Selectivities imprinted chiral phases

The molecular imprinting strategy can be applied for the recognition of different kinds of templates from small organic molecules to biomacromolecules as proteins. Some examples of separations investigated with MIP monoliths in CEC and LC are shown in Table 2. The influence of the imprinted monolithic phase preparation procedure and of the separation conditions on the selectivity and chromatographic efficiency have been widely studied [154, 157, 161, 166, 167, 192]. The performance of imprinted monoliths as chromatographic stationary phase has also been compared to that of the traditional bulk polymer packed column [149, 160]. It was shown that the monolithic phases yielded faster analyses and improved chiral separations. 

The latest results on imprinted chiral footprints [154] have shown that enantioselective catalysis (hydrolysis) does occur, and based on kinetic measurement the authors believe that this is due to an enantioselective mechanism. Kaiser and Andersson also chose aluminium doped silica as a polymeric material to obtain phenanthrene imprints and their work has been discussed earlier [52]. No selectivity towards the template was observed when imprinted silica was used as stationary phase. Only relative retention and capacity factors increased. Furthermore, even after careful extraction in a Soxhlet, the polymer still leaked phenanthrene. They also found that diazomethane yields a side reaction forming long alkyl chains. Finally they attempted to rej at the work of Morihara et al. [150-155]. but were not able to detect any selectivity using dibenzamide as the template and instead found that the template decomposes into at least five different products when adsorbed on the silica. Clearly further work is required on these systems. 

Sellergren B. Molecular imprinting by noncovalent interactions tailor-made chiral stationary phases of high selectivity and sample load capacity. Chirality 1989 1 63-68. 

Chiral CEC will be discussed in detail later in the book but is included here to exemplify the application of the high efficiencies obtained with electro-driven techniques which makes them attractive for chiral analysis where selectivity factors are sometimes small. CE has made use of chiral additives in the electrolyte whilst LC tends to utilise chiral stationary phases. Both options have been explored for chiral CEC [27,28,77]. The small amount of packing material necessary for capillaries allows the use of chiral stationary phases that would be prohibitively expensive for standard LC. Cyclodextrins, proteins, antibiotics and molecular imprinting have all been used to form chiral stationary phases [78-80]. After some less than encouraging peak efficiencies obtained using the chiral CEC approach, much improved chiral resolutions have been achieved using CEC compared to LC or CE [81-83]. 

Imprinted polymers have been used as stationary phase in liquid chromatographic separations and allowed the enantiomer separation for a wide range of compounds [445]. Improved chiral chromatographic resolution faetors have been archived by means of gradient-elution [448]. Imprinted polymer thin films have been tested as selective membranes [450,487], including enantiomer separation [451]. 

Molecular imprinted polymers (MIPs) attracted significant attention in the last decade as chiral stationary phases targeted for the separation of selected compounds used as templates during the synthesis of packing material [88]. 

Optical isomer separations that are carried out on a chiral layer produced from C-18 modified silica gel impregnated with a Cu(II) salt and an optically active enantiomerically pure hydroxyproline derivative, on a silica layer impregnated with a chiral selector such as brucine,on molecularly imprinted polymers of alpha-agonists,or on cellulose with mobile phases having added chiral selectors such as cyclodextrins have been reported mostly for amino acids and their derivatives. Mixtures of sorbents have been used to prepare layers with special selectivity properties. 

The high selectivity of M IPs is demonstrated when an optically active compound is imprinted the resulting MIP will normally resolve the racemate. Numerous reports on MIP chiral stationary phases have appeared [184—188]. Chiral templates studied include amino acids [26, 29, 120, 139, 189-192], peptides [139, 192, 193], carbohydrates [58, 194, 195] and dmgs [127, 196]. 

One of the most important features of molecular imprinting is to provide a means for creating robust polymeric recognition materials with predetermined ligand selectivity [5-7]. Thus, the main application of MIPs turned out to be in the field of separation sciences, as stationary phases in (electro)chromatographic analysis and chiral separations and as selective sorbents in solid phase... 

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