Amide dipole

Fig. 3. Alignment of amide dipoles in polyamide crystals (a) for a two-dimensional array of an odd nylon, nylon-7, (b) for a one-dimensional array of an odd—odd nylon, nylon-5,7 (c) for one-dimensional arrays of polyamides containing even segments an even nylon, nylon-6 an even—even nylon, nylon-6,6 ...

The measurement of the surface tension of SDS solutions at constant polymer additions was performed to investigate any possible interactions between SDS and the polymers used in these experiments. The results, shown in Figure 2, indicate no interaction between SDS and either PAA or PAM. Interactions between similarly charged surfactant and polyelectrolyte are not common as electrical effects frequently dominate to prevent any hydro-phobic or hydrogen bonding interaction. The hydrophilic nature of the amide dipole of polyacrylamides has been suggested (11) as a possible factor in preventing interaction with sodium dodecylsulfate,... 

Figure 17 Orientation of the side chains of the left- and right-handed a-helices of poly( -chlorobenzyl-L-aspartate). The solid arrows represent the direction of the C—Cl, ester, and amide dipoles, respectively.10...

Knowles (1991) proposed that triosephosphate isomerase catalyses its reaction by using the a-helix macro dipole supporting the hypothesis of Hoi et al. (1978). However, subsequent experimental studies of his group (Lodi and Knowles, 1993) indicated that the helix effect on His-95 is due to the hydrogen bonding provided by its end and not to the macro dipole. This work also considered a much smaller effect on His-103 that might be due to local amide dipoles near this residue. Apparently, the... 

Debyes,but lower than the gas phase dipole moment of formamide, 3.7 Debyes. (16) The small increase in the amide dipole with the addition of water indicates that water mobility is hindered in the presence of the amide group. 

Fig 11. Stereoview showing the orientations of amide dipoles around the cluster in Chromatium vinosum HiPIP. (Reproduced with permission from Ref. 169-. Jensen, G. M. Warshel, A. Stephens, P. J. Biochemistry 1994,33, 10911. Copyright 1994 American Chemical Society.)... 

In general, the differences in the oxidation levels used by the HiPIPs, in contrast to the low-potential ferredoxins, are thought to be principally due to differences in local amide dipoles, the more hydrophobic environment of the 4Fe—4S cluster, and fewer hydrogen bonds (20). 

It was shown that the folded conformation is favored by ca. 3 kcal/mcd in enthalpy and the entropy change is 3—4 e.u., vdiich is unfavorable to the folded form. The entropy loss on going to the folded conformation is readfly understandable in terms of the freezing out of all motion of the aromatic side chain in the folded form. The intramolecular interaction is not the process accompanyit the release of bound or ordered solvent molecules (hydroj obic interaction). It seems likely that the folded conformation of the aromatic cyclic dipeptides is stabilized by interaction of amide dipole and aromatic induced-dipole, dispersion forces between polarizable rr systems of the amide group and the aromatic ring, and some sbortHranged, h ily directional effects. As a consequence, the intramolecular interaction is characterized by invariant AH and AS with solvent and it is not destroyed even in amide solvent. 

Finally, we note that not all aqueous polymer solutions exhibit the foregoing behaviour. Thus, for example, poly(acrylic acid) and poly(acry-lamide) at low pH display positive values for the enthalpy and entropy of dilution parameters (Silberberg et ai, 1957). Day and Robb (1981) have confirmed calorimetrically that the heat of dilution of poly(acrylamide) in water is endothermic, althou, in contrast, they found that the substituted polyacrylamides poly(N-methyl acrylamide) and poly(N,N-dimethyl acrylamide) displayed negative heats of dilution. It was proposed that the endothermic character of the poly(acrylamide) solutions was a consequence of the energy required to separate the associated amide dipoles. 

As ATGEE possesses amide dipoles it does not, however, provide the most appropriate model for the amide group of the N-... 

In addition, %-% interactions between aromatic surfaces of base pairs (vertical direction) are believed to account for most of the thermal stability. In contrast, a-helices utilize three networks of hydrogen bonds between amide donors and acceptors (N-H 0=C) as well as the constituent amide dipole moments to guide (vertical direction) and stabilize the single helical configuration. Further stability of this structure in aqueous media is achieved by embedding a-helices inside a hydro-phobic environment, e.g., protein interior or membrane," which leads to stronger interactions when water is excluded. 

When the dipolar interactions are not included in the calculations, region III loses much of its stability, indicating that the extended peptide chain is mainly stabilized by the electrostatic interactions between the antiparallel amide dipoles. 

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