Solvent-induced folding

Limited Proteolysis of Solvent-Induced Folding Changes of /3-Lactoglobulin... 

In this case, a moderately water-soluble amphiphilic N-vinylcaprolaclam (NVC1) played the role of a fl-unit, and a well-water-compatible N-vinyl-imidazole (NVIAz) served as a P-unil. The polymerization was carried out in a medium of 10% aqueous dimethylsulfoxide (DMSO). The addition of DMSO to the reaction solvent was necessary because of insufficient NVC1 solubility in pure water. It was also shown that in this solvent mixture, the NVCl-homopolymers and NVCl/NVIAz-copolymers retained their LCST-behaviour [26,28]. Hence, the DMSO in the reaction solvent did not significantly suppress the hydrophobic interactions of the NVC1 units. The polymerization was initiated by the redox system (N,N,N, N -tetramethylethylenediamine (TMEDA) + ammonium persulphate (APS)) and was carried out at 65 °C (1st step). This condition was very important, since admittedly the temperature was higher than the phase separation threshold of the reaction bulk when the polymeric products were formed that is, under these thermal conditions, hydrophobically-induced folding as the NVCl-blocks appear was ensured. After completion of the reaction, the... 

In this section, we review our first examinations of tryptophan probing sensitivity and water dynamics in a series of important model systems from simple to complex, which range from a tripeptide [70], to a prototype membrane protein melittin [70], to a common drug transporter human serum albumin [71], and to lipid interface of a nanochannel [86]. At the end, we also give a special case that using indole moiety of tryptophan probes supramolecule crown ether solvation, and we observed solvent-induced supramolecule folding [87]. The obtained solvation dynamics in these systems are linked to properties or functions of these biological-relevant macromolecules. 

Although copper binds tighter than zinc to aU forms of the enzyme tested, zinc stabilizes the protein fold better as judged by solvent-induced denaturation experiments. In addition the dissociation rate constant for zinc is about 100 times slower than copper suggesting the zinc is kinetically trapped once folding has occurred. This may thus be a physiological means by which metal ion specificity is achieved. ... 

Solvent-induced conformational changes may also affect the tendency to chain cyclization. Bruice and Turner(3) found that succinate esters have intramolecular rate constants about 20-fold smaller in water than the phthalate esters do, whereas the rate constants for the two groups of esters are essentially the same in 1 M H2O in DMSO. The observation was interpreted in terms of extended conformation of the succinate esters in water and predominant cisoid conformation in the H2O - DMSO mixed solvent. 

In section 4.7 we treated the helix-coil transition in the vacuum. It was assumed that the main thermodynamic force to form the helix is the strong HB between the C=0 of the kih. residue and the NH of the k + 4)th residue. Because of some simplifying assumptions it was possible to treat the helix-coil transition as a 1-D problem. In section 8.6.3 we pointed out how the insertion of a 1-D model in water might invalidate the theory. We now discuss one possible solvent-induced effect on the selection of a preferential folding pathway—here, a transition from coil to helix. 

Second, if we insist on studying the specific effects caused by the addition of a specific solute, we need not repeat the treatment of all the previous examples. The reason is that we already know that the general modification from the vacuum theory to the solution theory involves the solvent-induced quantity 8G (if we are working in the T, P, N system, which in practice is the most important system). We also know that 5G can always be expressed as a combination of solvation Gibbs energies. For instance, for the association and the folding of proteins, we have... 

Hydrolysis of TEOS in various solvents is such that for a particular system increases directiy with the concentration of H" or H O" in acidic media and with the concentration of OH in basic media. The dominant factor in controlling the hydrolysis rate is pH (21). However, the nature of the acid plays an important role, so that a small addition of HCl induces a 1500-fold increase in whereas acetic acid has Httie effect. Hydrolysis is also temperature-dependent. The reaction rate increases 10-fold when the temperature is varied from 20 to 45°C. Nmr experiments show that varies in different solvents as foUows acetonitrile > methanol > dimethylformamide > dioxane > formamide, where the k in acetonitrile is about 20 times larger than the k in formamide. The nature of the alkoxy groups on the siHcon atom also influences the rate constant. The longer and the bulkier the alkoxide group, the lower the (3). 

Recently, an example of cycloamylose-induced catalysis has been presented which may be attributed, in part, to a favorable conformational effect. The rates of decarboxylation of several unionized /3-keto acids are accelerated approximately six-fold by cycloheptaamylose (Table XV) (Straub and Bender, 1972). Unlike anionic decarboxylations, the rates of acidic decarboxylations are not highly solvent dependent. Relative to water, for example, the rate of decarboxylation of benzoylacetic acid is accelerated by a maximum of 2.5-fold in mixed 2-propanol-water solutions.6 Thus, if it is assumed that 2-propanol-water solutions accurately simulate the properties of the cycloamylose cavity, the observed rate accelerations cannot be attributed solely to a microsolvent effect. Since decarboxylations of unionized /3-keto acids proceed through a cyclic transition state (Scheme X), Straub and Bender suggested that an additional rate acceleration may be derived from preferential inclusion of the cyclic ground state conformer. This process effectively freezes the substrate in a reactive conformation and, in this case, complements the microsolvent effect. 

Hbp C93 S-nitrosation could also be accomplished by exposure to RSNOs (GSNO or CysNO). The rates RSNO-dependent Hb-S-nitrosation was 10-fold larger in oxy-Hb than in deoxy-Hb. Conversely, the rate of spontaneous decay of deoxy-Hb-SNO was -20-fold larger than oxy-Hb-SNO. An explanation for this differential reactivity was presented in a subsequent study (Stamler et al, 1997) where protein modeling data based on the X-ray structures of Hb in T and R states indicated that in OxyHb the SNO of Cys (1 93 is protected from solvent. In contrast, in deoxyHb the SNO is highly exposed to solvent. The implication was that the NO+ on Cys (193-S-NO could be transferred to thiols in RBC and eventually effluxed to induce vasodilation under conditions of low 02 saturation. 

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