Principles of Samdims

Mrksich and co-workers developed a MALDI-based assay scheme making use of a target surface modification by self-assembled monolayers (SAMs) [22]. This combination of SAMs and MALDI is predominantly called SAMDI (self-assembled monolayers for MALDI). For SAMDI, a self-assembled monolayer with reactive end groups is used in order to covalently bind enzyme substrates to a surface. To

Fig. 8.12 Scheme of the SAMDI principle. To a self-assembled monolayer (SAM) with reactive end groups, enzyme substrates are immobilized. During incubation with an enzyme solution, the bound substrates are converted into the product compounds. The reaction is quenched by rinsing the surface. Finally, matrix is added, the solvent is evaporated, and the surface is analyzed by means of MALDI-MS.

Fig. 8.12 Scheme of the SAMDI principle. To a self-assembled monolayer (SAM) with reactive end groups, enzyme substrates are immobilized. During incubation with an enzyme solution, the bound substrates are converted into the product compounds. The reaction is quenched by rinsing the surface. Finally, matrix is added, the solvent is evaporated, and the surface is analyzed by means of MALDI-MS.

monitor enzymatic activity, the functionalized surface is incubated with the solution containing the enzyme of interest. After quenching the enzymatic reaction by rinsing the surface, matrix is added, and by means of MALDI, substrate consumption and product formation can be monitored (Fig. 8.12).

This hybrid set-up combines the high selectivity of a functionalized surface with the versatile ionization efficiency of MALDI. The SAMs applied in the approach presented by Mrksich et al. were designed in order to present a mixture of oligo(ethylene glycol) groups and substrates, e.g. peptides or carbohydrate li-gands, as terminal groups. It is crucial to use oligo(ethylene glycol), because it prevents non-specific interactions of proteins with the surface, thus ensuring that all interactions of proteins in solution occur with the immobilized substrates. To ensure a maximum of accessibility to the immobilized substrates, SAMs presenting the substrate and oligo(ethylene glycol) terminal groups in a ratio of 1:4 are created. In contrast to DIOS-MS, where the porous silicon replaces the MALDI matrix, SAMs in SAMDI only serve as surfaces for the selective immobilization of analytes. Ionization is later on supported by the addition of classical MALDI matrices. This method was used in several studies published by the same group.

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