The SERS Mechanism

The experimental facts (and myths) were critically reviewed and exposed here. The theoretical models were discussed. 

The present author can only reiterate his conclusion, stated in the Introduction, based on the evaluation of theory and experiment as given above There is no one mechanism at the root of SERS however, there is a mechanism which, in the large majority of systems, is the main contributor to the surface enhancement effect. That mechanism is a resonance mechanism. It is felt there is not enough evidence, yet, to determine which of the mechanisms belonging to this group is the important one, or which can be ruled out. The LFE mechanism certainly has a role, but a more minor one. Note, however, that a minor factor in SERS is a factor of a 100 or so, which may be the difference between a detectable and a nondetectable signal  

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The cyanide ion was one of the first adsorbed ions which was found to be surface enhanced (8,9) on silver electrodes. In this first stage of SERS investigation of the cyanide ion, it was assumed to be linearly bonded to Ag, through the carbon atom, perpendicular to the metal surface (9). However faced with the pressure of explaining the SERS mechanism, alternative structures were proposed. With the conception of the ad-atom model for SERS, Otto conjectured that the main SERS band was due... 

The reversible dependence cannot be explained on the basis of adsorption and desorption or chemical reactions alone but must reflect also some intrinsic dependence of the SERS mechanism on voltage. 

Besides the direct utility of SERS and the impact it had had on the way we think about metal interfaces, SERS and the SERS mechanisms have suggested many other effects, some of which have already been put to trial. This review would not be complete without mentioning them. 

The metal substrate evidently affords a huge ( 10 and even as high as 10 [84, 85]) increase in the cross-section for Raman scattering of the adsorbate. There are two broad classes of mechanisms which are said to contribute to this enhancenient [, and Ml- The first is based on electromagnetic effects and the second on cheniicaT effects. Of these two classes the fomier is better understood and, for the most part, the specific mechanisms are agreed upon the latter is more complicated and is less well understood. SERS enhancenient can take place in either physisorbed or chemisorbed situations, with the chemisorbed case typically characterized by larger Raman frequency shifts from the bulk phase. 

By changing Ser 221 in subtilisin to Ala the reaction rate (both kcat and kcat/Km) is reduced by a factor of about 10 compared with the wild-type enzyme. The Km value and, by inference, the initial binding of substrate are essentially unchanged. This mutation prevents formation of the covalent bond with the substrate and therefore abolishes the reaction mechanism outlined in Figure 11.5. When the Ser 221 to Ala mutant is further mutated by changes of His 64 to Ala or Asp 32 to Ala or both, as expected there is no effect on the catalytic reaction rate, since the reaction mechanism that involves the catalytic triad is no longer in operation. However, the enzyme still has an appreciable catalytic effect peptide hydrolysis is still about 10 -10 times the nonenzymatic rate. Whatever the reaction mechanism... 

In the chymotrypsiii mechanism, the nitrophenylacetate combines with the enzyme to form an ES complex. This is followed by a rapid second step in which an acyl-enzyme intermediate is formed, with the acetyl group covalently bound to the very reactive Ser . The nitrophenyl moiety is released as nitrophenolate (Figure 16.22), accounting for the burst of nitrophenolate product. Attack of a water molecule on the acyl-enzyme intermediate yields acetate as the second product in a subsequent, slower step. The enzyme is now free to bind another molecule of nitrophenylacetate, and the nitrophenolate product produced at this point corresponds to the slower, steady-state formation of product in the upper right portion of Figure 16.21. In this mechanism, the release of acetate is the rate-llmitmg step, and accounts for the observation of burst kinetics—the pattern shown in Figure 16.21. 

Since its discovery, SERS has received attention both from theoretical and experimental viewpoints (13). The large enhancement has been observed only for certain metals such as Ag, Cu and Au. The enhancement mechanism Is not quantitatively understood. Even so, very useful Information concerning adsorbed species of some electrode surfaces can be obtained In-sltu with this effect. With... 

The requirement for atomic-scale sites in the CT mechanism is thought to be due to the formation of adatom-adsorbate complexes. Such complexes require a coordination site on the adsorbate through which the strong interaction can occur, as was demonstrated by the controlled adsorption of isonicotinic acid and benzoic acid on thin island films of silver (see the work of Chen et ai, 1980). This clearly showed that SERS spectra could only be obtained from the molecules when coordinating sites were exposed to the Ag film. 

The detailed mechanism of inhibition of TEM-2 (class A) enzyme with clavulanate has been established (Scheme 1) [23,24], The inhibition is a consequence of the instability of the acyl enzyme formed between the /1-lactam of clavulanate and the active site Ser-70 of the enzyme. In competition with deacylation, the clavulanate acyl-enzyme complex A undergoes an intramolecular fragmentation. This fragmentation initially provides the new acyl enzyme species B, which is at once capable of further reaction, including tautomeriza-tion to an entity C that is much less chemically reactive to deacylation. This species C then undergoes decarboxylation to give another key intermediate enamine D, which is in equilibrium with imine E. The imine E either forms stable cross-linked vinyl ether F, by interacting with Ser-130 or is converted to the hydrated aldehyde G to complete the inactivation. 

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