Function of c-mos

Sagata, N., Oskarsson, M., Copeland, T Brumbaugh, J and Vande Woude, G. F. (1988). Function of c-mos proto-oncogene product in meiotic maturation in Xenopus oocytes. Nature 335 519-525. 

It should be noted that the NRE defined in these experiments is distinct from the previously described c-mos UMS sequence (Blair et al., 1984 Wood et al., 1984). The UMS is located approximately 1.4 kb upstream of the c-mos spermatocyte promoter and was identified because it blocked activation of c-mos transforming potential by insertion of retroviral promoters. It is thought to act as a transcriptional terminator, blocking transcription of c-mos initiated at upstream sequences. However, both the spermatocyte and oocyte transcription initiation sites are substantially downstream of the UMS. Moreover, the presence or absence of the UMS does not affect c-mos expression in either microin-jected oocytes (Pal et al., 1991) or transfected NIH 3T3 cells (Zinkel et al., 1992). It thus appears unlikely that the UMS functions as a negative regulator of c-mos transcription from either the spermatocyte or oocyte promoters in somatic cells. 

Also in need of further study is the function of Mos in male germ cells. It appears that Mos is expressed prior to meiosis of spermatocytes, consistent with the possibility that it acts in the initiation of meiosis and/or during progression from meiosis I to meiosis II. However, c-mos expression continues in postmeiotic spermatids, where it does not induce metaphase II arrest. This may be due to the absence of other components of cytostatic factor, but the role of c-mos expression in postmeiotic male germ cells remains unclear. 

In simple conjugated hydrocarbons, carbon utilizes sp2 hybrid orbitals to form a-bonds and the pure px orbital to give the it-MOs. Since the c-skeleton of the hydrocarbon is perpendicular to the wave functions of it-MO, only px AOs need be considered for the formation of it-MOs of interest for photochemists. Let us consider the case of butadiene with px AO contributed by 4 carbon atoms. The possible combinations are given in Figure 2.18. The energy increases with the number of nodes so that Et < < E3 < Et. 

The key process in the HF ab initio calculation of energies and wavefunctions is calculation of the Fock matrix, i.e. of the matrix elements Frs (Section 5.2.3.6.2). Equation (5.63) expresses these in terms of the basis functions and the operators //core, J and K, but the J and K operators (Eqs. 5.28 and 5.31) are themselves functions of the MO s i// and therefore of the c s and the basis functions Fock matrix to be efficiently calculated from the coefficients and the basis functions without explicitly evaluating the operators J and K after each iteration. This formulation of the Fock matrix will now be explained. 

Assume that the solution for the Hiickel secular determinant of a parent molecule has been determined, that is, that the eigenvalues Sj and the associated set of orthonormal HMOs ij/j are known. We now introduce a small perturbation of the system, say by adding a substituent or by replacing one of the C atoms by a heteroatom, for example N. To this end, we must first express the MO energies Sj as a function of the MO coefficients. Inserting Equation 4.6 into c, = < i// /7 i// > we obtain Equation 4.38. 

We have found by EPR that the yields of epoxide vary as a function of pentavalent Mo initially present on the grafted support. Indeed under the same activating conditionsd h at 80 C in ethylbenzene with a fixed RO2H/M0 ratio) the activity of the catalysts is definitely affected by the Mo(V)/Motot ratio which varies as a function of the different H202 Mo ratios adopted in the grafting procedure. TABLE 1 reports some typical results. 

Figure 2. Variation o/ DVD as a function of [i/Mos for spherical (c) and cylindrical (b) gels. D is the apparent diffusion coefficient obtained by assuming [L 0. D is the actual diffusion coefficient that can be calculated only if [l/Mos is known. Neglecting the shear modulus introduces only a small error for cylindrical samples.

The optimum value of c is determined by the variational principle. If c = 1, the UHF wave function is identical to RHF. This will normally be the case near the equilibrium distance. As the bond is stretched, the UHF wave function allows each of the electrons to localize on a nucleus c goes towards 0. The point where the RHF and UHF descriptions start to differ is often referred to as the RHF/UHF instability point. This is an example of symmetry breaking, as discussed in Section 3.8.3. The UHF wave function correctly dissociates into two hydrogen atoms, however, the symmetry breaking of the MOs has two other, closely connected, consequences introduction of electron correlation and spin contamination. To illustrate these concepts, we need to look at the 4 o UHF determinant, and the six RHF determinants in eqs. (4.15) and (4.16) in more detail. We will again ignore all normalization constants. 

If the wave function is variationally optimized with respect to all parameters (HF or MCSCF, but not Cl), the last term disappears since the energy is stationary with respect to a variation of the MO/state coefficients (Ho,Pi and P2 do not depend on the parameters C). 

Figure 10.1 shows 4 at various electrodes as a function of CO poisoning time at 26 °C. For the pure Pt electrode, the value of 4 decreases and reaches nearly zero after 30 minutes. In contrast, the Pt-Fe, Pt-Ni, Pt-Co, and Pt-Mo alloys retain high HOR activity for a prolonged period of time the reduction in 4 is negligibly small. Such CO tolerance of these alloys was found to be almost independent of the composition for example, alloying Pt with only 5 at% Fe resulted in excellent tolerance. However, Pt alloys with Ti, Cr, Cu, Ge, Nb, Pd, In, Sb, W, An, Pb, or Bi showed complete CO poisoning after a short time, while the combination of Pt with Mn, Zn, Ag, or Sn exhibited only limited CO tolerance. 

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