Spontaneous condensation

As indicated in Section 29.3, the conversion of the chlorosilane intermediates into polymers is accomplished by hydrolysis with water followed by spontaneous condensation. In practice there are three important stages ... 

In fact, perfluoroalkyl iodides can react with thiolates even in the absence of UV irradiation [55, 62, 63] However, the photochemical reaction is particularly useful for the transformation of aliphatic thiols [59], because the spontaneous condensation gives a considerable quantity of disulfide in that case (equations 53-55). 

Merck s thienamycin synthesis commences with mono (V-silylation of dibenzyl aspartate (13, Scheme 2), the bis(benzyl) ester of aspartic acid (12). Thus, treatment of a cooled (0°C) solution of 13 in ether with trimethylsilyl chloride and triethylamine, followed by filtration to remove the triethylamine hydrochloride by-product, provides 11. When 11 is exposed to the action of one equivalent of tm-butylmagnesium chloride, the active hydrogen attached to nitrogen is removed, and the resultant anion spontaneously condenses with the electrophilic ester carbonyl four atoms away. After hydrolysis of the reaction mixture with 2 n HC1 saturated with ammonium chloride, enantiomerically pure azetidinone ester 10 is formed in 65-70% yield from 13. Although it is conceivable that... 

Pyridine-containing tricyclic compounds have been produced via a sequence consisting of a Suzuki reaction and a subsequent annulation. Gronowitz et al. coupled 2-formylthienyl-3-boronic acid with 3-amino-4-iodopyridine. The resulting adduct spontaneously condensed to yield thieno[2,3-c]-l,7-naphthyridine 59 [47]. They also synthesized thieno[3,4-c]-l,5-naphthyridine-9-oxide (60) in a similar fashion [48]. Neither the amino nor the N-oxide functional group was detrimental to the Suzuki reactions. 

Condensation is generally a transient operation in which, as discussed by Ueda and Takashima(106), simultaneous heat and mass transfer are further complicated by the effects of spontaneous condensation in the bulk gaseous phase. After the creation of supersaturation in the vapour phase, nucleation normally occurs which may be homogeneous in special circumstances, but more usually heterogeneous. This process is followed by both crystal growth and agglomeration which lead to the formation of the final crystal product. As a rate process, the condensation of solids from vapours is less well understood than vaporisation(98). Strickland-Constable(107) has described a simple laboratory technique... 

The modified aldehyde group can spontaneously condense with vicinal peptidyl aldehydes or with e-amino groups of peptdidyl lysine. An example of such a cross-link found in collagen is shown in Figure 12. 

Morowitz presents an interesting (and controversial) theory of the beginning of cellular life the theory is based on the spontaneous condensation of amphiphilic molecules to form vesicles ( protocells ). 

An important preparation of pyrazines (303) is from a-amino ketones RCOCH2NH2 or their monooximes which spontaneously condense to give 2,5-dihydropyrazines (302). The a-amino ketones are often prepared in situ by reduction of isonitroso ketones, and the dihydropyrazines are usually oxidized to pyrazines before isolation icf Section 3.2.2.3.3). Catalytic reduction of a-azido ketones also leads to 2,5-dihydropyrazines (80OPP265). Similarly, a-nitro ketones may be reduced to the a-amino ketones which dimerize spontaneously (69USP3453278). 

Figure 3. Capillary hysteresis of nitrogen in cylindrical pores at 77 K Equilibrium desorption (black squares) and spontaneous condensation (open squares) pressures predicted by the NLDFT in comparison with the results of the Broekhoff and de Boer theory [10, 11],...

One consequence of the curvature dependence of the vapor pressure is capillary condensation, that is the spontaneous condensation of liquids into pores and capillaries. Capillary condensation plays an important role for the adsorption of liquids into porous materials and powders. It also causes the adhesion of particles. The condensing liquid forms a meniscus around the contact area of two particles which causes the meniscus force. ... 

Tropoelastin molecules are crosslinked in the extracellular space through the action of the copper-dependent amine oxidase, lysyl oxidase. Specific members of the lysyl oxidase-like family of enzymes are implicated in this process (Liu etal, 2004 Noblesse etal, 2004), although their direct roles are yet to be demonstrated enzymatically. Lysyl oxidase catalyzes the oxidative deamination of e-amino groups on lysine residues (Kagan and Sullivan, 1982) within tropoelastin to form the o-aminoadipic-6-semialdehyde, allysine (Kagan and Cai, 1995). The oxidation of lysine residues by lysyl oxidase is the only known posttranslational modification of tropoelastin. Allysine is the reactive precursor to a variety of inter- and intramolecular crosslinks found in elastin. These crosslinks are formed by nonenzymatic, spontaneous condensation of allysine with another allysine or unmodified lysyl residues. Crosslinking is essential for the structural integrity and function of elastin. Various crosslink types include the bifunctional crosslinks allysine-aldol and lysinonorleucine, the trifunctional crosslink merodes-mosine, and the tetrafunctional crosslinks desmosine and isodesmosine (Umeda etal, 2001). 

Molecular self-assembly is defined as the spontaneous condensation of molecules into ordered arrays by noncovalent forces [refs. 20,24]. For molecular self-assembly to be reversible, the free energy of interaction, AG°, between two molecules must be comparable to the average thermal energy, kT, of molecules. Molecular self-assembly ordinarily involves weak interactions (e.g. hydrogen bonds, van der Waals interactions, and hydrophobic interactions). Although one, two, or all three dimensions of the final array may extend into the millimeter... 

It has long been recognized that a small droplet will evaporate even when the gas surrounding it is fully saturated. Supersaturation of the gas is necessary to maintain the drop in equilibrium. Supersaturation is required because the probability of a net loss of a molecule from a convex surface is greater than the probability of net loss from a flat surface of infinite extent. A molecule that has left a small spherical droplet has a much more difficult time finding its way back than it would in finding its way back to a flat surface of infinite extent. Thus the high supersaturations necessary for spontaneous condensation are related to the size of the drop produced. 

As mentioned earlier, experiments indicate that spontaneous condensation is not significant until fairly high supersaturations are achieved. For example, supersaturations of slightly less than 5 are necessary with water vapor in particle free air for the formation of a visible fog by adiabatic expansion of moist air at 0°C. This supersaturation implies a critical droplet diameter of about 0.0015 xm and a cluster of several hundred molecules. 

Table 14.2 shows an estimate of the number of water embryos produced per cubic centimeter per second for various saturation ratios according to Eq. 14.11. For these computations the following constants were used ac = 1, pw = 1 g/cm3, T = 273 K, p = 4.58 mmHg, and y = 76.1 dyn/cm. According to Pruppacher and Klett (1978), a value of J 1 is necessary for spontaneous condensation to occur. 

Example 14.6 Determine the saturation ratio that corresponds to the diameter in Example 14.5. Hence, predict the minimum saturation ratio at which spontaneous condensation on ions will occur. Assume T = 0°C. From Eq. 14.13... 

Because more of the classic adsorption studies have been done with gaseous adsorbates, concentrations are usually expressed in terms of partial pressures, hence the terms concentration and partial pressure are used somewhat interchangeably throughout this section. For gaseous species, the term saturaiian means that any increase in analyte concentration will result in spontaneous condensation of any additional analyte from the gas phase. 

If Ca > Cr in Equation 4, then attractive forces can result. Forces measurements on DNA double helices spontaneously assembled by polycations provide evidence for attractive hydration forces (12, 13). The binding of several polycations result in spontaneous condensation of DNA. Packaging of genetic material in vivo and compaction of DNA for use in nonviral gene therapy exploit this attraction. The equilibrium spacing between helix surfaces in the condensed state varies between... 

Nucleosomes are connected to one another by linker DNA of variable length and the linker-binding histone HI protein (Fig. 1) (7). These long arrays of nucleosomes spontaneously condense to form helical arrays of nucleosomes, termed the 30-nm fiber after its apparent diameter (Fig. 1) (8). Additional condensation and compaction of chromatin occur through intemucleosomal interactions. One important internucleosomal interaction required for chromatin fiber formation is the interaction of a highly acidic patch of histone H2A with the histone H4 tail (8). Ultimately, these internucleosomal interactions form interphase chromatin with an unknown architecture (Fig. 1) (9). 

By the compression of 1 gr. of saturated water vapour at 100° to saturated water vapour at 101°, we have an emission of 1-2 cal. If this heat is not allowed to escape, we do not obtain saturated water vapour at 101°, but superheated steam at a correspondingly higher temperature. Conversely, if we allow saturated water vapour to expand adiabatically, condensation takes place. We may mention that this spontaneous condensation of water vapour, which for a slight expansion takes place only in the presence of dust particles or ions, has been successfully employed in counting the number of ions or nuclei in the vapour. 

Figure 10.59). A sulfhydryl group can spontaneously condense with another sulfhydryl group, forming a disulfide bond. In many cases, the formation of a disulfide bond involving two residues of cysteine in a protein is required for the normal functioning of the protein and is desirable. In other cases, the disulfide bond represents an abnormal structure and may be reversed by thioltransferase. 

Formaldehyde dehydrogenase catalyzes the oxidation of a number of aldehydes, including formaldehyde, to the corresponding acid. The enzyme is important as it catalyzes the detoxification of formaldehyde, a chemical present in small concentrations in most or all biological tissues. Formaldehyde, as well as other aldehydes, spontaneously condense with amino groups — via a Schiff base linkage — to form a condensation product. This type of condensation product is not desirable, and contributes to a small extent to the various types of damage inflicted upon the proteins of the body. 

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