Ester hydrolysis SAMs

Using dicyclohexyl-18-crown-6 it is possible to dissolve potassium hydroxide in benzene at a concentration which exceeds 0.15 mol dm-3 (Pedersen, 1967). The free OH- has been shown to be an excellent reagent for ester hydrolysis under such conditions. The related solubilization of potassium permanganate in benzene, to yield purple benzene , enables oxidations to be performed in this solvent (Hiraoka, 1982). Thus, it is possible to oxidize a range of alkenes, alcohols, aldehydes, and alkylbenzenes under mild conditions using this solubilized reagent. For example, purple benzene will oxidize many alkenes or alcohols virtually instantaneously at room temperature to yield the corresponding carboxylic acids in near-quantitative yields (Sam Simmons, 1972). 

Fig. 4 Structure of NHS ester-functionalized SAM on gold n = 2, 10, 15) and hydrolysis reaction in aqueous NaOH...

The impact of the pronounced conformational differences of these SAMs on their reactivities was assessed by GIR-FTIR and CA measmements for the well known ester hydrolysis in alkaUne medium. These measurements were performed in an ex situ mode for samples immersed in the appropriate solutions for variable periods of time followed by extensive rinsing. The kinetics was determined by measuring the decrease in the integrated intensity of the succinimidyl carbonyl band, as shown in Fig. 6a for a NHS-CIO SAM hydrolyzed in 1.00 X 10 M NaOH. The strong band at 1748 cm decreases m absorbance as the reaction progresses. The extent of the base-catalyzed reaction X can be expressed as a function of hydrolysis time... 

The deviation of the kinetics from the simple pseudo first order kinetics observed for the hydrolysis is certainly related to the differences in size and nucleophilicity of the attacking nucleophile. Similar to the induction period observed for the hydrolysis of NHS esters in SAMs of NHS-C15 on gold. 

In this paper we will first discuss the characterization of the SAMs studied, followed by the determination of reaction kinetics using FT-IR spectroscopy. After presenting the results of the new approach, namely inverted CFM, applied to in situ studies of the reaction kinetics of an ester hydrolysis, additional in situ AFM friction images will be shown. 

The ester hydrolysis was carried out in 1.0 M aqueous sodium hydroxide at room temperature. Average "macroscopic" kinetics were determined by FT-IR spectroscopy following the decrease of the integrated intensity of the v(C=0), v(C-O), and 5s(CH3) vibrations ex situ. SAMs of thiol 1 (and disulfide 2) reacted much more slowly than SAMs of the mixed disulfide 3 (Figure 3). The kinetics of the mixed disulfide 3 was exponential while 1 and 2 showed sigmoid behavior. 

One of the reactions catalyzed by esterases and lipases is the reversible hydrolysis of esters (Figure 1 reaction 2). These enzymes also catalyze transesterifications and the asymmetrization of meso -substrates (Section 13.2.3.1.1). Many esterases and lipases are commercially available, making them easy to use for screening desired biotransformations without the need for culture collections and/or fermentation capabilities. As more and more research has been conducted with these enzymes, a less empirical approach is being taken due to the different substrate profiles amassed for various enzymes. These profiles have been used to construct active site models for such enzymes as pig liver esterase (PLE) (EC 3.1.1.1) and the microbial lipases (EC 3.1.1.3) Pseudomonas cepacia lipase (PCL), formerly P.fluorescens lipase, Candida rugosa lipase (CRL), formerly C. cylindracea lipase, lipase SAM-2 from Pseudomonas sp., and Rhizopus oryzae lipase (ROL) [108-116]. In addition, x-ray crystal structure information on PCL and CRL has been most helpful in predicting substrate activities and isomer preferences [117-119]. 

Over the last decade, a considerable number of reactions has been studied (11,35) (i) olefins oxidation (38,39), hydroboration, and halogenation (40) (ii) amines silylation (41,42), amidation (43), and imine formation (44) (iii) hydroxyl groups reaction with anhydrides (45), isocyanates (46), epichloro-hydrin and chlorosilanes (47) (iv) carboxylic acids formation of acid chlorides (48), mixed anhydrides (49) and activated esters (50) (v) carboxylic esters reduction and hydrolysis (51) (vi) aldehydes imine formation (52) (vii) epoxides reactions with amines (55), glycols (54) and carboxyl-terminated polymers (55). A list of all the major classes of reactions on SAMs plus relevant examples are discussed comprehensively elsewhere (//). The following sections will provide a more detailed look at reactions with some of the common functional SAMs, i.e hydroxyl and carboxyl terminated SAMs. 

Compared to the hydrolysis reactions of NHS ester model compoimds in solution, [ 136] we observe a decrease in the apparent rate constants by 2-3 orders of magnitude for NHS - C2 and NHS - C10. More strikingly, the reaction of the NHS esters in the highly ordered SAM NHS - Cl5 shows a different overall kinetic profile. Instead of the expected pseudo first order (exponential) kinetics, sigmoid kinetics with a pronoimced induction period are found. 

Fig. 12 Schematic of base-catalyzed hydrolysis reaction in a SAMs of NHS-CIO and b ultrathin films of PNHSMA on oxidized silicon together with the definitions of surface and surface-near regions of the polymer film. The approximate depths in this tentative model were assigned based on the information depths of the techniques (CA 1 nm [148], IR the entire film, in other words 40 nm), the fact that only 25% of the NHS ester groups can be hydrolyzed, and that the reaction can be expected to start at the film-solution interface and to proceed homogeneously into the amorphous film. (Reprinted with permission from [128], copyright (2003), American Chemical Society)...

A variety of terminal functional groups and their chemical transformations on SAMs have been examined for example, (i) olefins—oxidation [23,24,131,132], hydroboration, and halogenation [23,24] (ii) amines—silyla-tion [145,146], coupling with carboxylic acids [22,146], and condensation with aldehydes [22,147] (iii) hydroxyl groups—reactions with anhydrides [148,149], isocyanates [150], epichlorohydrin [151], and chlorosilanes [152] (iv) carboxylic acids—formation of acyl chlorides [153], mixed anhydrides [154], and activated esters [148,155] (v) carboxylic esters—reduction and hydrolysis [156] (vi) thiols and sulfides—oxidation to generate disulfides [157-159] and sulfoxides [160] and (vii) aldehydes—condensation with active amines [161], Nucleophilic... 

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