Sulfur helical structure

Amorphous or "plastic" sulfur is obtained by fast cooling of the crystalline form. X-ray studies indicate that amorphous sulfur may have a helical structure with eight atoms per spiral. Crystalline sulfur seems to be made of rings, each containing eight sulfur atoms, which fit together to give a normal X-ray pattern. 

C.-H. Kiang e o/.[33] reported that the singlelayered coiled lubes were obtained by co-vaporizing cobalt with carbon in an arc fullerene generator. A single-layered helical structure with radii of curvature as small as 20 nm was seen. These helically coiled forms lend to bundle together. In the soot obtained with sulfur-containing anodes, they also found the 1.3-nm diameter lube coil around the 3.6 nm tube (see Fig. 14). This kind of structure was theoretically proposed in ref. [14]. 

Because of the conversion of orthorhombic sulfur to monoclinic form, the above values of melting points are difficult to observe, as the resulting allotropic mixture melts at only 115 C. Amorphous or plastic sulfur can be produced through the rapid cooling of molten sulfur. X-ray crystallographic studies show that the amorphous form may have a helical structure with eight atoms per turn. This form is metastable at room temperature and gradually reverts back to crystalline within hours to days but this conversion can be rapidly catalyzed. 

The Ag cations are coordinated to two sulfur atoms of different cavitands with Ag-S distances in the range 2.47-2.50 A. In the solid, efficient r-stack-ing of the P-phenyl groups with the picrate anions stabilizes the supramolecular complex (Fig. 10). The two cavitands are aligned along their common C4 axis and offset by about 45°, leading to a helical structure. The inner space is reduced by the occupancy of the sulfur atoms, and there is probably not enough room to accommodate small guests inside the cavity. 

Recently, series of carbon-sulfur [n]helicenes substituted with w-octyl groups at the a-positions of the terminal thiophene rings were prepared (Fig. 15.16) [64, 87]. The helical structures of [7]helicene 44 and [ll]helicene 45 were confirmed by X-ray crystallography. Multiple short S-S contacts were found, especially for racemic [ll]helicene 45. Asymmetric synthesis of [ll]helicene 45 provided enantiomeric excess of either the (-)- or the (+)-enantiomer for the monoannelation or, unprecedented, triannelation approach (Fig. 15.16). Also, selective diannelation of octathiophene 47, followed by monoannelation of decathiophene 46, provided an efficient synthetic route to (-)-[ll]helicene 45, avoiding protection/deprotection steps [64],... 

Keratin in the cortex comprises 85% or more of the mass of the hair shaft. Cortical keratin is composed of two types of structural proteins, matrix proteins and fibrous proteins. 2-1 Matrix proteins have a high sulfur content and contain polypeptides with a molecular weight of approximately 10 to 28 kDa. Fibrous proteins are embedded in matrix proteins and are characterized by a low sulfur content. They have a molecular weight of approximately 40 to 58 kDa. Also, matrix proteins have a nonhelical structure and are readily soluble at pH 4.5 in 0.5 M KCl, whereas fibrous proteins exhibit a helical structure and are insoluble in this same solution. 

Somewhat later11 this flat chain structure was rejected on the grounds that the known geometrical properties of -S-S-S-S- systems should lead to a helical structure, as observed in two congeners of sulfur, selenium,... 

A crystalline phase of sulfur has been synthesized by workers (3, 4, 5, 6, 7) at pressures above 21 kb at temperatures in excess of 210°-240°C. This phase is insoluble in CS2. Lind and Geller (8) showed that this high pressure phase, designated fibrous sulfur, is monoclinic (P2) and consists of right and left handed helices. This helical structure resembles slightly the helicoidal zigzag chain structures of selenium and tellurium at ambient conditions. Fibrous sulfur has several properties characteristic of a modest semiconductor such as a steep negative temperature coeflBcient of resistance and a photoelectric eflFect. It shows also an EPR absorption peak and does not revert at ambient conditions (for up to 4 years) to ordinary octameric sulfur (6,7),... 

They are insoluble because the peptide chains are linked by disulfide bonds. Many keratins contain coils of a-helixes. Some keratines, however, were found to consist of complicated j -helical structures. This apparently has not been fully explained. Wool keratin was shown to range in molecular weight from 8,000-80,000. The extensibility of a-keratins is believed to be due to the helical structures. The extent of keratin hardness (in claws, horns, and nails) is believed to be due to the amount of sulfur links. 

HPLC is one of the most universal methods for determining the enantiomeric composition of substances and mixtures in a short time frame. Its application is not restricted to molecules in which chirality is based on a quaternary carbon atom with four different substituents it can also be employed for compounds containing a chiral silicon, nitrogen, sulfur, or phosphorus atom. Likewise, asymmetric sulfoxides or aziridines, the chirality of which is based on a lone electron pair, can be separated. Chirality can also be traced back to helical structures, as in the case of polymers and proteins to the existence of atropiso-merism, the hindered rotation about a single bond, as observed, for example, in the case of binaphthol, or to spiro compounds. 

Nonrepetitive but well-defined structures of this type form many important features of enzyme active sites. In some cases, a particular arrangement of coil structure providing a specific type of functional site recurs in several functionally related proteins. The peptide loop that binds iron-sulfur clusters in both ferredoxin and high potential iron protein is one example. Another is the central loop portion of the E—F hand structure that binds a calcium ion in several calcium-binding proteins, including calmodulin, carp parvalbumin, troponin C, and the intestinal calcium-binding protein. This loop, shown in Figure 6.26, connects two short a-helices. The calcium ion nestles into the pocket formed by this structure. 

Figure 15.9 The structure of right-handed and left-handed Sot helices in fibrous sulfur (see...

The concentration of this species in liquid sulfur was estimated from the calculated Gibbs energy of formation as ca. 1% of all Ss species at the boihng point [35]. In this context it is interesting to note that the structurally related homocyclic sulfur oxide Sy=0 is known as a pure compound and has been characterized by X-ray crystallography and vibrational spectroscopy [48, 49]. Similarly, branched long chains of the type -S-S-S(=S)-S-S- must be components of the polymeric S o present in liquid sulfur at higher temperatures since the model compound H-S-S-S(=S)-S-S-H was calculated to be by only 53 kJ mol less stable at the G3X(MP2) level than the unbranched helical isomer of HySs [35]. 

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