Trivalent nitrogen - configuration

Azapeptides are peptides in which one (or more) of the a-carbons have been replaced by a trivalent nitrogen atom [87,88]. This transformation results in a loss of asymmetry associated with the a-carbon and affords a structure that can be considered intermediate in configuration between D- and L-amino acids [89]. Interest in this a-carbon replacement units derives from its ability to provide resistance to enzymatic cleavage and its capacity to act as a selective inhibitor of cysteine and serine proteases. Complete substitution of all asymmetric trisubstituted a-carbons is represented by pure azapeptides , termed azatides [90]. 

Azapeptides, -NH-CHRi-CO-NH-NR -CO-NH-CHR -CO-, a dass of backbone-modified peptides in which the a-CH of one or more amino add residues in the peptide chain is isoelectronicaUy replaced by a trivalent nitrogen atom. This alteration results in a loss of asymmetry associated with the a-CH, and yields a structure that can be considered intermediate in configuration between d- and t-amino acids. This a-carbon replacement is connected with the capability to provide resistance to enzymatic deavage, and the capacity to act as selective inhibitor of serine and cysteine proteases [J. Gante, Synthesis 1989, 405 J. Magrathetal.,J. Med. Chem. 1992,35,4279 R. Xing et al., J. Med. Chem. 1998, 42,1344 E. Wieczerzak et al., J. Med. Chem. 2002, 45, 4202]. 

A similar situation occurs in trivalent phosphorus compounds, or phosphines. It turns out, though, that inversion at phosphorus is substantially slower than Inversion at nitrogen, so stable chiral phosphines can be isolated. (R)- and (5)-metbylpropylphenylphosphine, for example, are configurationally stable for several hours at 100 °C. We ll see the Importance of phosphine chirality in Section 26.7 in connection with the synthesis of chiral amino adds. 

Figure 5. Niels Bohr came up with the idea that the energy of orbiting electrons would be in discrete amounts, or quanta. This enabled him to successfully describe the hydrogen atom, with its single electron, In developing the remainder of his first table of electron configurations, however, Bohr clearly relied on chemical properties, rather than quantum theory, to assign electrons to shells. In this segment of his configuration table, one can see that Bohr adjusted the number of electrons in nitrogen s inner shell in order to make the outer shell, or the reactive shell, reflect the element s known trivalency.

Multiple bonding occurs in the nitrogen molecule, N2. Traditionally, nitrogen is described as trivalent, and the molecule is depicted as N=N, with three bonds linking the two atoms to each other. This is explained in the following way. The outer electron configuration of nitrogen is 2s 2p. Instead of... 

Bohr s account of the periodic table also came under attack from chemical evidence. The element nitrogen, for example, was attributed an electronic configuration of 2, 4,1, as noted above. This grouping of electrons suggested that 1 or 5 electrons were more loosely bound than the others and imphed either penta- or univalence, neither of which is the case in practice, as nitrogen is predominantly trivalent. 

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