Single helix

A single helix is a coil a double helix is two nested coils The tertiary structure of DNA in a nucleosome is a coiled coil Coiled coils are referred to as supercoils and are quite common... 

Parallel Day 1 South-facing slope is heated—single helix 2 Upslope flow on both heated slopes—double helix... 

Withers, J. G., Tube-Side Heat Transfer and Pressure Drop for Tubes Having Helical Internal Ridging and Turbulent/Tran-sitional Flow of Single-Phase Fluid Part 1 and Part 2 Single Helix Ridging, Heat Trans. Eng, V. 2, Jufy-Sept., Oct.-Dec. (1980) p. 49. 

Construction of the DNA chip (Nicolini 1996) requires a single-helix DNA immobilized at the surface of the device and sensitive to the binding of the complementary strand. 

Fig. 15. Stereo drawing of a bent helix (glyceraldehyde-phosphate dehydrogenase residues 146-161) with an internal proline. The proline ring produces steric hindrance to the straight a-helical conformation as well as having no NH group available for a hydrogen bond. A proline is the commonest way of producing a bend within a single helix, as well as occurring very frequently at the comers between helices.

Using computer modeling, jointly with x-ray fiber diffraction data, the molecular architectures of two different gel-forming polysaccharides have been examined. Preliminary results indicate that the neutral and doubly branched capsular polysaccharide from Rhizobium trifolii can form a 2-fold single helix of pitch 1.96 nm or a half-staggered, 4-fold doublehelix of pitch 3.92 nm. The molecules are likely to be stabilized by main chain — side chain interactions. 

There are two distinct stereochemical possibilities for the helix which are consistent with the intensity distribution. One of them is a 2-fold single-helix of pitch 19.6A and the other a 4-fold, half-staggered, parallel, double-helix of pitch 39.2A. The doublehelix could be right- or left-handed. In all cases, there is considerable conformational mobility about the (1- 6) linkage of the disaccharide side chain. Preliminary models have been built for each possibility and, due to insufficient diffraction data, detailed x-ray refinements have not been conducted for any of them. 

Figure 2. Two mutually perpendicular views of the 2-fold single-helix of RTFS. The sidechain conformation is gauche plus in (a) and irons in (b). (Reproduced with permission from Ref. 16. Copyright 1987 Gordon and Breach.)...

Using the pitch, symmetry, monomer geometries and other stereochemical constraints, a number of types of molecular model can be constructed. Typical dilemmas are whether the molecular helix is left- or right-handed, whether the molecule is a single helix or coaxial double-helix (and in the later case whether the two chains in the duplex are parallel or antiparallel), or whether, if there are two or more molecules in the unit cell, the molecules are parallel or antiparallel. Solution of a structure therefore involves refinement and adjudication All candidate models are refined until the fit with the measured x-ray amplitudes or steric factors allows one model to be declared significantly superior to the others by some standard statistical test. 

Examples of the various helical forms found in nature are the single helix (RNA), the double helix (DNA), the triple helix (collagen fibrils), and complex multiple helices (myosin, F-actin). Generally, these single and double helices are fairly readily soluble in dilute aqueous salt solution. The triple and complex helices are soluble only if the secondary bonds are broken. 

The helix-coil interconversion occurs through various elementary processes, among which the most important may be the following two types of process. One is associated with the growth of a helical sequence, i.e. appearance and disappearance of a single helix unit at the ends of a helical sequence. These are... 

Fig. 2. Principal configurations of elements used in bimetal thermometers (a) flat spiral, (b) single helix, (c) multiple helix...

Starting at the far left, we see a water molecule, two common amino acids, alanine and tryptophan, a segment of a DNA double helix, a segment of a protein single helix, and the folded polypeptide chain of the enzyme copper, zinc superoxide dismutase or SOD. 

Both in theory and in practice there exist eight gluco-pyranose homopolymers, and some of the molecular conformations of three of these, i.e. cellulose and amylose (l.,2., 3,4), and (1+3)-8-D-glucan (5.,6.,.7) have been established by x-ray analysis. Although (1+3)-a-D-glucan is among the five homopolymers previously unsolved by x-ray diffraction, possible chain conformations were predicted with computers to be an extended ribbon (8.,9.) a single helix (9.), or a double or triple helix (10). 

---