Left-handedness

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). 

About 50 years ago, physicists were amazed to discover that the universe, which had previously been regarded as completely symmetrical, had a certain preference for left-handedness. It had been considered impossible that basic natural laws would distinguish between left and right. This assumption formed the basis for the physical law of the conservation of parity according to this, the sum of the parities before and after each physical process must be equal. In other words the mirror image of each physical phenomenon is also a real phenomenon (Ball, 1994). 

The influence of the weak interaction on chemical reactions can be calculated since it favours left-handedness, it has an effect on the energy content of molecules and thus on their stability. In the case of the amino acids, the L-form would be more stable than the corresponding D-form to a very small extent. Theoretical calculations (using ab initio methods), in particular by Mason and Tranter (1983), indicated that the energy difference between two enantiomers due to the parity violation is close to 10 14J/mol (Buschmann et al., 2000). More recent evidence suggests that the... 

Everyone has preferred behaviours. All things being equal, we tend to do the things that we are most comfortable with doing and avoid doing the things that come less naturally to us. The most obvious example is our innate right- or left-handedness. If one lost the use of one s dominant hand, one would, with... 

The two chromatids in Fig. 26-13G are said to be coiled with opposite handedness. Can you draw this conclusion from Fig. 26 alone What are the biological implications for mitosis 280 Does the DNA have a differing chirality at the molecular level Compare this observation with the existence of snail shells or flowers497 with both right and left handedness within the same species. 

The right- or left-handedness of a helix is the same as a conventional screw or bolt. When turned clockwise, a right-handed screw advances. The same is true of a helix or a helical spring. 

Pasteur showed that optical activity was related to molecular right- or left-handedness (chirality). Later, van t Hoff and LeBel proposed that the four valences of carbon are directed toward the corners of a tetrahedron. If the four attached groups are different, two arrangements are possible and are related as an object and its nonsuperimposable mirror image. Enantiomers differ only in chiral (or handed) properties, such as the direction of rotation of plane-polarized light. They have identical achiral properties, such as melting and boiling points. 

The helical hand of the individual stems in chiral but racemic polymers is a very severe and therefore critical criterion in the crystallization process. The constraints apply for each stem and are dictated by the symmetry of the unit-cell. Contrary to the stem length (which, if incorrect, can be healed or adjusted by later structural reorganization), helix chirality involves a flip of a coin type of decision - right- or left-handedness. 

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],... 

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. 

Sex differences have also been found in the pattern of cerebral lateralization, with some studies reporting women to be less lateralized for some cognitive functions and language, as displayed, for example, by a more symmetric pattern at the dichotic listening test (McGlone, 1980 Hiscock et al., 1993). However, controversy exists as other studies have not replicated these findings or have found left-handedness to be more common in males (Sommer et al., 2008). 

It is possible to imagine a universe where space itself has an attribute of left-handedness or right-handedness, or where space does not but materials do. But if we set these possibilities aside and use ordinary ideas about symmetry, it follows that at any point where stresses exist inside a continuum, there are three orthogonal planes across which the tangential stress is zero these planes suffer only normal stresses. The planes themselves are principal planes, their normals are the three principal directions at the point and the normal-stress magnitudes are the principal stress magnitudes. (See Figure 6.1.)... 

A Fig. 3.16 A pair of enantiomers consists of two molecular species which are mirror images of each other and are non-superposable. (a) Helical Se o has either a right- or left-handedness, (b) The six-coordinate complex [Cr(acac)3] contains three identical didentate, chelating ligands the labels A and A describe the absolute configuration of the molecule (see Box 3.2). 

A and A prefixes enantiomers of octahedral complexes containing three equivalent didentate ligands (tris-chelate complexes) are among those which are distinguished using A (delta) and A (lambda) prefixes. The octahedron is viewed down a three-fold axis, and the chelates then define either a right- or left-handed helix. The enantiomer with right-handedness is labelled A, and that with left-handedness is A. 

The components of a Cartesian pseudotensor satisfy the same transformation law as do those of a true Cartesian tensor of the same rank, except when the transformation alters the right- or left-handedness of the axes (an improper rotation), as for the case of reflection in a plane (inversion). In this case the two transformation laws differ by an algebraic sign. 

A recurrent theme arising throughout the analysis pertains to the screwlike properties of particles and of their intrinsic right- and left-handedness (Sections 1I,C,1 II,C,2 III,C and IV,B). Such properties reflect an inseparable coupling between the translational and rotational motions of the particle. 

Coren S, Halpern DF (1991) Left-handedness - a marker for decreased survival fitness. Psychological Bulletin 109 90-106. 

The different chemical shifts that arise from enantiotopic atoms and groups in chiral solvents can be explained by assuming that the chiral solvent or additive must be part of the solvation shell of the solute molecule. Thus, the presence of chiral molecules nearby will result in different intermolecular interactions depending, on the right- or left-handedness of the prochiral atoms or groups. 

Use of a Ml or proper orthogonal group puts the additional property of preservation of right- or left handedness on the change of frame some authors [12, 23-26] (motivated usually... 

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