Dehydrated hydrates

Figure 2 The aconitase reaction a reversible isomerization by means of subsequent dehydration-hydration reactions.

In particular, the dehydration/hydration catalysis in intermediary metabolism, by proteins that contain an asymmetric cubane, is well established. The present picture is predominantly based on the extensive studies with aconitase for which a detailed reaction mechanism has been proposed. The dehydration and hydration steps in this mechanism are well understood however, the details of the flip of the intermediate are still to be determined. 

Fig. 6 Macromolecules are readily encapsulated in lipid vesicles in a single cycle of dehydration-hydration [53]. Such wetting-drying cycles would have commonly occurred in the prebiotic environment at intertidal zones...

FIGURE 5 Spectra of hydrated-dehydrated-hydrated alumina-supported samples. 

Moving one step further back in the synthetic sequence towards cyclopentenoftes identifies the alkynic alcohols as viable precursors. Since the subsequent dehydration-hydration-cyclization steps are all acid-... 

The use of new nitrating agents has led to the preparation of many anhydrous nitrates (nitrato compounds) which cannot be obtained from aqueous solution or by dehydrating hydrated salts. They include ... 

Very recently, a process for dehydrating hydrated rare earth halides, typically hexahydrates, with phosgene was patented [1612a]. 

Hydroxybutyryl-CoA epimerase EC 5.1.2.3 Epimerization at 3-position through a dehydration/ hydration mechanism... 

TFase and vice versa. Furthermore, the transfer of the lO-formyl group does not appear to proceed through a dehydration-hydration sequence involving an amine function on either enzyme since 0 is not exchanged into the 1-carbon unit during transfer [73]. 

In the case of the porous [R(phos)3] 2H20 networks, the adsorption isotherms have been recorded to test the intake capacity for N2, H2O, MeOH, and EtOH, of which 1.1,1.76, 0.8, and 0 molecule(s) per formula unit can enter into the pores of the material, respectively. The network is stable to several dehydration/hydration cycles, maintaining its homo-chiral structure (Yue et al., 2006). Another clever approach to porous materials able to host small molecules starts by producing a mixture of monometallic helical complexes with different chirality from lanthanide perchlorate and tris(2-benzimidazoylmethyl)amine L53 (Jiang et al.,... 

The beef heart enzyme (M, = 80,000) (117) is a component of the citric acid cycle. Its active form contains one [4Fe-4S] cluster. Although such a cluster is normally associated with electron transfer, the enzyme catalyzes the nonredox reaction of citrate-isocitrate interconversion via a dehydration-hydration pathway. The current state of understanding of cluster structures and reactions of beef heart aconitase has been thoroughly reviewed by Emptage (130). When isolated aerobically, aconitase is inactive and contains one [3Fe-4S] cluster. Upon incubation of the reduced protein with Fe(ll), the fully active enzyme is generated. When a 3-Fe center is reduced to [3Fe-4S]°, Reaction 10 builds a 4-Fe cluster in a nonredox process. The Mossbauer spectra in Fig. 8 address the question of subsite specificity in this reaction of aconitase (124). If the externally supplied iron is Fe, the resultant spectrum reveals the intrinsic (original) Fe atoms... 

Propargyl alcohol, HC2CCH2OH is an important intermediate in the synthesis of 1,4-butandiol, furan, and THF from acetylene (Fig. 15). HC2CMe2(OH) is used in the synthesis of aryl- and diaryl-acetylenes or of carbamates. Dehydration-hydration reactions of cluster-bound propargyl alcohols will be discussed in this section, after a short glance at their behavior on mono- and binuclear complexes. 

Dehydration-hydration. Alkynols pros zeolite HSZ-360 at lower and higher temperani... 

The equilibrium thermodynamics of stoichiometric hydrates has been described by several authors. The overview presented here is intended both to review the basic thermodynamics of crystalline hydrate formation/stability and to highlight the intrinsic differences between polymorphic systems and hydrate systems (a discussion of the kinetics of dehydration/hydration will be given in Section IV). The following description is a hybrid based on the work of Grant and Higuchi [7] and that of Carstensen [8]. 

Expanded channels (non-stoichiometric) Lattice planes Dehydrated hydrates Metal-ion coordinated water... 

Dehydrated hydrates may in principle belong to any of the classes just discussed, but the cases with which the author is familiar (findings not yet published) have all been either channel hydrates or clathrate type structures where water is the guest instead of the host in a cavity and in a nonstoichiometric amount. This subclass deals with crystals that dehydrate even at relatively high partial pressures of water. Therefore, the hydrate that forms in solution dehydrates almost immediately on removal from the mother liquor. When dehydration leaves an intact anhydrous structure that is very similar to the hydrated structure but with lower density, it is classified as a dehydrated hydrate. If there already exists an anhydrous crystalline form of the molecule, the dehydrated hydrate is classified as a polymorph. 

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