Page 23 - The Biochemistry of Inorganic Polyphosphates

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The structures of condensed phosphates 7
Table 1.1 Compositions of synthetic samples of Graham’s salt (Dirheimer,
1964). The phosphorus contents of the poly- and cyclophosphates are expressed
as a percentage of the total phosphorus contents of the compounds.
Polyphosphates and cyclophosphates Sample 1 Sample 2
High-molecular-weight polyphosphate 68.1 75.1
Polyphosphates (n ∼ 5–10) 17.3 13.6
Tetrapolyphosphate plus cyclotriphosphate 7.8 7.0
Tripolyphosphate 4.5 2.8
Pyrophosphate 2.3 1.5

polyphosphates with recurrence periods from 2 to 24 phosphate residues are shown in
Figure 1.4.

1.1.3 Branched Inorganic Phosphates, or ‘Ultraphosphates’

High-molecular-weight condensed phosphates which, unlike the linear polyphosphates,
contain ‘branching points’, i.e. phosphorus atoms which are linked to three rather than
two neighbouring phosphorus atoms, are known as branched phosphates (or ‘ultraphos-
phates’). Such phosphates have a branched structure, a fragment of which is shown in
Figure 1.5. In this type of structure, the individual polyphosphate chains are linked to form
a ‘network’, which is the reason for the name given to this type of condensed phosphates.
The existence of this group of phosphorus compounds was observed in some samples of
both Kurrol’s and Graham’s salt, as identified by chemical methods (Van Wazer and Holst,
1950; Strauss and Smith, 1953; Strauss et al., 1953; Strauss and Treitler, 1955a,b; Thilo,
1956, 1959; Van Wazer, 1958). In samples of Graham’s salt with very long chains (of the
order of several hundred phosphorus atoms), approximately one in every thousand phos-
phorus atoms is a branching point (Strauss and Smith, 1953; Strauss et al., 1953; Strauss
and Treitler, 1955a,b). The presence of branching in polyphosphate chains, or in other
words, the presence of a reticular structure, can be detected by the decrease in the vis-
cosity of aqueous solutions which occurs following dissolving the compounds in water
(owing to the rapid hydrolysis of the lateral bonds, which are very unstable). Figure 1.6
shows how the proportion of lateral bonds in Graham’s salt increases as the chain length is
increased.
Although branched phosphates have not yet been found in living organisms (perhaps as
a consequence of their unusually rapid hydrolysis in aqueous solution, irrespective of pH,
even at room temperature), it is believed that their presence in biological materials cannot
be excluded.
Information on the chemical compositions of the condensed inorganic phosphates, to-
gether with descriptions of their chemical and physico-chemical properties, can be found
in several papers, reviews and monographs (Thilo, 1950, 1955, 1956, 1959; Van Wazer,
1950, 1958; Ebel, 1951; Griffith et al., 1973; Ohashi, 1975; Corbridge, 1980). We shall