SERRS processes

Guerrin, M., Simon, M., Montezin, M., Haftek, M., Vincent, C., and Serre, G., Expression cloning of human comeodesmosin proves its identity with the product of the S gene and allows improved characterization of its processing during keratinocyte differentiation,. /. Biol. Chem., 273, 22640, 1998. 

The classical Raman effect produces only very weak signals. There are two techniques which very successfully enhance this effect. The resonance Raman spectroscopy RRS is making use of the excitation of molecules in a spectral range of electronic absorption. The surface-enhanced Raman spectroscopy SERS employs the influence of small metal particles on the elementary process of Raman scattering. These two techniques may even be combined surface-enhanced resonance Raman effect SERRS. Such spectra are recorded with the same spectrometers as classical Raman spectra, although different conditions of the excitation and special sample techniques are used (Sec. 6.1). 

Fig. 13.2 Schematic representation of the process required to achieve detection of labeled DNA by SERRS. Represented is (a) an oligonucletide labeled with R6G which is already positively charged, therefore no surface modification is required to allow adsorption onto a silver nanoparticle surface and excited with laser light to give a SERRS spectrum and (b) an oligonucleotide modified with a negatively charged dye label, in this case HEX, which requires further modification of the DNA with propargylamine to introduce a region of positive charge to allow adsorption to the metal nanoparticle...

The first block, INTERACTION, is devoted to the calculation of electronic energies determining the potential energy surface (PES) on which the nuclear morion takes place. The second l)lock, DYNAMICS, is devoted to the integration of the scattering equations to determine the outcome of the molecular process. The third block, OBSER WBLES. is devoted to the reconstruction of the ol)serr al)le properties of the beam from the calculated dynamical quantities. All these blocks reejuiro not only different skills and expertise but also specialized computer software and hardware. 

Fundamental questions related to the electronic configuration of the open or colored forms and the number and structures of the photomerocyanine isomers are considered on the basis of the results of continuous-wave (stationary) and time-resolved (picosecond, nanosecond, and millisecond) Raman experiments. For spironaphthoxazine photochromic compounds, the Raman spectra may be attributed to the TTC (trans-trans-cis) isomer having a dominant quinoidal electronic configuration. Surface-enhanced resonance Raman spectroscopy (SERRS) is demonstrated as a new analytical method for the study of the photodegradation process in solution for nitro-BIPS derivatives. The development of this method could lead to the identification of the photoproducts in thin polymer films or sol-gel matrices and ultimately to control of degradation. 

These results may be compared with those of recent SERS experiments performed by Schneider et al.,54 who postulated that SERS spectra ofnitro-BIPS derivatives in methanolic solutions originated from neutral open merocyanine species. The presence of open forms in solutions of 7, in the absence of any UV light, probably arises from solvatochromism, which is an efficient process in the opening of nitro-substituted spiro compounds in polar solvents.55 Moreover, as discussed above, the detection of photomerocyanines, even at trace levels, is favored with respect to that of the nonresonant closed form of the photochromes by a SERRS effect excited at 5l4.5nm. It should be noted that in the case of cyanine dyes, SERRS spectra, in Ag colloids, from 10 I7M solutions have been recently reported.56... 

Figure 6.18 Resonant SERS-melting profiles of the polymerase chain reaction products for wild type (squares) and AF508 mutation (triangles) using the 1347cm" SERRS band for CyS. A schematic of the dehybridization process of the polymerase chain reaction products is shown. The first derivatives of the melting profiles are shown in the inset. The mutation has a melting potential of-0.44V,...

Flavins are an important cofactor involved in many biological electron transfer processes [258[. They commonly occur as FAD, flavin mononucleohde, and riboflavin. For example, FAD is the cofactor in GOx. Cotton and Holt could even detect it at a very sensihve level of 10" M on a roughened Ag electrode by SERRS although in their study on GOx it was an interference [295]. Flavins can undergo oxidahon-reduction in either a single two-electron transfer step or two... 

Disentangling interfadal redox processes of proteins by SERR spectroscopy. Chemical Society Reviews,... 

VS. SCE, measured by the ratio of the SERRS band intensities at 1370 cm Fe(III) and 1360 cm Fe(II) agrees with the average of the anodic and cathodic potentials (—0.72 V and —0.58 V), see Fig. 30, 31. Furthermore, the irreversible character of the cyclic voltammetric redox process at a silver electrode, characterized by a potential peak separation larger than 0.06 V can also be illustrated by the non-Nem-stian behaviour of the ratio of the RR band intensities. This behaviour may reflect differential rates of adsorption and desorption of the oxidized and reduced species. 

As discussed before in the case of nucleic acids the authors have also considered the incidence of the interfacial conformation of the hemoproteins on the appearance of the SERRS signals from the chromophores. Although under their Raman conditions no protein vibration can be observed, the possibility of heme loss or protein denatura-tion are envisaged to explain a direct interaction of the heme chromophores with the electrode surface in the case of the adsorl Mb. extensive denaturation of Cytc at the electrode appears unlikely to the authors on the basis of the close correspondence of the surface and solution spectra. Furthermore, the sluggish electron transfer kinetics measured by cyclic voltammetry in the case of Cytc is also an argument in favour of some structural hindrance for the accessibility to the heme chromophore in the adsorbed state of Cytc. This electrochemical aspect of the behaviour of Cytc has very recently incited Cotton et al. and Tanigushi et al. to modify the silver and gold electrode surface in order to accelerate the electron transfer. The authors show that in the presence of 4,4-bipyridine bis (4-pyridyl)disulfide and purine an enhancement of the quasi-reversible redox process is possible. The SERRS spectroscopy has also permitted the characterization of the surface of the modified silver electrode. It has teen thus shown, that in presence of both pyridine derivates the direct adsorption of the heme chromophore is not detected while in presence of purine a coadsorption of Cytc and purine occurs In the case of the Ag-bipyridyl modified electrode the cyclicvoltammetric and SERRS data indicate that the bipyridyl forms an Ag(I) complex on Ag electrodes with the appropriate redox potential to mediate electron transfer between the electrode and cytochrome c. 

The examples of SERS and SERRS measurements in the biochemical field reviewed in this article were selected to illustrate the sensitivity, molecular specificity of adsorption process, accuracy, ease of sample preparation, and significant manifold apphcations of Raman analysis by SERS and SERRS for biomolecules in the adsorbed state. 

Developments in SERS and SERRS have yet to make an impact upon process analysis but some potential means of using these enhanced sensitivity methods have been proposed. 

MUDA exhibit a small hysteresis between the forward and backward potential scans, suggesting that the redox process is reversible and that cyt. C3 retains its native structure. The SERRS spectra of cyt. C3 (both oxidized and reduced forms) immobilized on 11-MUDA clearly indicate that the hemes are in the 6cLS state. The formal potential of cyt. C3 immobilized on the 11-MUDA modified silver electrode was monitored by the shift of the oxidation state marker band and is -0.20 V versus NHE, which is about 0.10 V more negative than that observed on a bare silver electrode [11]. 

Both traditional electrochemical and ER techniques cannot provide insight into molecular processes at the electrode interface, which includes conformational transformations of the electroactive species coupled with the ET reactions as has been reported for cyt. c [71, 72, 85]. SERRS can provide both structural and kinetic information on the ET process of cyt. c at an electrode surface. The time-resolved SERRS technique was developed to elucidate the heterogeneous ET rate of cyt. c with different conformations at a silver electrode surface by monitoring the change in the SERRS intensity with time after applying a potential step [92, 93]. The... 

Future research aimed at refining SERS or SERRS spectrophotometry should be oriented towards procedures allowing surfaces to be prepared and characterized in a reproducible manner, where as many as possible of the experimental variables (especially surface charge density) can also be controlled. Furthermore, attention needs to be paid to the structural perturbations of molecules in the adsorbed state. These perturbations could occur not only as a consequence of the adsorption process, but also through photodegradation reactions occurring during laser irradiation. 

The second possible arrangement illustrated in Figure 2A is where the laser excitation is set off the frequency of the adsorbate resonance and at the maximum of the plasmon resonance (2). For resonance experiments on the molecule alone, this would be described as a preresonant condition and often SERRS undertaken in this way is written as SE(R)RS. More orientation information is to be expected and additional bands have been observed and assigned as due to mechanisms of surface enhancement. However, in this preresonant condition, the selectivity of resonance still applies. Thus, it is possible to pick out individual molecules in the presence of a matrix of interferents, but the effect will now be more dependent on the angle of the adsorbate to the surface. For many surface studies this is a key point and consequently this experimental process may be preferred for surface analysis. 

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