in
natural waters, amino acids are interesting because of their acid-base behaviour, which
can be considered to be intermediate between that of carboxylic acids and amines (De
Stefano et al. 1995). Of the twenty amino acids that commonly occur in proteins, about
half contain side chain donor atoms that are potentially capable of forming stable
complex species with metal ions, and several studies have been described in literature
on this subject (Silien and Martell 1964, 1971; Martell and Smith 1997). Most of these
studies concern the co-ordination chemistry of transition metal ions, and relatively
few data regarding their complexation with alkali and alkaline earth metal ions have
been published (Evans and Guevremont 1979; Casale et al. 1989; De Robertis et al. 1991;
De Stefano and Gianguzza 1991; De Stefano et al. 1995). While taking into account the
above considerations on the dependence of amino acid assimilation on sea water salinity, because of their importance in defining the chemical speciation of amino acids
in natural waters, we also thought it would be worthwhile examining the interactions
of this class of compounds with sodium, calcium and magnesium (i.e. the cation macro
constituents of natural waters).
The protonation constants of glycine, alanine, histidine, aspartic, glutamic and
iminodiacetic acids were studied in different aqueous media and at different ionic
strengths (De Stefano et al. 1995, 2000b). These studies showed that: (a) carboxylic
groups interact weakly with Na + and rather strongly with Ca 2 + and Mg2+,
(b) unprotonated aminogroups interact weakly with Ca 2 + and Mg2+ and (c) protonated
aminogroups interact weakly with cr and SO~-. Moreover, by studying the dependence
of protonation constants on ionic strength and complexing ability, amino acids were
grouped into three different types: the first includes glycine, alanine and serine; the
second lysine and histidine; and the third aspartic, glutamic and iminodiacetic acids.
Apparent protonation constants (logf3H*) for glycine, alanine, histidine, and
glutamic and aspartic acids (De Stefano et al. 1995, 2000b) and for serine, valine, leucine, threonine and methionine (Fiol et al. 1995a,b, 1998) determined in artificial sea
water are shown in Tables A9.4 and A9.5, respectively, in Sect. A9.2 of the appendix
(in Sect. A9.1 of the appendix formulas and abbreviations of all amino acids considered here are reported).
The dependence on salinity of Mog f3r values [Mogf3r = 10gf3t(NaCI) -logf3t(SSWE)]
is shown in Fig. 9.9, for the first (Fig. 9.9a) and the second (Fig. 9.9b) protonation step
of glycine, histidine and glutamic acid. As can be seen, gly and his behave in a similar
manner (in particular as regards 10gf3r\ whilst protonation constants for glutamic
acid are significantly lower. This is due to the presence of a second carboxylic group,
which binds cations (see formation constants for alkali and alkaline earth complexes
below) more strongly than aminogroups.