N-helix

Variations on the a helix in which the chain is either more loosely or more tightly coiled, with hydrogen bonds to residues n + 5 or n + 3 instead of n + 4 are called the n helix and 3io helix, respectively. The 3io helix has 3 residues per turn and contains 10 atoms between the hydrogen bond donor and acceptor, hence its name. Both the n helix and the 3to helix occur rarely and usually only at the ends of a helices or as single-turn helices. They are not energetically favorable, since the backbone atoms are too tightly packed in the 3io helix and so loosely packed in the n helix that there is a hole through the middle. Only in the a helix are the backbone atoms properly packed to provide a stable structure. 

Top right) The tetramer viewed from the top. The transition segment between the N-helix and the C-helix is shown in yellow and blue and the C-terminus is shown in green and purple. The hydrophobic strips are in the center and the N-helices are on each side. The tetramer has 2-fold symmetry about each of the three axes. 

Hoi, W.G.J. (1985a) The role of the n-helix dipole in protein function and structure. 

M. Miyasaka, A. Rajca, M. Pink, S. Rajca, Cross-conjugated Oligothio-phenes Derived from the (C2S)n Helix Asymmetric Synthesis and Structure of Carbon-Sulfur [lljHelicene, J. Am. Chem. Soc. 2005, 327, 13806-13807. 

Other structures are also possible, such as the 10-atom ring (310) or the 16-atom ring (n) helix however, these do not occur to any great extent in nature. Another feature of a helix geometry is that the amino acid side chains are located outside the a-helical core in planes perpendicular to the a helix axis. The core of the a helix is hydrophobic. 

N-helix insertions can potentially enhance membrane deformations... 

Figure 3 Steady-state shapes upon binding of the Amphiphysin N-BAR domain dimer plots show upper leaflet contours of membranes with different bending rigidities and with N-helix insertions of various depths. The membrane patches have -0.4e/nm2 average surface charge densities (corresponding to 0.3 PS lipid fractions) on both layers. The orientation of the BAR domain used in these calculations is the same as in Figure 2. For all systems, a nonzero spontaneous curvature c0 domain was defined for a membrane patch inside the BAR projection area shown in panel L and extending 20 A away from the projected zone. The values for c0 in the range of 0-1/70 A 1 were used.

Urea-formaldehyde resin solutions are shown to be dominated by physical associations rather than primary chemical bonding. These physical associations, or colloidal dispersions, are directly related to the thermodynamic balance of secondary bond formation between resin and solvent systems. Steric and entripic evaluations of molecule configuration have shown that linear urea-formaldehyde oligomers resemble polypeptides, and have the potential to form both 3-sheets and n-helixs, While the exact configuration of the associations is not known, their presence has been confirmed by x-ray analysis, which shows that urea-formaldehyde resins are crystalline in solid form. 

It is weU known that E Z photoisomerization of azobenzene-containing LC molecules can lead to a nematic-to-isotropic transition as well as photochromism [167]. It was reported that UV irradiation of a nematic mixture doped by chiral azobenzene bent-core LCs leads to the N -I transition and shift in selective reflection band position of the N phase [177]. These chiral bent-core molecules can induce the N helix upon doping them into a nematic base mixture, and their helical twisting power (HTP) is given by P = UPC where P is the helical pitch length, and C is the concentration of a chiral dopant. The P value for the exclusively E isomer is maximum and decreases with the increase in the ratio of the Z isomer. UV irradiation causes E Z conversion, therefore increasing the helical pitch and shifting the selective reflection band of the N phase. 

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