Solvent aminoacids

Other important derivatives for the preparation of (i-aminoacids are the corresponding P-aminonitriles. Lipase-catalyzed N-acylations of racemic cis-2-aminocyclopentane and cyclohexane carbonitriles with 2,2,2-trifluoroethyl butanoate have been successfully carried out in organic solvents and ionic liquids [53], PSL yielding better results than CALB (Scheme 7.29). 

Sauvagnat, B., Lamaty, F., Lazaro, R. and Martinez, J., Poly(ethylene glycol) as solvent and polymer support in the microwave assisted parallel synthesis of aminoacid derivatives, Tetrahedron Lett., 2000, 41, 6371. 

Although it is reported that the U-5C-4CR can work well with nucleophiles other than methanol, such as primary or secondary amines, the only examples reported in the literature are those where trifunctional a-aminoacids such as lysine [67] or homoserine [66] or bifunctional aldehydes such as glycolaldehyde [65] are employed. In these cases, the side-chain amino or hydroxy group acts as the nucleophile and opens the cyclic intermediate generating the corresponding lactams or lactones. A less nucleophilic solvent such as trifluoroethanol is usually employed, in order to maximize the intramolecular attack. The observed stereoselectivities are, apart from a few examples [66], usually not very high this could be due to different factors (a) the side chains of the a-amino acids are not very bulky (b) the intramolecular nucleophilic attack could be faster than the methanol attack and the cyclic intermediate could not equilibrate to the thermodynamically favored isomer (c) the intramolecular nucleophilic attack on the more stable diastereoiso-meric cyclic intermediate could be kinetically less favored. 

Substrate channeling is a process by which two or more sequential enzymes in a pathway interact to transfer a metabolite (or intermediate) from one enzyme to another without allowing free diffusion of the metabolite into bulk solvent. (Ovadi, 1991 Srere, 1987 Anderson, 1999). The substrate tunneling is one of fundamental process of regulating enzymatic processes in cells. Glycolysis, biosynthesis of nucleic acids, aminoacids, and fatty acids are found to be among these processes. 

Other Examples of the Use of Principal Properties Characterization by principal properties has been reported for classes of compounds in applications other than organic synthesis Aminoacids, where principal properties have been used for quantitative structure-activity relations (QSAR) of peptides [64], Environmentally hazardous chemicals, for toxicity studies on homogeneous subgroups [65]. Eluents for chromatography, where principal properties of solvent mixtures have been used for optimization of chromatographic separations in HPLC and TLC [66],... 

This transformation suggests that when an aminoacid molecule is introduced from vacuum into the middle of a polar medium, which is the case of the physiological cellular media, a severe change is produced in its properties, its geometry, its energy, the charges which its atoms support, the dipolar molecular moment, etc., set off and conditioned by the presence of the solvent. 

That molecule of water forms two hydrogen bonds so much with the neutral glycine as zwitterionic, and when the glycine is transformed from the neutral configuration to the zwitterion the interchange of two atoms of hydrogen is produced between the aminoacid and the molecule of solvent. Thus, we reproduce from a theoretical point of view the process of protonic Table transfer of an aminoacid with the participation of the solvent (intermolecular mechanism). Table 2.1.2 shows the relative energies of the three solute-solvent structures analysed, as well as that of the system formed by the amino acid and the molecule of solvent individually. 

A more visual check of the process submitted to study is achieved by means of hybrid QM/MM Molecular Dynamics, which permits snapshots to be obtained which reproduce the geometry of the aminoacid surrounded by the solvent. In Figure 2.1.11 are shown four of these snapshots, corresponding to times of 200,270,405 and 440 femtoseconds after commencement of the process. 

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