Coordination environment, enhanced

Probing Metalloproteins Electronic absorption spectroscopy of copper proteins, 226, 1 electronic absorption spectroscopy of nonheme iron proteins, 226, 33 cobalt as probe and label of proteins, 226, 52 biochemical and spectroscopic probes of mercury(ii) coordination environments in proteins, 226, 71 low-temperature optical spectroscopy metalloprotein structure and dynamics, 226, 97 nanosecond transient absorption spectroscopy, 226, 119 nanosecond time-resolved absorption and polarization dichroism spectroscopies, 226, 147 real-time spectroscopic techniques for probing conformational dynamics of heme proteins, 226, 177 variable-temperature magnetic circular dichroism, 226, 199 linear dichroism, 226, 232 infrared spectroscopy, 226, 259 Fourier transform infrared spectroscopy, 226, 289 infrared circular dichroism, 226, 306 Raman and resonance Raman spectroscopy, 226, 319 protein structure from ultraviolet resonance Raman spectroscopy, 226, 374 single-crystal micro-Raman spectroscopy, 226, 397 nanosecond time-resolved resonance Raman spectroscopy, 226, 409 techniques for obtaining resonance Raman spectra of metalloproteins, 226, 431 Raman optical activity, 226, 470 surface-enhanced resonance Raman scattering, 226, 482 luminescence... 

Mitchell and Valero (1982, 1983) studied VO-phthalocyanine (VO-PC) and a Schiff base complex, VO-salen, as model vanadium compounds. The VO-PC provides a metal coordination environment comprised of four nitrogens, similar to the porphyrin. The benzo rings at the /3-pyrrolic positions contribute to enhanced aromaticity in the metal ligand. 

The differences in catalytic reactivity between Ti-HMS, Ti-MCM-41, and Ti-SBA-3 cannot be attributed to differences in Ti siting. XANES and EXAFS studies showed that the titanium center adopts primarily tetrahedral coordination in all three catalysts12. Also, the coordination environment is very similar for the three catalysts, as judged from the similarities in the EXAFS features. UV-VIS adsorption spectra showed no phase segregation of titania, the spectral features being consistent with site-isolated titanium centers. Because the framework walls of HMS tend to be thicker than MCM-41, the superior reactivity of Ti-HMS cannot be due to an enhancement in the fraction of Ti available for reaction on the pore walls. Thicker walls should bury more titanium at inaccessible sites within the walls. The most distinguishing feature is the greater textural mesoporosity for Ti-HMS. This complementary textural mesoporosity facilitates substrate transport and access to the active sites in the framework walls. 

Many simple complexes have been prepared as models of active sites of biomolecules. For example, a reactive five-coordinate thiolate Co complex (Figure 24) was prepared to model the active site of nitrile hydratase, a Co or Fe metalloenzyme that promotes the conversion of nitriles to amides. The synthesized model complex is facile in its uptake and release of azide and thiocyanate, indicating that an appropriate nonleaving group environment enhances ligand displacement sufficiently for catalytic paths in non-redox active Co metalloenzymes. Other examples have appeared earlier in this report. 

The NIR emission intensity of the lanthanide porphyrinate complexes follows the trend Yb > Nd > Er. This agrees with observations on other luminescent lanthanide complexes and reflects the fact that the efficiency of nonradiative decay increases as the energy of the luminescent state decreases. The emission yields of the ternary lan-thanide(III) monoporphyrinate complexes with hydridotris(pyrazol-l-yl)borate or (cyclopen-tadienyl)tris(diethylphosphito)cobaltate as a co-ligand are generally higher than those of other Yb(III), Nd(III), and Er(III) complexes because the coordination environment provided by the porphyrinate in combination with the tripodal anion effectively shields the Ln + ion from interacting with solvent (C-H) vibrational modes that enhance the rate of nonradiative decay. 

The goal of many biomolecular NMR studies is characterization of global molecular structure. In metallo-biomolecules, and in particular, for paramagnetic species, it is sometimes preferable to use NMR to perform a more focused study of the metal ion coordination environment and the metal electronic structure. Metal sites show great variation in the effects on chemical shifts and line widths and thus often call for tailored approaches. In this section, characteristics of some of the metalloproteins metal sites most frequently studied by NMR are summarized. Examples have been selected to illustrate approaches described in this chapter such as metal substitution, use of pseudocontact shifts, RDCs, relaxation enhancement, and detection of nuclei other than H. 

Recently, the use of pthalocyanines and porphyrins by Ishikawa and coworkers enabled tiie synthesis of several so-called double-decker or triple-decker lanthanide complexes, described as stacked Ji-conjugate molecules. The photophysical properties of lantiianide ious depeud markedly on their coordination environments. Since a multidecker framework can effectively keep solvent molecules away from Ln centers, it is assumed that enhanced luminescent properties can be achieved witii these types of architectures. We discovered that the use of two conjugated Schiff-base ligands H2Lh can stabihze novel multidecker framewoiks (Scheme 1). 

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