Solvent perturbation

The solvent perturbation method depends on the different behavior of neutral and cationic acids when organic solvents are added to aqueous buffers, as indicated by the following examples  

Therefore, the experimental protocol must include the comparison of the behavior of the p/fa in both neutral acid and cationic acid buffers in the presence and absence of solvent. One must run all four experiments, because there are some variations in the behavior of different neutral add buffers (such as diethylamonate and phenolsulfonate) and cationic acid buffers (such as Ttis and glycine) which must be corrected by control experiments. Table 2 shows the expect changes in apparent pRa as a result of solvent perturbation (Cleland, 1977). 

In the particular case of alcohol dehydrogenase, outlined in Section 15.3, the combination of temperature studies with solvent perturbation will separate the neutral amino acids from the cationic ones, provided the group is ejqiosed to the solvent. Thus, by combining the knowledge of pK and the heat of ionization for a given amino add side chain with a solvent perturbation method, a fairly accurate identification of an amino acid can be achieved. 

The solvent perturbation method is a hazardous one, since one must always take care that the denaturation, or a time-dependent denaturation of enzyme. 

Candour, RD. Schowen, RL. (eds.) (1978) Transition states in biochemical processes, Plenum Press, New York and London. 

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McMorrow D, Kasha M (1984) Intramolecular excited-state proton transfer in 3-hydroxy-flavone. Hydrogen-bonding solvent perturbations. J Phys Chem 88 2235-2243... 

The dissolution of a solute into a solvent perturbs the colligative properties of the solvent, affecting the freezing point, boiling point, vapor pressure, and osmotic pressure. The dissolution of solutes into a volatile solvent system will affect the vapor pressure of that solvent, and an ideal solution is one for which the degree of vapor pressure change is proportional to the concentration of solute. It was established by Raoult in 1888 that the effect on vapor pressure would be proportional to the mole fraction of solute, and independent of temperature. This behavior is illustrated in Fig. 10A, where individual vapor pressure curves are... 

There is a small solvent dependence for [Ru(L)3]2+complexes which depends on the structure of L. The emission spectra of the Ru(bpy)32+ is mildly sensitive to media, the Ru(phen)32+ much less so, and the Ru(Ph2phen)32+ is virtually media independent. This decreasing sensitivity to solvent perturbation is a consequence of the excitation being localized in the metal a-diimine portion of the complex (-N=C-C=N).(27) The more extended the complex, the greater the shielding of the excited portion and the smaller the solvent perturbations of the emission spectrum. In particular, the bulky phenyl groups are extremely effective at shielding the excited state from environmental perturbations. 

Kasha s tests for identification of n- n and it -> n transitions. Solvent perturbation technique is a useful way to identify transitions as n- n Or it - for complex molecules. While comparing bands of different orbital promotion types in hydioxylic solvents such as water and ethanol with those in hydrocarbon or nonpolar solvents, if the band shifts towards the high frequency or shorter wavelength side (blue-shift) then the transition is probably n -> it. If there is a small red shift, the transition is likely to be 7t -> it. The effect of solvents on the n - re transition in acetone and pyrimidine is shown in the Table 3.3. 

Such solvent perturbations are largely responsible for the variation of photochemical behaviour of a molecule in different solvents, because the order of energy levels may change with change of solvent. Many such examples will be presented in appropriate places. 

Sogami, M. and Ogura, S. 1973. Structural transition in bovine plasma albumin. Location of tyrosyl and tryptophyl residues by solvent perturbation difference spectra. 

At 40% dioxane the fluorescence has increased about 50% over the value in water. Ribonuclease-A shows exactly the same effect in what is essentially a fluorimetric solvent perturbation experiment (308). All other physical evidence indicates that no structural change in RNase-A has occurred in this solvent at neutral pH and room temperature. There appears to be no explanation other than the alteration of fluorescence of accessible tyrosyl residues. 

The solvent perturbation technique has been used to study the absorption bands attributable to singlet-triplet transitions in benzo-[6]thiophene, using ethyl iodide as the perturbing solvent.173 Attempts to obtain information on the -electron conjugation in the 5-membered ring of benzo[6]thiophene by examination of the vibrational structure of the electronic spectrum were unsuccessful.171 UV evidence suggests that the valence shell of the sulfur atom can expand to a 10-electron structure in benzo[6]thiophene.156... 

Crystals suitable for protein x-ray studies may be grown by a variety of techniques, which generally depend on solvent perturbation methods for rendering proteins insoluble in a structurally intact state. The trick is to induce the molecules to associate with each other in a specific fashion to produce a three-dimensionally ordered array. A typical protein crystal useful for diffraction work is about 0.5 mm on a side and contains about 1012 protein molecules (an array 104 molecules long along each crystal edge). Note especially that, because protein crystals are from 20 to 70% solvent by volume, crystalline protein is in an environment that is not substantially different from free solution. 

The ASEP/MD method, acronym for Averaged Solvent Electrostatic Potential from Molecular Dynamics, is a theoretical method addressed at the study of solvent effects that is half-way between continuum and quantum mechanics/molecular mechanics (QM/MM) methods. As in continuum or Langevin dipole methods, the solvent perturbation is introduced into the molecular Hamiltonian through a continuous distribution function, i.e. the method uses the mean field approximation (MFA). However, this distribution function is obtained from simulations, i.e., as in QM/MM methods, ASEP/MD combines quantum mechanics (QM) in the description of the solute with molecular dynamics (MD) calculations in the description of the solvent. 

We now return to the task of formulating the solvent perturbation operator in Eq. (9-1). To formulate ourselves in terms of matrix elements, we introduce a real and orthogonal basis set for the quantum chemical region, =i, When the discussion turns to the quantum chemical method, the details of this basis will be dealt with, but for the moment the discussion is kept general. The permanent electrostatic contribution to Vsoiv., called Vperm., comes from the interaction between the quantum chemical charge distribution and the point-charges of the solvent. In other words,... 

Unfortunately, evaluating this formula exactly would still require that we know the fully coupled solute-solvent dynamics because it calls for Fext(t) = Fext(q((t)), but since the solvent perturbs the solute vibration only weakly, a perturbative treatment suffices (just as it does quantum mechanically). To leading order, Fext(t) = F Oj, what the solvent force would be if the solute s vibrational mode were held fixed. Thus, the average rate of solute-solvent energy transfer in the steady state is... 

Solvent-perturbation study of detergent binding AA-peak at 2450 A is not imidazole-based... 

Try AA-peak at 2900 A is connected with catalytic site Classification of tyrosyls by solvent perturbation method A-spectra show different modes of detergent-protein interaction at low and high detergent concentrations... 

The acid transformation of serum albumin has been studied further with the solvent-perturbation technique (Section VI,G ) by Leonard and Foster (1961), and by Herskovits and Laskowski (1962). The results of these studies support the idea that the tyrosyl groups of serum albumin do become progressively more exposed to solvent, both in the N-F transformation and in the molecular expansion at slightly lower pH. However, it is not yet possible to make unique assignments for the causes of the spectral perturbations observed by Williams and Foster (1959). 

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