Trifunctional reagents

Figure 6.2 The trifunctional reagent sulfo-SBED reacts with amine-containing bait proteins via its NHS ester side chain. Subsequent interaction with a protein sample and exposure to UV light can cause crosslink formation with a second interacting protein. The biotin portion provides purification or labeling capability using avidin or streptavidin reagents. The disulfide bond on the NHS ester arm provides cleavability using disulfide reductants, which effectively transfers the biotin label to an unknown interacting protein.

Figure 18.1 A trifunctional reagent for studying protein interactions by mass spec. The bis-NHS ester arms crosslink interacting proteins, while the discrete PEG-containing biotin arm can be used to isolate or detect the conjugates using (strept)avidin reagents.

One way of obtaining the more useful cross-linked polyurethanes is by using a trifunctional reagent. Thus either the TDI can react with a triol or the propylene oxide can be polymerized in the presence of a triol. Then the isocyanate-alcohol reaction would of course give a cross-linked urethane. 

Gl and q3 are the molar fractions of OH groups in mono- and trifunctional reagents, respectively,... 

It should be emphasized that these reactions involving two trifunctional reagents afford monomeric compounds in high yields instead of polymeric products. There is no question that the driving force of these reactions which generate the tricyclic silatrane skeleton is the formation of the transannular Si- —N bond. 

A polymeric surface structure can result in slower mass transfer of the analyte in the polymer coating compared with the more brush- or bristlelike bonding of monomeric phases and thereby lead to higher efficiencies with monomeric phases. However, Thurman and Mills [75] note that the trifunctional reagent yields a phase that is more stable to acid because the... 

Figure 3.12 Schematic illustration of the possible products formed by the reaction of a silica surface with a trifunctional reagent (alkyltriethoxysilane).

In figure 3.11 a mono-ethoxysilane is used as an example. Alternatively, a trifunctional reagent such as a triethoxy-alkylsilane may be used, to yield what is commonly referred to as a polymeric material. The various possible products from the reaction of trifunctional silane molecules with the silica surface are shown in figure 3.12. 

The term polymeric phases arises from the fact that trifunctional reagents may just as well react with each other as with the silica surface under the influence of (inevitably present) traces of water. Hence, the resulting material is not necessarily a well-defined monomolecular layer. Moreover, for every silanol group that disappears during the reaction, two new ones are potentially formed once the product is brought in contact with water. Many of these newly formed silanol groups can subsequently be removed by reaction with a small monofunctional silane (e.g. trimethylchlorosilane, TMCS). Also,... 

This so-called end-capping process is common practice in the synthesis of bonded phases for RPLC, whether mono- or trifunctional reagents are used. This is done in order to keep the number of remaining silanols to a minimum. 

The student should remember from qualitative analysis that white zinc sulfide dissolves readily in dilute HC1, that yellow cadmium sulfide may be dissolved in concentrated HC1, but that black mercuric sulfide (one of the least soluble salts known) requires the action of a trifunctional reagent such as aqua regia (Exercise 1). 

In most cases, di- and trifunctional reagents will polymerize in the presence of water forming linear (for difunctional) and branched alkoxysilanes (for trifunctional modihers). Trace amounts of water are always present on the surface of polar silica. 

Figure 3-11 illustrates possible pathways of silica surface modification with mono-, di-, and trifunctional reagents. 

Capping of 2c may be achieved by trifunctional reagents, but the best results are obtained using its 1,3,5-trimethyl ether as starting material . As a recent example the... 

Apparently, trifunctional reagents will react with adjacent silanol groups and derivatize them, which in turn creates a more hydrophobic surface on the silica gel. The greater stability of the sorbent at low pH is an important consideration for the isolation of acidic compounds. For example, organic acids require that the pH be lowered to two pH units below the for adequate sorption thus, the pH must be lowered to -2.0 for most carboxylic acids with a pAT of about 4. In this case, the trifunctional-derivatized silicas are recommended because trifunctional SPE bonded phases will be more stable at acid pH. 

---