Hydrogel carbamates

PHEMA has been effected. Reaction of p-nltro-benzenesulfonyl Isocyanate with PHEMA or copolymers containing HEMA and varying amounts of styrene followed by reduction of the nitro group with sodium dlthlonlte produced p-amlnobenzenesulfonyl carbamates of hydrogels (p-ABSC-HEMA). The precise composition of the hydrogel carbamates was determined by nonaqueous titration of the carbamate moiety with tetrabutylammonlum hydroxide In Isopropanol/dlmethylformamlde. 

Preparation of Hydrogel p-Nitrophenyl Carbamates. A 20 mL solution of hydrogel (2-3.0 g) in pyridine was treated with 3 g... 

Preparation of Hydrogel Arylsulfonyl Carbamates. To a solution of hydrogel (1-4.0 g) in dry pyridine (20-40 mL) was added eitherp-toluenesulfonyl Isocyanate (pTSI), 3.0 g, or p-nitro-benzenesulfonyl Isocyanate (p-NBSI), 0.5-3.0 g and the mixture was heated at 70 under nitrogen for 4 hr. The adduct was isolated by precipitation in benzene or ethanol. Repreclpltatlon from acetone or DMF yielded the desired products, IR (film) 3170 cm (-NH), 1530 cm" (-NO2), 1340, 1180 cm" (-SOj). The precise degree of substitution was determined by non-aqueous titration (Table II). 

Reduction of Hydrogel p-Nltrobenzenesulfonyl Carbamates. A solution of p-NBS carbamate, 1.0 g, in 20 mL of THF was blended with 15 mL 0 10% aqueous Na2S20i. The two phase system was stirred vigorously and heated to gentle reflux for 4-6 hr. The reduction was monitored by observing the disappearance of nitro absorbance at 1530 cm Evaporation of the THF phase, followed by exhaustive washing with distilled water yielded the reduced adduct. Loss of product during the Isolation procedure due to partial solubility in water was normally observed. The extent of reduction is reported in Table II. 

A sample of the hydrogel carbamate (50-80 mg) was dissolved (or slurried) in 3 mL DMF. The solution was titrated in a micro-titration vessel equipped with a 5.5 mm combination electrode. 

To activate the hydrogel adducts for enzyme immobilization, it was necessary to reduce the nitro function to the corresponding amine. From the many techniques available to effect this modification, we selected sodium dithionite as the mildest procedure with the least potential problems with residual reagent. Since the hydrogels were quite hydrophilic it was possible to use a two phase solvent system (THF/H2O) to effect the reduction. The arylsulfonyl carbamates were reduced selectively under these conditions no reduction of p-nitrophenyl carbamate was observed. 

Figure 2. Gas chromatogram of the N-HFB Orpropyl derivatives of the hydrolysate of trypsin CVB hydrogel sulfonyl carbamate (3f). Conditions 12 ft. X 1/4 in. glass column packed with 3% OV-1 on Chromasorb-W initial temp was 100° ford min, programed to 250° at 4°/min.

Figure 4. Active site titration recorder trace of 17.0 mg trypsin CVB hydrogel sulfonyl carbamate (If) being titrated by 19.75 mL of 0.12 mM NPGB in pH 8.3 veronal buffer burst, 0.060 A.

For example, Ossipov et al. (2007) synthesized a poly(vinyl alcohol) (PVA) hydrogel by mixing two aqueous poly (vinyl alcohol) components. The two PVA derivatives are functionalized through carbamate linkages with aldehyde (PVA-AL) and hydrazide (PVA-HY) as shown in Fig. 10.15. 

Ossipov and Hilborn derivatized one of the very important commercially available water-soluble polymers, namely polyvinyl alcohol (PVA), in order to prepare cross-linked hydrogels using an efficient carbonyldiimidazole (CDI)-mediated coupling, they introduced small amounts (1-5 mol%) of pendant azide and alkyne functionalities independently on to the polymer through a carbamate linkage. Cross-linked hydrogels were then prepared by reacting the PVA-azide and PVA-alkyne in the presence of Cu catalyst (Scheme 7.38) [65]. 

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