Cholesterol imprinting

Fig. 8 (a) Schematic representation of an ordered assembly of cyclodextrin molecules with each one binding a part of the template, (b) Binding modes of (a) cholesterol, (b) 4-hydroxyazobenzene and (c) 4-phenylphenol to the guest-binding sites in the cholesterol-imprinted beta-cyclodextrin polymer. Reprinted with permission from [89]. Copyright 1999 American Chemical Society... 

Fig. 7.2. Binding of structural analogues to cholesterol-imprinted polymer. Cholesterol (2), cholestane (3), cholesteryl acetate (4), epicholesterol (cholest-5-ene-3a-ol) (5) and cholest-5-ene-3-one (6), all 2 mM in hexane. Adapted from [9].

In order to demonstrate the imprinting effect, a control polymer, using phenyl (4-vinyl) phenyl carbonate as the template, was prepared under the same condition as the MIP. This was chemically identical to the cholesterol-imprinted polymer after template cleavage, but would have a much smaller imprint cavity associated with the phenol in the recognition site, too small to accommodate the cholesterol molecule. An additional control, prepared in the absence of either template was also made, to test the effect of the hydrolysis conditions on the polymer matrix. Both of these polymers in their hydrolyzed state and the cholesterol-imprinted polymer in its unhydrolyzed state (template still bound in the polymer) were unable to bind cholesterol. 

Figure 8 (a) Binding of cholesterol to cholesterol-imprinted and control polymers, from a 2 mM solution of cholesterol in hexane, as a function of polymer concentration, (b) Binding of cholesterol and various cholesterol analogues (2 mM) to the cholesterol-imprinted polymer, prepared by the sacrificial spacer method. Reprinted with permission from Journal of the American Chemical Society. Copyright 1995 American Chemical Society (Ref. 10). 

One consequence of the uniform binding site distribution and high capacity of these polymers is that they may be rather better suited to the preparation of chromatographic (HPLC) stationary phases than noncovalently imprinted materials. In fact this was the conclusion arrived at by Hwang and Lee [15], who compared the chromatographic performance of cholesterol-imprinted polymers prepared by the semi-covalent (carbonate spacer) method with noncovalent MIPs incorporating... 

Hwang, C.C. Lee, W.C. Chromatographic characteristics of cholesterol-imprinted polymers prepared by covalent and non-covalent imprinting methods. J. Chromatogr. 

Figure 8 Cholesterol imprinted core-shell nanoparticles. Reprinted in part with permission from Ref. 41. Copyright (2000) John Wiley Sons, Inc.

Various novel imprinting techniques have also been presented recently. For instance, latex particles surfaces were imprinted with a cholesterol derivative in a core-shell emulsion polymerization. This was performed in a two-step procedure starting with polymerizing DVB over a polystyrene core followed by a second polymerization with a vinyl surfactant and a surfactant/cholesterol-hybrid molecule as monomer and template, respectively. The submicrometer particles did bind cholesterol in a mixture of 2-propanol (60%) and water [134]. Also new is a technique for the orientated immobilization of templates on silica surfaces [ 135]. Molecular imprinting was performed in this case by generating a polymer covering the silica as well as templates. This step was followed by the dissolution of the silica support with hydrofluoric acid. Theophylline selective MIP were obtained. 

The self-assembly of an imprinted layer on the surface of a transducer was realized through the adsorption of the template on gold, Si02, or ln02 surfaces followed by treatment with an alkylthiol or organosilane (Hirsch et al. 2003). The first example of this type of sensor was reported in 1987 by Tabushi and coworkers (1987). Octadecylchlorosilane was chemisorbed in the presence of n-hexadecane onto tin dioxide or silicon dioxide for electrochemical detection of phylloquinone, menaqui-none, topopherol, cholesterol, and adamantane. Another MlP-based sensor was... 

Asanuma H, Kakazu M, Shibata M, Hishiya T, Komiyama M. Molecularly imprinted polymer of beta-cyclodextrin for the efficient recognition of cholesterol. Chem Commun 1997 1971-1972. 

Huval CC, Bailey MJ, Braunlin WH, Holmes-Farley SR, Mandeville WH, Petersen JS, Polomoscanik SC, Sacchiro RJ, Chen X, Dhal PK. Novel cholesterol lowering polymeric drugs obtained by molecular imprinting. Macromolecules 2001 34 1548-1550. 

Whitcombe MJ, Rodriguez ME, Villar P, Vulfson EN. A new method for the introduction of recognition site functionality into polymers prepared by molecular imprinting synthesis and characterization of pol3mieric receptors for cholesterol. J Am Chem Soc 1995 117 7105-7111. 

Whitcombe et al. observed a strong decrease of the cholesterol concentration, and hence excellent binding for cholesterol with p(EGDMA-co-CVPC) shell/ pMMA core particles in isohexane. When the carbonate ester was not hydrolyzed and hence no imprinted sites existed in the shell, only 2.7 pmol cholesterol per g particles were bound corresponding to the non-specifically adsorbed templates. Slightly less cholesterol was adsorbed to particles prepared without CVPC at all thus formed with a pure pEGDMA shell (<2 pmol g-1). 

Fig. 7.1. The sacrificial spacer methodology. Imprinting of cholesterol via the 4-vinylphe-nyl carbonate ester (1). AIBN = 2,2 -azobisisobutyronitrile. Adapted from [9].

Recently, Komiyama and co-workers have reported the preparation of novel cyclodextrin derived imprinted carbohydrates for the efficient recognition of cholesterol [34]. jS-Cyclodextrin was cross-linked in the presence of cholesterol using different diisocyanates as the cross-linking agents. These cyclodextrin matrices, after removal of the template, exhibited selective preference towards cholesterol compared to the non-templated cross-linked cyclodextrin. 

Fig. 12.2. Electron micrograph of EDMA beads covalently imprinted with cholesterol made by conventional suspension polymerisation in water using poly(vinyl alcohol) as stabiliser and a dioctyl phthalate/decane mixture as porogen (M.J. Whitcombe, unpublished).

In a short communication, Cooper et al. [28] report the utilisation of two novel fluorescent functional monomers (Fig. 20.10) in EDMA-based MlPs. Initially, a polymerisable phenolic compound was treated with cholesterol chloroformate (as per Whitcombe et al. [29]) and used to prepare a covalently imprinted polymer. Details are lacking, but this procedure was not successful both the imprinted and the non-imprinted polymers had a similar affinity for cholesterol and no changes in... 

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