Amides bond dipoles

In this context, the solvent influence on the C—N rotational barrier in N,N-dimethylformamide, Mc2N—CH=0 <-> Me2N+=CH—0 , is noteworthy [280]. For this rotation, the Gibbs free energy of activation in the gas phase AG =81 kJ/mol) is much smaller than in polar HBD solvents such as water AG = 92 kJ/mol). Thus, the rate of amide bond rotation decreases as the polarity and the HBD ability of the solvent increases. This can be attributed to the change in dipole moment on rotation, whereby a polar solvent stabilizes the ground state with the higher dipole moment fi = 3.8 D) in preference to the less dipolar activated complex [280]. 

The exploitation of cross-correlation effects in high magnetic fields has introduced a new form of NMR spectroscopy called transverse relaxation-optimised spectroscopy or TROSY. The cross-correlation of the optimised dipole-dipole (DD) and chemical shift anisotropy (CSA) relaxation mechanisms leads to differential transverse relaxation rates for the two components of the l5N- H doublet in undecoupled spectra of l5N-labelled proteins. For one component, DD and CSA relaxation constructively add to produce very efficient relaxation, leading to a broad line, whereas for the other component, the two relaxation mechanisms constructively interfere, leading to a narrow line when the two mechanisms are nearly equal. There is no optimum field where DD and CSA relaxation are equal for all amide bonds, because DD relaxation between the amide protons and other nearby protons differs for each residue.72 Clearly, the overall effectiveness of TROSY is optimized when the non-exchangeable protons in the macromolecule... 

Fluoroalkenes have been recognized as one of the possible nonhydrolyzable mimetics of amides on the basis of the similarity of charge distribution of the amide bond with the fluoroalkene moiety (Figure 6.2) [31] and their dipole moments (3.7 and 0.97 D for N-methylacetamide and ( )-2-fluoro-2-butene, respectively) [32]. A comparison of the receptor-binding ability of neuropeptide substance P (SP) with some related isosteres with fluoroalkenyl moiety is shown in Table 6.5, which demonstrates the effectiveness of replacement of the amide bond with fluoroalkenyl moiety [33]. 

By comparison of the calculated electrostatic potentials (77) of trans-2-butene and 2-fluoro-2(Z)-butene with N-methyl acetamide as simple models of the peptidic bond and its isosteres (see figure 1) the fluoroolefin clearly is the better replacement of the amide bond, since it not only mimics its steric but also, at least in part, its electronic feature. Calculating dipole moments Abraham (73) came to similar results, but attempts to synthesize the corresponding dipeptide isostere 2 have been until now unsuccesful (14). As part of our ongoing program in fluoroorganic chemistry we developed two general methods for the preparation of these compounds. 

Electrophilic activation of the amide by placing the negative end of bond dipoles near the oxygen of the carbonyl. 

This refined resonance model was inspired by high-level calculations, and it reflects changing views on the importance of bond dipoles (i.e., C -0 ") in structure and reactivity. Bond dipoles are also important in rationalizing the Z-E preference. There are two strong bond dipoles in a secondary amide C -0 " and As shown, the Z... 

The spectroscopic analysis and various physical, dielectric, and electrical properties of odd-numbered nylons have been sumtxtarized. Odd-numbered nylons sb polymorphism. Nylon-11 shows at least live different crystal forms. The y form seems to be the most common polymorph in many odd-numbered nylons. The piezoelectric and ferroelectric behaviors fA odd-numbered nylons such as nylon-7 and nykm-11 are comparable or belter than for PVDF and depend on the crystal form. The desired stability of the piezoelectric response in odd-munbered nylons is related to the ferroelectric reorientation of the amide group dipoles followed by densely packed hydrogen-bonded sheets in the crystalline regions induced by poling and annealing. 

---