Handedness complexes

In the held of thermotropic cholesterics, the most promising approach seems to be that reported by Nordio and Ferrarini22 23 for calculating helical twisting powers. It allows one to tackle real molecules with rather complex structures and to describe them in detail. The model is currently being extended to include a better description of nematic solvents and specific solute-solvent interactions. Once tested also for conformationally mobile molecules, this model could allow the prediction of the handedness of single-component cholesterics, and, in the held of induced cholesterics, very interesting information on solute molecules could be obtained. 

In a recent paper, Krasnow and co-workers (120) applied a Rec A protein coating to DNA knots and catenanes to enhance visualization of the helical DNA segments and, in particular, to determine the absolute handedness of the knots. The Rec A protein is known to bind cooperatively to duplex DNA, forming a stiffened complex about 100 A in diameter in the presence of ATPase (121). 

Besides hairpin turns and broader U-tums, a protein chain may turn out and fold back to reenter a P sheet from the opposite side. Such crossover connections, which are necessarily quite long, often contain helices. Like turns, crossover connections have a handedness and are nearly always right-handed (Fig. 2-25).117/219 Most proteins also contain poorly organized loops on their surfaces. Despite their random appearance, these loops may be critical for the functioning of a protein.220 In spite of the complexity of the folding patterns, peptide chains are rarely found to be knotted.221... 

Upper Left A pair of mitotic sister chromatids in a section stained with an antibody to topoisomerase II. Notice that the two chromatids are coiled with opposite helical handedness. Lower Left Meiotic chromosomes of a lily at the pachytene stage in which sister chromatids are connected along their length in a synaptonemal complex. From Kleckner, N. (1996) Proc. Natl. Acad, Sci U.S.A. 93, 8167-8174... 

Figure 26-13 Synaptonemal complexes. (A) Aligned pairs of homologous chromatids lying 0.4 pm apart in Allium cepa. Arrows indicate "recombination nodules" which may be involved in initiating formation of crossovers. Portions of meiotic chromosomes of lily are shown at successive stages (B) Pachytene. (C) Portion of diplotene nucleus. (D) A bivalent at diplo-tene. (E) Two bivalents at diakinesis. Pairs of sister chromatids are coiled with appropriate handedness. (F) Sister chromatid cores are far apart in preparation for separation. A chiasma is present between the two central strands. (B) through (F) courtesy of Stephen Stack.279,279d (G) Pair of sister chromatids coiled with opposite handedness at metaphase. These are immun-ostained with anti-topoisomerase II antibodies. From Boy de la Tour and Laemmli.280 Courtesy of U. K. Laemmli.

Molecules of inheient structural asymmetry aie anisotropic they are optically active and exhibit optical rotation in solution. The typical optically active center is a carbon atom with four different substituents. In addition, any structural dissymmetry that results in a spatial left- and right-handedness will cause optical activity. Compounds of these types of come in a right-hand l R) and left-hand (L) form. When equal amounts of these two forms are mixed (racemic mixtures) there is no optical rotation because the activity of the two forms exactly cancel. Internal compensation of optically active centers m complex molecules is also found. Left- and right-handed optical isomers were first studied by Pasteur well over 100 years ago. and extensive surveys are found in most organic chemical texts. 

Inorganic complexes. Molecules that can support optical activity are said to be chiral, and to possess chirality (meaning handedness, since the human hand is chiral). 

As mentioned in the previous paragraphs, the striking relationship between the chirality of the individual molecular components and the corresponding helical handedness of the DC8,9PC tubules, has led scientists to believe that the tubule formation is driven by the molecular chirality. However, according to more recent studies the process of tubule formation is more complex than previously thought. It initially involves the formation of enantiopure La -phase vesicles which are then transformed to -phase helical ribbons composed of a nearly racemic mixture of left and right handed helices.90 In the few minutes following the sphere-to-tubule transition, monomeric lipids from the saturated... 

A pro-chiral molecule may turn chiral after undergoing a symmetry breaking chemical reaction on a surface. An example where covalent bonds to surface atoms are formed is the reaction of frans-2-butene with Si(100) [30]. As with purely adsorption-induced chirality, the relative alignment of the prochiral reactant with respect to the surface plane determines the handedness of the adsorbate complex (Scheme la). 

It is reasonable to assume that the molecules in these domains have opposite handedness. Doping the SU layer with one TA enantiomer suppresses completely the formation of one mirror domain and installs global homochirality [28]. The opposite TA enantiomer suppresses the opposite SU enan-tiomorph (Fig. 33). Since hydrogen bonds between the bisuccinate molecules cannot be expected to play a role, one must consider a substrate-mediated mechanism. That is, a chiral footprint onto the surface acts as a chiral bias and suppresses opposite handedness in the adjacent adsorbate complex. A chiral footprint reconstruction has also been proposed for the TA/Ni(110) system [110]. The same type of homochirality inductions have been shown for (S, S)- or (.R,.R)-TA-doped (R, S)-TA monolayers on Cu(110) [29]. 

Recently, the importance of the structure of chiral metal complexes on the handedness of the mesophases induced in a nematic LC was exemplified [114]. The chiral metal complexes 10 and 11—in which the alkyl substituents are aligned almost perpendicularly to the C2 axis in the former and parallel in the latter—show very different induction phenomena. Not only are the induced helicities in the nematic LC of opposite sense for the two compounds, but the helical twisting power of 10 is much higher than that of 11. The reason for these differences is the way in which the molecules are incorporated into the host nematic phase and exert their force upon it to create the twist between the layers. 

---