P group

The group of peptides known as tachykinins include substance P, substance K or neurokinin A, and neuromedin K, ie, neurokinin B, as well as a number of nonmammalian peptides. All members of this family contain the conserved carboxy-terrninal sequence Phe-X-Gly-Leu-Met-NH2, where X is an aromatic, ie, Phe or Tyr, or branched aliphatic, eg, Val or lie, amino acid. In general, this C-terminal sequence is cmcial for tachykinin activity (33) in fact, both the methionineamide and the C-terminal amide are cmcial for activity. The nature of the X residue in this sequence determines pharmacological identity (34,35) thus the substance P group contains an aromatic residue in this position, while the substance K group contains an aliphatic residue (33). 

Steric effects also pl y a significant role. With iV-(/ ,/ -disubstituted-ethyl)-AfJ rj r-trimefliylammonium ions, syn elimination is important when the P substituents are aryl or branched. As the p groups become less bulky, the amount of syn elimination diminishes. This effect is demonstrated by the data below. ... 

Sokolov, A. V., and Shirokovski, V. P., Group Theoretical Methods in the Quantum Physics of Solids (Spatial Symmetry), Soviet Physics—Uspekhi, 3, 651 (1961). 

Wigner, E. P., Group Theory and its Application to the Quantum Mechanics of Atomic Spectra, Academic Press, New York, 1969. 

Ordinary vinylie substrates react very poorly if at all by these mechanisms, but substitution is greatly enhanced in substrates of the type ZCH=CHX, where Z is an electron-withdrawing group such as HCO, RCO, EtOOC, ArS02, NC, F, and so on, since these P groups stabilize the carbanion ... 

NMR studies on polymers containing aspartic acid showed the presence of a relatively high proportion of P-peptide bonds (Andini et al 1975), i.e., the peptide bond involved the P-group of the acidic amino acid rather than the a-carboxyl group. 

Parent generation (P,) fed diets 360-mg/kg P, group had reduced fertility and hatchability, significant... 

Until now, only two families of ligands have been realized where both P groups are attached to side chains, probably because the resulting metal complexes have relatively large chelate rings which usually are not suitable for enantioselective catalysis. A cursory inspection of the ligands depicted in Fig. 25.14 shows that, due to steric bulk of the ferrocene backbone, both diphosphines probably have sufficiently restricted flexibility so that good stereocontrol is still possible. 

Oliver, J. P. Group 13 organometallic chalcogen derivatives their stmctures and behavior in solution. J. Organomet. Chem. 1995, 500, 269-281. 

An axial or equatorial preference of the YH (Y=N, P) group can be dictated by a modification of the heterocyclic ring. This may be accomplished by substituting a more electronegative or electropositive atom for X=CH2 in the system shown below ... 

A summary of all calculations is given in Table 5.8. Dunning basis sets are available for p group elements only. Basis set comparisons for other atoms therefore omit this category. Pople bases appear first, followed by the Dunning, ANO, and ECP bases. The mean absolute deviations (MADs) are listed. 

P group elements. Molecules with p group elements already have been studied with the P3 approximation and they probably will remain inviting objects of study with this method. Errors obtained for the p group elements (Table 5.5) are somewhat larger than those found for organic molecules. Groups VI and VH are especially problematic. 

Table 5.5 Ionization energies and their MADs (eV) computed with the best basis sets for p group elements.

Aside from the results for the individual atoms, some trends in basis set performance may be observed. Pople basis sets produced results that were fairly accurate, especially for alkali and alkaline earth metals. Although the results are much less accurate for the p group elements, they are certainly within acceptable error for this simple approximation. The steady decrease in errors observed in the progression from the P3/6-31G to the P3/6-311++G(3df,3pd) level for nontransition elements also attests to the sound design of these basis sets. 

Basis set3 Alkali and alkaline earths P group elements TVansition metals ... 

The performance of the P3 method used in conjunction with ECPs is also encouraging. Among the p group metals, the CEP-4G set is the most accurate (MAD of 0.67 eV), with the SHC potential of Goddard and Smedley performing best for the alkalis and alkaline earths (MAD of 0.34 eV). The SDD sets succeed in all three cases and produce errors that are competitive with all of the other ECPs. 

The nitrido and phosphido complexes of TMs have been the subjects of intensive experimental studies in the recent years. Of particular interest has been the issue of Lewis basicity of the nitrogen and phosphorus atoms in the TM=N and TM=P groups. Table 7.17 lists the BDEs calculated at the MP2/II, B3LYP/B and CCSD(T)/B levels of theory for LnMN-X and LnMP-X, where X is a group-13 Lewis acid or a chalcogen atom [86, 87]. 

Now for the glycerates. 1,3 bis-phosphoglycerate [compound (iii)] is the only molecule with two attached P groups. When we number the carbon atoms in an aliphatic organic compound we invariably start at the most oxidized carbon (drawn at the top of the chain), so carbon 2 of the glyceric acid derivatives must be the middle... 

In an asymmetric p-diketone there should be a preference by the enol proton for one of the carbonyls over the other and attempts have been made to determine which it is by C-nmr spectroscopy (Shapet ko et al., 1975 Lazaar and Bauer, 1983). With the additional help of O-nmr spectroscopy it has been possible to demonstrate convincingly that the enol group prefers the carbonyl with a p-group in the following order CF3 > Ph > Bu > Me (Geraldes et al., 1990). 

Figure 4a. The structure of cortiuncto-l(PMe2Ph)2(Pt2BgH 14)] showing the central linear P-Pt-Pt-P group coordinated by an group and a bridging B Hs).

If [TO4] groups are linked to form chains, multiplicity is the number of linked chains the observed multiplicities in this case are m = 2,3, 4, and 5. Analogously, multiplicity is defined as the number of mutually linked sheets. If several [TO4] groups are linked in a chain, periodicity (p) is the number of groups that defines the structural motive (i.e., after p groups, the chain is obtained by simple translation). Dimensionality of [TO4] groups is the degree of condensation of a crystal structure. The condensation of an infinite number of tetrahedra may lead to infinite unidimensional chains d = 1), bidimensional sheets d = 2), and tridimensional lattices d = 3). The dimensionality of an isolated [TO4] tetrahedron is zero d = 0). 

In skin-painting studies in mice, catechol increased the carcinogenic effects of benzo[< ]pyrene (B(skin tumors was compared with that obtained from groups treated with B([Pg.129]

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