Solvents protonic

Dlugosz M, Antosiewicz JM (2005) Effects of solute-solvent proton exchange on polypeptide chain dynamics A constant-pH molecular dynamics study. J Phys Chem B 109 13777-13784. 

Ribonucleotide reductase differs from the other 5 -deoxyadenosyl-cobalamin requiring enzymes in a number of respects. Hydrogen is transferred from coenzyme to the C2-position of the ribose moiety without inversion of configuration. Also since lipoic acid functions in hydrogen transfer, exchange with solvent protons takes place. Furthermore, exchange between free and bound 5 -deoxyadenosylcobalamin occurs rapidly during catalysis. Evidence for a Co(I)-corrin as an intermediate for this reduction is presented in our section on electron spin resonance. 

Analysis of the data in Table XVIII suggests that silene formation is kinetically the most favorable process. However, according to experiment, metallated silenes are formed. This is related to the fact that in polar solvents proton transfer from the carbon atom to silicon is intermolecular, which leads to a considerable decrease in the reaction barrier. We believe that when the migration of substituents from the carbon atom to silicon is suppressed, for example, by the introduction of two alkyl radicals, the elimination of phosphines resulting in silene formation becomes the most probable process. 

Figure 1. The 100-MHz NMR spectrum of c s-Os(CO)kHg(0.05 M), K[Os-(CO)j,H](0.08 Nty, and hexamethyldisiloxane (0.01 M) (used as an internal line width standard) in CDSCN. Chemical shifts are illustrated in 8. The signal at 81.93 is due to residual solvent protons.

The reduction of 2-oxoacids bound to different chiral auxiliaries gave the 2-hydroxyacid derivatives in a 64 to 76% yield and 42 to 86% de depending on solvent, proton donor, supporting electrolyte, temperature, and substituent R in the oxoacid. The results are in accordance with an ECE reduction of the 2-oxoamide to an enolate anion, which subsequently undergoes a face-selective protonation to the hydroxy acid [346, 347]. 

XAFS measurements on the beautiful blue ion Cr(H20) + suggest tetragonal distortion with Cr —O = 1.99A (equatorial) and Cr —O = 2.30A (axial).Solvent proton relaxation measurements on Cr(II) in CHjOD indicate (a) two exchanging coordinated CHjOD,... 

From all these considerations, it results that it is usually very easy to obtain information on the presence or absence of protons in the first coordination sphere of a paramagnetic metal ion from the analysis of solvent proton relaxivity, but it may be hard to obtain their number, or to have information on second coordination sphere protons. 

The NMRD profile of chromium(III) aqua ion (Fig. 18) is characterized by slow exchanging water protons, as clearly shown by the fact that the solvent proton relaxivity at low fields increases with increasing the temperature. The occurrence of slow exchange hinders any increase in relaxivity below 300 K, thus explaining the fact that the contact dispersion disappears in the low temperature profiles, whereas it is well shown in the high temperature profiles, as already discussed in Section I.C.8. 

The smaller contribution to solvent proton relaxation due to the slow exchanging regime also allows detection of second and outer sphere contributions (62). In fact outer-sphere and/or second sphere protons contribute less than 5% of proton relaxivity for the highest temperature profile, and to about 30% for the lowest temperature profile. The fact that they affect differently the profiles acquired at different temperature influences the best-fit values of all parameters with respect to the values obtained without including outer and second sphere contributions, and not only the value of the first sphere proton-metal ion distance (as it usually happens for the other metal aqua ions). A simultaneous fit of longitudinal and transverse relaxation rates provides the values of the distance of the 12 water protons from the metal ion (2.71 A), of the transient ZFS (0.11 cm ), of the correlation time for electron relaxation (about 2 x 10 s at room temperature), of the reorienta-tional time (about 70 x 10 s at room temperature), of the lifetime (about 7 x 10 s at room temperature), of the constant of contact interaction (2.1 MHz). A second coordination sphere was considered with 26 fast exchanging water protons at 4.5 A from the metal ion (99), and the distance of closest approach was fixed in the range between 5.5 and 6.5 A. 

The contrast enhancing efficiency of a contrast agent is commonly expressed in terms of reiaxivity, i.e., the increase in the solvent proton longitudinal and transverse relaxation rates normalized to a one millimolar concentration of the chelated paramagnetic metal (10-12) ... 

Fig. 16. Cross-relaxation or Z-spectra derived from the solvent proton spectra of cross-linked bovine serum albumin gel supported in a ternary solvent system consisting of 8.8% H2O, 8.7% acetone, and 8.8% methanol and 63% D20.The offset axis represents the frequency offset of the off-resonance preparation pulse which partly saturates the immobilized spin. In this case the 3 s preparation had an amplitude of 880 Hz at a Larmor frequency of 500 MHz (87).

H-NMR and C-NMR spectroscopy demonstrated a three-component equilibrium (83G91) in ( 03)280 solutions of the diastereomeric monooximes of 5-methyl-2-phenacylcyclohexanone 54 and 55. The same methods revealed the ring-chain equilibrium 56A 56B (X = H2, R = Ph) in solutions of 5-hydroxy-5-methyl-2-phenylisoxazolidines (89KGS823). An increase in the solvent proton-accepting ability shifted the equilibrium in favor of the open-chain tautomer 56A in CCI4, Kj = 1.0 and in ( 03)280,... 

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