Page 21 - The Biochemistry of Inorganic Polyphosphates
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The structures of condensed phosphates 5
O O O O
M O P O P O P O P O M
OM OM OM OM
Figure 1.2 Structure of a linear condensed phosphate (PolyP), where M is H or a monovalent
+
metal cation.
is a mixture of condensed sodium phosphates containing cyclic phosphates (including
cyclohexaphosphate),butwhichismainlycomposedofhighlypolymerizedlinearpolyphos-
phates (Van Wazer and Griffith, 1955; Thilo and Sonntag, 1957).
1.1.2 Polyphosphates
Polyphosphates (PolyPs) have the general formula M (n + 2) P n O (3n + 1) . Their anions are com-
posed of chains in which each phosphorus atom is linked to its neighbours through two oxy-
gen atoms, thus forming a linear, unbranched structure which may be represented schemat-
ically as shown in Figure 1.2. The degree of polymerization, n, can take values from 2 to
6
10 , and as the value of n increases, the composition of the polyphosphates, i.e. the cation-
to-phosphorus ratio, approximates to that of the cyclophosphates, which explains the belief
which prevailed until recently that ‘polyphosphate’ and ‘metaphosphate’ were equivalent
terms. Polyphosphates in which n = 2–5 can be obtained in the pure, crystalline state (Van
Wazer, 1958), but members of this series in which n has higher values have been obtained
in appreciable amounts only in admixtures with each other.
In contrast to the cyclophosphates, they are designated as ‘tripolyphosphates’,
‘tetrapolyphosphates’, etc., although the mono- and dimeric compounds are still called by
their old names of ‘orthophosphate’ (P i ) and ‘pyrophosphate’(PP i ), respectively. In addition,
4
the highly polymeric, water-insoluble potassium polyphosphate (n ∼ 2 × l0 ), which has a
fibrous structure of the asbestos type, is still called Kurrol’s salt. We may mention in passing
that the facile preparation of Kurrol’s salt (by fusion of KH 2 PO 4 at 260 C), and the ease
◦
with which it is converted into the water-soluble sodium form by means of cation-exchange
materials, has led to its frequent preparation and use in chemical and biochemical work as
an inorganic polyphosphate.
2
Even better known is Graham’s salt, the vitreous sodium polyphosphate (n ∼ 10 ) ob-
◦
tained by fusion of NaH 2 PO 4 at 700–800 C for several hours, followed by rapid cooling.
Graham’s salt is a mixture of linear polyphosphates with different chain lengths. Fractional
precipitation from aqueous solution by means of acetone (Van Wazer, 1958) affords less
heterogeneous fractions with different molecular weights. For example, a sample of Gra-
ham’s salt, in which the chains on average have 193 phosphorus atoms (i.e. n ∼ 193), can
be separated by this method, as shown in Figure 1.3.
As can be seen from this Figure, the sample contains molecules of different sizes. The
fraction of highest molecular weight has n ∼ 500, i.e. its molecular weight is of the order of