Solvent-Sensitive Probes

Bindingof ANS to BSAor human serum albumin (HSA) is one experimoit you can do at home. Prepare aqueous solutions of ANS (about 10 M) and BSA (about 10 mg/ml) and observe them under a UV handlamp. Little 

Membranes can also be labeled with more water-soluble probes like fluorescein or rhodamine. In such cases the probes can be forced to localize in the membrane by attachment to long acyl chains or to the phospholipids 

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The use of solvent-sensitive probes has a long history in biochemical research. We now describe a few examples to illustrate the diversity of applications. 

Let us now return to the question of solvolysis and how it relates to the stracture under stable-ion conditions. To relate the structural data to solvolysis conditions, the primary issues that must be considered are the extent of solvent participation in the transition state and the nature of solvation of the cationic intermediate. The extent of solvent participation has been probed by comparison of solvolysis characteristics in trifluoroacetic acid with the solvolysis in acetic acid. The exo endo reactivity ratio in trifluoroacetic acid is 1120 1, compared to 280 1 in acetic acid. Whereas the endo isomer shows solvent sensitivity typical of normal secondary tosylates, the exx> isomer reveals a reduced sensitivity. This indicates that the transition state for solvolysis of the exo isomer possesses a greater degree of charge dispersal, which would be consistent with a bridged structure. This fact, along with the rate enhancement of the exo isomer, indicates that the c participation commences prior to the transition state being attained, so that it can be concluded that bridging is a characteristic of the solvolysis intermediate, as well as of the stable-ion structure. ... 

A sensitive probe of electrostatic interactions in the distal pocket is provided by the structural and vibrational properties of the Fe-CO unit [9], The bound CO ligand exhibits three main infrared (IR) absorption bands, denoted Ao, A, and A3, with vibrational frequencies 1965 cm-1, 1949 cm, and 1933 cm, respectively. These bands, which change relative intensity and wave-number depending on temperature, pressure, pH, or solvent [10], are used to identify functionally different conformational substrates of MbCO, denoted taxonomic substates [11], Nevertheless the relationship between the A states and specific structural features of the protein has not yet been clarified. 

The chlorine atom cage effect was used as a highly sensitive probe for studying the effect of viscosity and the possible role of solvent clusters on cage lifetimes and reactivity for reactions carried out in supercritical fluid solvents. The results of these experiments provide no indication of an enhanced cage effect near the critical point in SC-CO2 solvent. The magnitude of the cage effect observed in SC-CO2 at all pressures examined is well within what is anticipated on the basis of extrapolations from conventional solvents (Fletcher et al., 1998). 

Photolysis of iodine. Unlike DBK, molecular iodine photodissociates via a one bond scission. Also the wavelength of the laser pulse used by Otto et al. (19) results in a relatively mild, low energy separation of the photogenerated radicals. Therefore 12 photodissociation should be a more sensitive probe for the characterization of solvent cage effects. 

In this manner, the electronic transitions of some molecules are sensitive probes of the solute environment. Since the probe molecules selected by Kamlet and Taft have n electronic states which are more polar than the ground state, a change in the polar-ity/polarizability of the solvent medium changes the electronic energy gap, and thus the position of the absorption band. Kamlet and Taft have developed an empirical relationship between measured solute absorption maxima in a solvent and the polarity/polar-izability of that solvent ... 

Solvent strength in the critical region. All of the experiments were performed with the dye phenol blue which has been well-characterized both experimentally and theoretically in liquid solvents (20,21,22). Since the dipole moment of phenol blue increases 2.5 debye upon electronic excitation (8), it is a sensitive probe of the local solvent environment. For example the absorption maxima occur at 550 and 608 nm in n-hexane and methanol, respectively. The excited state is stabilized to a greater extent than the ground state as the "solvent strength" is increased, which is designated as a red shift. 

The electrochemical quartz crystal microbalance is a versatile technique for studying several aspects of electroactive polymer film dynamics. For rigid films, it is a sensitive probe of mobile species (ion and solvent) population changes within the film in response to redox switching. For non-rigid films, it can be used to determine film shear moduli. In the former case, one simply follows changes in crystal resonant frequency. In the latter case, the frequency dependence of resonator admittance in the... 

When Pt2(pop)44- is irradiated at 450 nm in the presence of DNA, strand scission occurs, and scavenger studies show that no diffusible intermediates are involved (22). Thus, scission must involve abstraction of hydrogen atoms from the sugar functionalities by the metal complex itself. Because Pt2(pop)44- is an anion, binding of the complex to DNA does not occur. This presents the possibility that Pt2(pop)44- may be a particularly sensitive probe of solvent accessibility and hence nucleic acid structure. 

The solvent-dependent chemical shift observed for this cation is over 2600 ppm [238, 239]. In comparison, the known solvent chemical shift for Li is only 6 ppm [244], for Na 20 ppm [245, 396], and for Cs it is only 130 ppm [246]. The remarkable solvent sensitivity of the chemical shift of makes it an exceptionally useful probe... 

The proton spin-lattice relaxation times for solvent water are strongly perturbed if the water is in rapid exchange with a paramagnet. In particular, Mn is a strong relaxer for water protons and thus nuclear magnetic resonance (NMR) spectroscopy provides a sensitive probe for the presence of exchangeable water molecules bound to Mn in Mn proteins. 

To determine the shape of the hydrophobic barrier of bilayer membranes, fatty acids and PC molecules spin labeled with nitroxides at various positions along the lipid chains were diffused into vesicles and their solvent-sensitive isotropic coupling constants were measured [54]. Results are plotted in Figure 5 in terms of distance of the probe from the bilayer center. Also shown is the profile of the dielectric constant along the membrane normal evaluated from the fluorescence lifetime distribution of fluorescence probes in PC liposomes [55]. These data correlate well with results from neutron diffraction studies that map the positional distribution of water and lipid moieties along the bilayer normal [56]. 

Nuclear Magnetic Resonance (NMR). Alkali metal NMR is of interest because it is a sensitive probe to monitor the immediate chemical environment and the mobility of alkali metal ions in aqueous or nonaqueous solvents. Sodium-23 NMR method (26.46 MH ) was first employed by Komoroskl and Mauritz (33) to study cation binding in a perfluorinated sodium salt membrane (EW=1100) as a function of water content and temperature. A profound chemical shift and an increase in line width were observed with decreasing water content. These effects reversed as temperature increased. It was interpreted that as the amount of water or temperature is reduced, a larger fraction of sodium ions is bound to the membrane at any given instant. This causes the observed line width and the chemical shift to approach the values for the completely bound species. 

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