Structural perturbation with adsorption

The enzymatic activities of O -chymotrypsin in solution and adsorbed at the different surfaces are presented in Fig. 11, where the specific enzymatic activity (defined as activity per unit mass of protein) is plotted as a function of temperature. The enzyme loses activity due to adsorption. On the hydrophobic Teflon and PS surfaces, the activity is completely gone, whereas on the hydrophilic silica surface, or-chymotrypsin has retained most of its biological function. These differences are in agreement with the adsorption isotherms and the circular dichroism spectra. The influence of the hydrophobicity of the sorbent surface on the affinity of the protein for the sorbent surface, as judged from the rising parts of the adsorption isotherms (Fig. 8), suggests that the proteins are more perturbed and, hence, less biologically active when adsorbed at hydrophobic surfaces. Also, the CD spectra indicate that adsorption-induced structural perturbations are more severe at hydrophobic surfaces. 

The electroactivity of the adsorbed states of this group of compounds is evidently associated with the presence on the adsorbed layer of a pendant functionality which is electroactive in the unadsorbed molecule and is attached to the surface in such a way that the pendant is virtually unperturbed structurally or electronically by the surface. In contrast, a molecule such as hydroquinone which is electroactive prior to adsorption but is strongly perturbed by adsorption (that is, by direct covalent attachment to the Pt surface) is not reversibly electroactive in the adsorbed state (Fig. 26). Accordingly, adsorbed DMBM is reversibly electroactive under almost all conditions, while adsorbed HQ is not reversibly electroactive under any conditions thus far studied. In between these two extremes is THBP, which is reversibly electroactive in one of its adsorbed states but not in the other two. 

Structural perturbations are to be distinguished from what are essentially phase changes. The perturbations represent relatively minor deviations from bulk liquid, and adsorption isotherms are usually nearly invariant if x is plotted against P/P°. That is, the heat of adsorption from the vapor phase is very close to that of condensation. By contrast, and as an example, in the case of water on PE, while this near invariance in isotherm shape holds around 20°C, the isotherms at -9°C and -24°C are very flat, with an Xq of only about l.sX. It would appear that the adsorbed film... 

Adsorbates can physisorb onto a surface into a shallow potential well, typically 0.25 eV or less [25]. In physisorption, or physical adsorption, the electronic structure of the system is barely perturbed by the interaction, and the physisorbed species are held onto a surface by weak van der Waals forces. This attractive force is due to charge fiuctuations in the surface and adsorbed molecules, such as mutually induced dipole moments. Because of the weak nature of this interaction, the equilibrium distance at which physisorbed molecules reside above a surface is relatively large, of the order of 3 A or so. Physisorbed species can be induced to remain adsorbed for a long period of time if the sample temperature is held sufficiently low. Thus, most studies of physisorption are carried out with the sample cooled by liquid nitrogen or helium. 

The existence of multiple peaks for molecular desorption has been attributed to lateral interactions among adsorbed species 62-64). As discussed previously, adsorption onto the surface lattice may occur preferentially in next nearest neighbor sites to form p(2 x 2) structures. Even at low coverages, attractive forces may cause adatoms to occupy next nearest neighbor positions, so that clusters of adsorbate form which have local twofold periodicity 65) with respect to the surface. Such effects are entirely consistent with the perturbations of the surface electronic wave functions due to adsorption 66-68) which show that these binding sites represent the... 

These kinetics studies required development of reproducible criteria of subtraction of foe H-O-H bending band of water, which completely overlaps foe Amide I (1650 cm 1) and Amide II (1550 cm"1) bands (98). In addition, correction of foe kinetic spectra of adsorbed protein layers for foe presence of "bulk" unadsorbed protein was described (99). Examination of kinetic spectra from an experiment involving a mixture of fibrinogen and albumin showed that a stable protein layer was formed on foe IRE surface, based on foe intensity of the Amide II band. Subsequent replacement of adsorbed albumin by fibrinogen followed, as monitored by foe intensity ratio of bands near 1300 cm"1 (albumin) and 1250 cm"1 (fibrinogen) (93). In addition to foe total amount of protein present at an interface, foe possible perturbation of foe secondary structure of foe protein upon adsorption is of interest. Deconvolution of foe broad Amide I,II, and m bands can provide information about foe relative amounts of a helices and f) sheet contents of aqueous protein solutions. Perturbation of foe secondary structures of several well characterized proteins were correlated with foe changes in foe deconvoluted spectra. Combining information from foe Amide I and m (1250 cm"1) bands is necessary for evaluation of protein secondary structure in solution (100). 

Second, at low coverages, the vibrational perturbation induced by adsorption on cationic sites located on different faces of the same microcrystal is primarily determined by the coordinative unsaturation of the cation (which in turn is a complex function of the structure of the face). This statement implies that the vibrational spectra of diatomic molecules adsorbed on low-surface-area materials (in which the crystallites exhibit only a few dominant faces) are usually characterized by the presence of a small number of narrow peaks—one for each exposed face. Therefore, vibrational spectra of adsorbed species provide morphological information that can be compared with information derived from HRTEM and SEM studies of the same microcrystals. 

A way to ascertain the coordination state of the anchored ions and, hence, their accessibility to adsorbing molecules is represented by the quantification of their capacity to adsorb molecular probes such as CO (and NO) and by the measurement of the IR spectrum of the resulting surface carbonyls (and nitrosyls). Thus, we obtain information about the propensity of the surface ions to insert ligands in their incomplete coordination spheres and, hence, indirect information about the location and structure of sites existing prior to adsorption. Of course, as mentioned previously, the probing with complexing molecules is always associated with a perturbation of the surface structures (this phenomenon is equivalent to surface relaxation). This unavoidable effect must be considered when structures of sites prior to adsorption are proposed. 

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