Left-handed preference

FIGURE 10.41 (a) Gramicidin forms a double helix in organic solvents a helical dimer is the preferred strnctnre in lipid bilayers. The strnctnre is a head-to-head, left-handed helix, with the carboxy-termini of the two monomers at the ends of the strnctnre. (b) The hydrogen-bonding pattern resembles that of a parallel /3-sheet. 

Now that about 70 different disulfides have been seen in proteins and more than 20 of those have been refined at high resolution, it is possible to examine disulfide conformation in more detail, as it occurs in proteins. Many examples resemble the left-handed small-molecule structures extremely closely Fig. 46 shows the Cys-30-Cys-115 disulfide from egg white lysozyme. The x > Xs and x dihedral angles and the Ca-Ca distance can be almost exactly superimposed on Fig. 45 the only major difference is in Xi All of the small-molecule structures have Xi close to 60°. Figure 47 shows the Xi values for halfcystines found in proteins. The preferred value is -60° (which puts S-y trans to the peptide carbonyl), while 60° is quite rare since it produces unfavorable bumps between S-y and the main chain except with a few specific combinations of x value and backbone conformation. 

For acyclic and monocarbocyclic subunits preference in lowest numbers is given to the carbon atoms through which they are attached to the main chain of the CRU. The point of attachment at the left-hand side of the subunit as written in the CRU must have locant 1. 

The nominal internal diameters correspond approximately to the internal diameters of the pipeline components (DIN 2402 - Feb. 1976). The left-hand column of the nominal internal diameter series is preferred in practice. 

In the case of flexible molecules all chiral conformers contribute to the observed CD spectrum. This usually leads to substantial reduction of the magnitude of the exciton Cotton effect. Nevertheless, if the conformation of the main contributor is established independently (e.g.. by molecular mechanics or NMR spectroscopy), its absolute configuration can be deduced from the exciton Cotton effect. Thus, for 3-(l-naphthalenyl)phthalide 5, the preferred conformation is 5 a. The negative couplet is in accordance with the left-handed screw between the phthalide 1 Ld and the naphthalene lBb transition dipole moment vectors, when the absolute configuration is R121. 

The disulfide group is inherently chiral, with the preferred torsion angle about the S-S bond being -90°. Both right- and left-handed disulfide bridges occur in proteins.305 306... 

Choosing units so that G equals h equals c equals 1. The left-hand side of this equation represents the geometry of spacetime, while the right-hand side is dependent on the rest of the matter of the universe. The left-hand side is classical, and an ordinary function of the spacetime points, while the right-hand side is a quantity, which depends on quantum operators. Thus the two sides are different and cannot be set equal to one another [28]. Isham [29] prefers to write Eq. (18) in the following form ... 

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