Supercritical fluids have different properties compared to ordinary fluids

Abstract: Supercritical fluids have different properties compared to ordinary fluids and can function as life-sustaining solvents on other worlds.
can function as a life-sustaining solvent on other worlds. Even on Earth, some bacterial
species have been shown to be tolerant to supercritical fluids. Special properties of supercritical fluids
The special properties of supercritical fluids, including various types of selectivity (e.g., steric, regio- and
chemoselectivity) have recently been recognized in the field of biotechnology and are used to catalyze reactions that do not occur in water.
Catalyzing reactions that do not occur in water. A suitable example is enzymes, when they are exposed to
enzymes are exposed to supercritical fluids, such as supercritical carbon dioxide: the enzymes become more stable.
Enzymes become more stable because they are conformationally rigid in the dehydrated state. In addition.
Enzymes exhibit a "molecular memory" in anhydrous organic solvents, i.e., the ability to
i.e., the ability to "remember" the conformational or pH state of the previous solvent to which they were exposed. Planets
Planetary environments with supercritical fluids, particularly supercritical CO2, are present even on Earth (below the seafloor).
on Earth (below the seafloor), on Venus, and possibly on super-Earth type exoplanets.
These planetary environments may provide possible habitats for exotic life.

Open Access
Life 2014, 4 332
Keywords: anhydrous solvents; biotransformation; carbon dioxide; enzymes; geochemistry.
Habitat; molecular memory; organic solvents; scCO2
1. supercritical carbon dioxide and its special properties as a supercritical fluid
Supercritical fluids (SCFs) have quite different properties compared to those of actual fluids. For example
For example, supercritical water is relatively non-polar and acidic [1]. Supercritical fluids cannot be defined as
A supercritical fluid cannot be defined as a liquid or a gas, but as a substance that is in a state above its critical temperature (TC) and critical pressure (TC) ("supercritical state").
A substance above its temperature (TC) and critical pressure (PC). For example, supercritical carbon dioxide (scCO2; critical point: 7.38 MPa, 304 mm).
critical point: 7.38 MPa, 304 K/31.1 °C and 73.8 bar) is a nonpolar medium with a large quadrupole moment [2].
Its density can be varied as a function of temperature and pressure [3]. At critical pressure, its
compressibility is at its maximum at critical pressure, and small changes in the thermal parameters can lead to large changes in its local density.
local density (Figure 1).
Not surprisingly, SCFs have gained the attention of enzymologists as non-aqueous solvents for enzyme-catalyzed reactions.
Since the 1980s, SCFs have gained the attention of enzymologists as non-aqueous solvents for enzyme-catalyzed reactions and have been used in various biotechnological fields.
Due to their numerous advantages, they have been used for various biotechnological applications [4]. Not only are enzymes able to function in SCF, but
they also show interesting new properties such as alteration of substrate specificity and enantiomer selectivity.
inhibition of side reactions, increased stability, and "molecular memory" [5]. In general, SCFs are different from common solvents
from ordinary solvents in that SCFs have both a liquid-like solubilization capacity and maintain high
diffusivity and low viscosity of the gas phase. Near the critical point, a small change in temperature or pressure
or pressure can lead to significant changes in solubility, partition coefficient, dipole moment and dielectric constant.
dielectric constants. Controlling these properties is relatively easy because small changes in pressure or temperature near the critical point can change the solubility, partition coefficient, dipole moment, and dielectric constant.
Small changes in pressure or temperature near the critical point can change the reactivity of biochemical processes because the solvent strength of supercritical fluids is high.
The strength of a supercritical fluid can be changed by varying the pressure and temperature [6].
The change in properties from the subcritical fluid to the supercritical state is particularly noteworthy for the common compounds water and carbon dioxide.
The changes in properties are particularly noteworthy for the common compounds water and carbon dioxide. These include (1) high solubility of gases in supercritical mixtures
supercritical mixtures, (2) the miscibility of gases such as O2 and H2 in supercritical fluids, (3) high diffusivity and variable density, and (4) high solubility.
diffusivity and variable density, and (4) high solubility [1,7]. As a conclusion, Ikushima
As a conclusion, Ikushima presented the rationale for supercritical fluids as suitable media for chemical and biochemical processes under certain conditions.
Under certain conditions, supercritical fluids can be used as appropriate media for chemical and biochemical processes [6].
Here, we focus on supercritical carbon dioxide (scCO2), which has attracted special attention in the field of research and technology due to its "green" properties.
It has attracted special attention in research and technology due to its "green" (i.e., sustainable) properties. scCO2 is chemically relatively
inert (e.g., it is "immune" to free radical chemical reactions) and is a non-corrosive solvent with low toxicity [8].
Unlike water, ScCO2 is an easily accessible supercritical state (7.38 MPa, 304 K/31.1 °C and 73.8 bar).
Moreover, as a solvent, it can be miscible with fluorides and organics [9,10] (Figure 1). In addition, CO
dioxide is at the maximum oxidation number of carbon (+IV; chemically completely oxidized state) and is therefore inert to further oxidation.
is therefore inert (i.e., non-combustible) to further oxidation. Therefore, supercritical carbon dioxide can
Therefore, supercritical carbon dioxide can be used as a solvent for "difficult" chemical transformations, such as the direct reaction of hydrogen and oxygen
to form hydrogen peroxide [11] or various selective free radical reactions [3].
Life 2014, 4 333
Figure 1. Schematic p-T phase diagram of carbon dioxide. Note that if the temperature and pressure of a substance
temperature and pressure of a substance are higher than the Tc and Pc of a particular substance, then the substance is defined as a
supercritical fluid. There are four different phases of carbon dioxide; the standard solid, liquid and gas phases, and the supercritical fluid.
The gas phase as well as the supercritical phase. Carbon dioxide transitions to supercritical
occurs relatively easily at the critical point of 7.38 MPa, 304 K/31.1°C and 73.8 bar.
2. Enzyme activity in anhydrous organic solvents, including scCO2
Organic solvents are usually volatile carbon-containing compounds that exist as liquids at room temperature.
in liquid form at room temperature. On the other hand, supercritical fluids, such as scCO2, can provide an environment that is farther from water than organic solvents.
even further from water than organic solvents [12], but share many characteristics with organic solvents
many common characteristics of organic solvents. Both represent a medium in which the vast majority of industrially relevant
biotechnological synthetic enzyme reactions are carried out in this medium, as many insoluble substrates can be converted by the enzyme.
Enzymes undergo transformation in non-aqueous media [13]. However, organic solvents are also extremely toxic to living cells.
However, organic solvents are also extremely toxic to living cells because of their ability to disrupt cell membranes, which can lead to leakage of macromolecules including RNA.
macromolecules including RNA and proteins [14]. Curiously, bacteria that are resistant to organic solvents, such as
various Pseudomonas strains, especially Pseudomonas putida can withstand this harsh environment
due to the presence of various adaptive mechanisms (e.g., their ability to alter the chemical composition of their membranes).
they are able to change the chemical composition of their membranes, in addition to other adaptive techniques) [15,16].
Enzymes of terrestrial organisms require a specific amount of bound water to be active [17]. Most of the enzymes of
activity of most enzymes decreases after transfer to non-aqueous solvents. The mechanism of inactivation
mechanisms likely include destructive changes in the active site, blocking access to substrates
unfavorable substrate desolvation, and the effects of transition state instability and restricted conformational mobility.
of conformational mobility [18]. Indeed, enzymes in anhydrous environments are very stiff and through this property alone, their activity is usually diminished.
By this property alone, their activity is usually weakened. Water is necessary for flexibility as a lubricant for the molecule and as an essential part of the enzyme.
An important part of the enzyme surface must be hydrated in order for catalysis to occur [19].
The three-dimensional structure of enzymes is greatly altered under conditions of extreme dehydration.
The three-dimensional structure of an enzyme is greatly altered under conditions of extreme dehydration, leading to its denaturation and thus loss of its activity. However, if
conditions are not as harsh, the structure of the protein may be preserved to a large extent. For example, scCO2 may
dissolve 0.3% to 0.5% (w/w) of water, depending on pressure and temperature [20].
Life 2014, 4 334
Kalibanov was one of the first to realize that the water bound to the enzyme determines the
catalytic activity rather than the total water content of the system [5]. In other words, the enzyme itself
was "not interested" in more than a few water molecules or a small hydrated layer to form the optimal or even maximum activity.
Even the maximum activity. The optimal water content required for a particular biotransformation depends
on the enzyme and the solvent [21]. Therefore, in order to maintain the biocatalytic capacity of terrestrial organisms
in unfavorable solvent environments, such as scCO2 or any other non-aqueous medium or supercritical fluids, maintain biocatalytic capacity
with minimum water content/solubility is very important [22]. Enzymes that are completely dry are
inactive - the threshold value is generally considered to be 0.2 g H2O/g enzyme [23]. The role of such water
"structural" water is not only to maintain the structure of the enzyme, but also to facilitate the disruption of non-covalent bonds and hydrogen
The disruption of non-covalent and hydrogen bonds during catalysis, which has a very strong impact on the reaction kinetics.
The reaction kinetics have a very significant impact. About


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