CLECs

As an example, let s consider an octahedral d] complex, such as one containing a Ti3+ ion. In a free Ti3+ ion, all five 3d-orbitals have the same energy and the d-clec-tron is equally likely to occupy any one of them. However, when a Ti3+ is dissolved in water, six H20 molecules surround it and form a [Ti(H20)h 31 complex. The six point charges representing the ligands lie on opposite sides of the central metal ion along the x-, y-, and z-axes. From Fig. 16.25, we can see that three of the orbitals (dxy, d, and d,x) have their lobes directed between the point charges. These three d-orbitals are called f2g-orbitals. The other two d-orbitals (dz2 and dx, y2), have lobes... 

Chiral ligand-exchange chromatography (CLEC) ° separates enantiomers by the formation of diastereomeric metal complexes. In a first instance the technique was mainly used for the separation of amino acids. Impressive results of the first separations gave rise to intensive investigation in the field and numerous publications appeared in the literature, which have been reviewed. 

The preparative capacity of CLEC columns is reasonable. However, the presence of copper in the mobile phase is not convenient. Copper can be removed using a iminodiacetic acid-based resin column. ... 

Compared to other chiral separation techniques, the practical importance of CLEC is relatively low because of its complexity. Nevertheless, CLEC is one of the best-investigated techniques from the theoretical point of view. Many theoretical concepts developed in CLEC were of general interest for the explanation and prediction of chiral recognition in all chromatographic systems. 

Coulombic efficiency in the first charging cycle comparable to that of the commercial electrolytes for lithium ion cells.The potential plateau at 0.80 V, characteristic of the reductive decomposition of PC, was completely eliminated due to the presence of ClEC, while a new process was observed at 1.70 V. When taking the irreversible capacity in the first cycle as a metric, the optimum concentration of ClEC was determined to be 30 vol although in a... 

Similar structural modification was also performed on PC. Trifluoropropylene carbonate (TFPC) was synthesized in the hope that a novel electrolyte free of linear carbonates could be formulated with improved safety in the case of fire. - Like ClEC and EEC, it is liquid at room temperature with a high flash point (134 °C), but its high viscosity results in slower ion transport within the electrolytes, because the maximum ion conductivity at room temperature... 

For this reason ClEC and FEC were treated as cosolvents in this review, instead of as additives as they were in most literature sources. 

Crosslinking of enzyme crystals with e.g. glutaraldehyde results in the so-called CLECs produced by Altus Biologies inc. 

CLECs are insoluble and have a high operational stability in aqueous systems as well as in organic solvents (Wang et al, 1997). 

A high enantioselectivity has been reported with lipase-CLECs (Persichetti et al,... 

Based on preliminary results from Helfferich130, further developments by Davankov and co-workers5 131 133 turned the principle of chelation into a powerful chiral chromatographic method by the introduction of chiral-complex-forming synlhetie resins. The technique is based on the reversible chelate complex formation of the chiral selector and the selectand (analyte) molecules with transient metal cations. The technical term is chiral ligand exchange chromatography (CLEC) reliable and complete LC separation of enantiomers of free a-amino acids and other classes of chiral compounds was made as early as 1968 131. 

Enantioselective resolution techniques all rely on the formation of diastereomeric [SO-SA] molecule associates, whereas in CLEC the additional incorporation of a metal ion w hich can be chelated by the analyte is essential. 

The aqueous mobile phase conditions, e.g., buffer pH, type of buffer, ionic strength, nature and concentration of competing ligands, type and amount of organic modifier and temperature, also influence the overall observed enantioselectivity of CLEC systems. 

Many of the parameters above have a direct influence on the efficiency of CLEC systems. Hence, they must be considered if the full analytical preparative potential of CLEC is to be utilized. 

Figure 19. Resolution of analytes by chiral ligand exchange chromatography (CLEC). A hydroxy acids (reprinted with permission from ref 138) B dansyl amino acids (reprinted with permission from ref 139),...

The overall observed retention of the enantiomers, and thus the elution order, is based on several kinetically and thermodynamically controlled parameters concerned with stereorecognition nonstereoselective interactions of all partners SO(R), SA(R S), and particularly of the [SO(RI-SA(KI] and [SO(K)-SA(Si] complexes with the achiral stationary phase, also play a role (Figure 21). Therefore the retention order may be reversed for a specific pair of enantiomers depending on whether a covalently bound CSP or a CMPA is applied, but using the same chiral molecule (part) as chiral selector. These general principles, shown schematically for a CLEC system, are further complicated by the complexity of the entire system, hence they are difficult to anticipate and each case must be studied individually. 

However, the CMPA concept for CLEC. introduced by Lindner and co-workers139 for the resolution of dansyl amino acids using a chiral triamine and Zn(II) as transient metal ions parallel to the work of Gil-Av146, allowed the further development and successful use of amino acids and their derivatives together with Cu(II) as CMPAs in reverse-phase systems for the direct resolution of, for example, a-amino acids146. 

The first cross-linked HNLs were reported by Costes et al. [72]. They compared MeHNL-CLECs with Celite -immobilized MeHNL. By cross-linking, the stability of the enzyme was improved, particularly in polar organic solvents. Furthermore, the cross-linked crystals could be reused without significant loss of activity. After six consecutive batches, 70% of the initial activity was retained, whereas the immobilized enzyme showed virtually no remaining activity (<1%). Nevertheless, crystallization and cross-linking cause a considerable loss of activity compared to the immobilization on Celite [72]. 

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