Water and carbon dioxide, however, are a problematic pair, at least at terran sea-level atmospheric pressure. The carbon of carbon dioxide is a good electrophilic center. But carbon dioxide itself is poorly soluble in water (0.88 v/v at 293 K and 1 atm) and dissolves at pH 7 primarily in the form of the bicarbonate anion. Bicarbonate, however, has its electrophilic center shielded by the anionic carboxylate group and so is intrinsically unreactive as an electrophile. Thus, the metabolism of carbon dioxide is caught in a conundrum. The reactive form is insoluble; the soluble form is unreactive.

Terran metabolism has worked hard to manage this conundrum. The reactivity of the vitamin biotin was discussed nearly three decades ago in this context.3 Biotin is metabolically costly, however, and cannot be used to manage carbon dioxide and its problematic reactivity in large amounts. Although the enzyme ribulose bisphosphate carboxylase attempts to manage the problem without biotin, the problematic reactivity of carbon dioxide competes with the problematic reactivity of dioxygen. Even in highly advanced plants, a sizable fraction of the substrate intended to capture carbon dioxide is destroyed through reaction with dioxygen.4 Terran life, in nearly a billion years, has not found a compelling solution to this problem, which may be universal. Indeed, if we do encounter nonterran carbon-based life that lives in water, it will be interesting to see how it has come to manage the unfortunate properties of carbon dioxide.

6.2
IF NOT WATER, THEN WHAT SOLVENT?

Nature presents a large number of atomic and small molecular species that might be discussed as biosolvents. Table 6.1 lists some of these, together with their freezing and normal (i.e., at 1 atmosphere) boiling points. It is important to note another contribution of pressure to physical properties. The physical properties of the substances listed the Table 6.1 are described by a phase diagram that relates the state of a material (solid of various types, liquid, or gas) to temperature and pressure. Above a critical point in the phase diagram, the substance is a supercritical fluid, neither liquid nor gas. Table 6.2 shows the critical temperatures and pressures for some substances common in the solar system.

The properties of supercritical fluids are generally different from those of regular fluids. For example, supercritical water is relatively nonpolar and acidic. Further, the properties of a supercritical fluid, such as its density and viscosity, change with changing pressure and temperature, dramatically as the critical point is approached. Thus, carbon dioxide is not listed in Table 6.1 because it has no liquid phase at terran atmospheric pressure. Carbon dioxide has a critical temperature of 304.2 K and pressure of 73.8 atm, however. It is therefore a supercritical fluid above that pressure, and may even exist as a potential biosolvent for rocky planets having the approximate mass of Earth (or Venus).

TABLE 6.1 Freezing and Boiling Points (at 1 atm) of Some Solvents

Solvent

Freezing Point (K)

Boiling Point (K)

Ammonia

195

240

Dihydrogen

14

20

Dinitrogen

63

77

Ethane

101

184

Formamide

273

495

Helium

4

Hydrazine

275

387

Hydrogen cyanide

260

299

Hydrogen fluoride

190

293

Hydrogen sulfide

192

213

Methane

91

112

Neon

25

27

Sulfuric acid

283

563

Water

273

373



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