Handedness right/left

There are also other helix forms of DNA that differ in their helical parameters, such as handedness (right, left), number of base pau per turn, pitch, helix diameter, etc. 

Don t confuse conformation with configuration. Students confuse these terms all of the time. Conformations are the different positions that a molecule can twist into, but configuration is a matter of right-handedness or left-handedness (R or S). 

R.B. King, Chirality and handedness The Ruch shoe-potato dichotomy in the right-left classification problem. Ann. N.Y. Acad. Sci. 988, 158-170 (2003)... 

Isotactic poly(x-olcfin)s crystallise in a helical conformation, and, in the case of polypropylene, with three units per turn [4,5], Isotactic polypropylene has a melting point of 175°C and does not dissolve in boiling n-heptane [6,7], Note that, depending upon the configuration of the tertiary carbon atom of the polymer main chains, the poly(x-olefin) helices will be characterised by right-handedness or left-handedness. It should be mentioned that the helical structure of the poly(x-olcfin) chain per se is sufficient for the appearance of chirality of such a macromolecule [8], Figure 3.3 presents the helical conformation of chains of isotactic poly(a-olefin)s in the crystalline state (with three units per turn - the case of polypropylene) [5],... 

Isoblock polypropylene is characterised by the appearance of stereosequences of m diads bridged by pairs of r diads [20,21], Such polypropylene, containing a small number of long blocks, is usually referred to as isotactic polypropylene. Note that the polymer chains in this case contain helix sequences of the same handedness (right or left) in the chain, which is due to the same relative configuration of tertiary carbon atoms in these sequences. Stereoblock polypropylene, which is also called block-isotactic polypropylene,... 

The dominance of right-handedness over left-handedness occurs in all races and cultures. Despite this fact, even identical twins can exhibit differences in hand preference. Pictured are Matthew (right-handed) and Zachary (left-handed), identical twin sons of the author. 

The structural forms of DNA can differ in four aspects the "handedness" (right or left), the length of the helix turn, the number of base pairs per turn, and the difference in size between the major and minor grooves. The most common structural form of DNA is the B-form (Fig. 4.5). 

Roald Don t forget to talk about the puzzle of why all living systems use handed (chiral) molecules of a given handedness (chirality), left or right. 

Fig. X.l. The coordinate system used in the multipole expansion, (a) Interparticle distances. The large black dots denote the origins of the two Cartesian coordinate q/stems, labelled a and b, respectively. We assume particle 1 always resides close to a. particle 2 always close to b. The figure gives a notation related to the distances considered, (b) Tvo Cartesian coordinate systems (and their polar counterparts) one associated with the centre a, the second one with centre b (the x and y axes are parallel in both systems, the x axes are collinear). Note that the two coordinate systems are not on the same footing the z axis of a points towards b, while the coordinate system b does not point to a. Sometimes in the literature we introduce an alternative coordinate system with equal footing by changing zj —zj (then the two coordinate systems point to each other), but this leads to different handedness ( right- or left-handed ) of the systems and subsequently to complications for chiral molecules. Let us stick to the non-equivalent choice .

The preferred conformation of poly(oxy methylene) is nearly all-gauche and the crystal structures reported are a trigonal form (I), which is the most stable, and a less stable orthorhombic (II). The chains in the unit cell are of the same handedness, and left- and right-handed molecules evidently appear in dif erent crystal lamellae. 

Sketch of the racemic and chiral structures of the SmCP phase of achiral banana-shape molecules in antiferroelectric (at E = 0) and ferroelectric (at E > Etj, 5 V/ om) states. Left column Racemic structure, in which the chirality alternates in the adjacent layers. The APE domains are synclinic with coexisting opposite tilt directions. Right column chiral structure, in which the adjacent layers have the same handedness, but domains of different chirality coexist. The shading illustrates the bent or tilted shape of the molecules (brighter parts are closer to the reader). R (L) is the chirality descriptor corresponding to right (left(-handed layer conformations. 

Chiral (Section 9.2) Having handedness. Chiral molecules are those that do not have a plane of symmetry and are therefore not superimposable on their mirror image. A chiral molecule thus exists in two forms, one right-handed and one left-handed. The most common cause of chirality in a molecule is the presence of a carbon atom that is bonded to four different substituents. 

Fig. 9. The right-handed (R) and left-handed (L) three-fold helices of i-PP. For each handedness, the two different orientations (up or down) with respect to the reference axis are shown. The heights of the methyl groups are expressed ic c/6 units...

If a molecule is nonsuperimposable on its miixor image, the mirror image must be a different molecule, since superimposability is the same as identity. In each case of optical activity of a pure compound there are two and only two isomers, called enantiomers (sometimes enantiomorphs), which differ in structure only in the left-and right-handedness of their orientations (Fig. 4.1). Enantiomers have identical physical and chemical properties except in two important respects ... 

Ever since Pasteur s work with enantiomers of sodium ammonium tartrate, the interaction of polarized light has provided a powerful, physical probe of molecular chirality [18]. What we may consider to be conventional circular dichroism (CD) arises from the different absorption of left- and right-circularly polarized light by target molecules of a specific handedness [19, 20]. However, absorption measurements made with randomly oriented samples provide a dichroism difference signal that is typically rather small. The chirally induced asymmetry or dichroism can be expressed as a Kuhn g-factor [21] defined as ... 

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