Single-stranded Helicates

Single-stranded helical co-ordination around a single metal ion was reported many years ago,and other examples of complexes of this type have continued to appear together with reports of single-stranded dimetallic and oligometallic ° systems. 

Chambron, C. Dietrich-Buchecker and J.-P. Sauvage, Topics Current Chem., 1993, 165, 131. E.C. Constable, Prog. Inorg. Chem., 1994, 42, 67. 

Tulchinsky, L.I. Demina, S.V. Lindeman and Y.T. Struchkov, Inorg. Chem., 1994, 33, 5836 and references therein. 

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Arseniev, A. S., Barsukov, I. L., Bystrov, V. F., Lomize, A. L. and Ovchinnikov, Y. A. (1985). 1H-NMR study of gramicidin-A transmembrane ion channel head-to-head right handed single stranded helices, FEBS Lett., 186, 168-174. 

The complex [(tpy)Ru(/i-276)Ru(tpy)]" + has been prepared and characterized both in solution and the solid state structural data confirm a single-stranded helicate and reveal that the ethynyl bridge suffers from severe bending. Other alkyne-containing bridging ligands impose rigidity... 

Fig. 14. Conformational map for helices with i to i-4 hydrogen bonding (variants of the a-helix). The shaded areas represent those conformations which allow acceptable hydrogen bonding. Conformations inside the area (marked by 1) enclosed by the line (as shown more distinctly in Fig. 13) are sterioally allowed for single-stranded helical polyglyoine. Only conformations lying inside both areas satisfy simultaneously all criteria. The positions of the right- and left-handed standard a-helices are shown by circles, those of the cu-helioes are shown by triangles (Leach et al., 1966b).

Fig. 23. Energy contour diagram for single-stranded helical polyglycine. The units of energy are kcal mole-1. The symbols R and L indicate the location of the standard right-and left-handed a-helical conformations, B that of the B helix of myoglobin, cu that of the to-helix, 3io that of the 3io helix, and II that of the polyglycine II structure (Scott and Scheraga, 1966c).

An infinite ID complex [Ag( 14)( 15) with a single-stranded helical chain structure was obtained through the reaction of 6-amino-1-naphthalenesulfonic acid (H-14) and 2-methylpyrazine (15) with Ag2CC>3.58 Furthermore, when tetra- and hexaethyleneglycol-spaced ligands, 1,1 l-(3,6,9-trioxaundecane)diyl-bis(nicotinate) (16) and l,17-(3,6,9,12,15-pentaoxaheptadecane)diyl-bis(nicotinate) (17), were reacted with silver salts, double-stranded helical complexes (as exhibited in Fig. 11.24) were formed.59... 

The quinquepyridine ligand 7 forms a dinuclear, single-stranded helicate with ruthenium(II) ions (8) [133]. The crystal structure shows two different six-coordinate metallic sites, with one ruthenium(II) coordinated to a ter-pyridine subunit of 7 and the second ruthenium(II) bound to the remaining bipyridine subunit (Fig. 10). The helical twist of the strand (74.9°) results... 

Fig. 10 Representation of the single-stranded helicate 8 formed from quinquepyridine ligand 7 and ruthenium(II) ions [133], Ligand 7 is shown in blue to illustrate the helical twist...

Besides having a much lower molar mass than DNA, RNA generally forms only single-strand helices. RNA is often found associated with proteins inside cells. The most prevalent bases in RNA are the same as those in DNA, except that uracil is present instead of thymine. Three common types of RNA are ribosomal (rRNA), transfer (tRNA), and messenger RNA (mRNA). They are all involved in protein synthesis, controlling the sequence of amino acids that make up the primary structure. Thus the base sequence in RNA is related to the amino acid sequence in the protein that is made from it. 

Cu2+ ions also disorder single-stranded helical structures by forming crosslinks between and within polynucleotide strands (4). 

Hosseini and co-workers [49] described the synthesis of an infinite single strand helical supramolecular network 27 4 by mixing equimolar amounts of the calix[4]-arene derivative 27, bearing four pyridine groups as H-bond acceptors, with... 

R. Parthasarathy, M. Malik, and S. M. Fridey (1982) X-ray structure of a dinucleoside monophosphate a2 p5 c that contains a 2 -5 link found in (2 -5 )oligo(a)s induced by interferons Single-stranded helical conformation of 2 -5 -linked oligonucleotides. Proc. Natl. Acad. Sci. USA 79, pp. 7292-7296... 

A further example of a stereospecific self-assembled circular helical structure was prepared from the chiral ligand 43 and silver ions. According to the X-ray crystal structure determination, the spontaneously formed sixfold circular single-stranded helicate [Ag6(43)6](Pp6)6 44 (Figure 26) is formed as a single diastereoisomer. The... 

Structure and Thermodynamic Properties of Single-Strand Helical Polynucleotides... 

Single-strand helices, stabilized by base stacking (6, 13, 25, 30). 

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