Biomedical Engineering Reference
In-Depth Information
natural products, including rapamycin, cyclosporine A, digioxin, and FK506, have
successfully been immobilized onto a diazirine-functionalized glass surface with their
affinity toward the binding proteins retained.
13.3.3
In Situ Synthesis Approach
An alternative method for SMM fabrication is the in situ synthesis of small-molecule
libraries. The entire synthetic process is executed directly on the array surface. With
this approach, synthetic chemists can eliminate the tedious cleavage and spotting
process. It should be noted that the chemistry used for the in situ synthesis of a
small-molecule library has a direct impact on the quality of the array. Therefore, it
must be extremely efficient and clean.
In 2004, Shaginian et al. described a light-directed approach for the in situ synthe-
sis of SMMs [52]. Three orthogonal safety-catch photolabile protecting groups were
used for the selective deprotection of multiple reactive sites (Figure 13.5a). With this
method in hand, the authors successfully constructed a 2
2 SMM without using
a photomask. Bowerman et al., on the other hand, developed a four-step cycle for
the in situ synthesis of peptoids with MeNPOC group, which is depicted in Figure
13.5b [53]. In this design, MeNPOC glycolic acid was first linked to the amine-
functionalized surface. UV irradiation was then applied to remove the MeNPOC
group and revealed the hydroxyl group. The hydroxyl group could be activated by
tosyl chloride and reactedwith a primary amine to complete amonomer unit synthesis.
In this approach, the light-sensitive MeNPOC group can be deprotected selectively at
specific positions with a digital photolithograph technique. In theory, up to 100,000
peptoids can be synthesized in situ using this method. In the next stage of the research,
the group extended their strategy to construct a cyclic peptide array, using differ-
ent chemically orthogonal groups [54]. A cyclic peptide, Glu(Fmoc)-Dpr(ivDde)-
Dpr(NPPOC)-Dpr)-Gly-OH, was covalently anchored onto amine-coated slides.
NPPOC group was first removed for the introduction of the first diversity. The Fmoc
and ivDde groups were selectively removed, and free amines were released accord-
ingly. The amine group could be protected by the NPPOC group and further modified
by other carboxylic acids in a light-controlled manner.
An in situ synthesis approach has also been adopted for the construction of an
oligosaccharide array. As illustrated in Figure 13.5c, Ban and Mrksich prepared a dis-
accharide array by directly assembling the carbohydrates on a phenol-functionalized
surface [55]. The phenol group served as a nucleophile which could link the first car-
bohydrate building block covalently onto a slide surface. The protecting group of the
first carbohydrate was removed selectively to reveal a hydroxyl group. The hydroxyl
group could react with the second carbohydrate building block to yield the desired
disaccharide. With this approach, the authors constructed an array of 24 disaccharides
and used the array to profile the substrate specificity of
×
-1,4-galactosyltransferase
in a label-free manner.
Yet another creative design was made by Anderson et al. The team created a
polymer microarray by spotting different combinations of 25 unique monomers onto
pHEMA-coated slides [56]. With long-wavelength UV irradiation, 576 different
