Single-stranded helical chain structure

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... 

Double helical structures may be constructed from complementary single-stranded polynucleotide chains sharing a common helical axis according to the procedure outlined below. The two strands of the complex are assumed to be regular helices defined by a common set of backbone and glycosyl torsion angles. The data presented here are limited to model poly(dA) poly(dT) double helices stabilized by Watson-Crick base pairs between anti parallel strands. 

A recently discovered subset of triple-stranded /l-helices from bacteriophage tail proteins (alternatively termed triple-stranded /1-solenoids ) represents another distinct group of /1-fibrous folds (Fig. 3B). In these structures, three identical chains related by threefold rotational symmetry wind around a common axis. These chains form unusual parallel /1-sheets with no intra- and only intermolecular -structural hydrogen bonding. Kajava and Steven (this volume) survey the distinguishing structural features of the known triple-stranded /1-solenoids, also documenting their notable diversity and differences in comparison to the single-stranded /1-solenoids. 

All the above-mentioned proteins have single-stranded folds based on solenoidal windings of one polypeptide chain. Recently, however, several triple-stranded /1-helices (alternatively, triple-stranded /l-solenoids ) have been described in bacteriophage tail proteins (Kanamaru et al., 2002 Smith et al., 2005 Stummeyer et al., 2005 van Raaij et al, 2001). In these structures, three identical chains wind around a common axis and their coils have an axial rise of 14.5 A, that is, 3 x 4.83 A (for details see Sections IV and V.D). In this chapter, triple-stranded /l-solenoids will be abbreviated as TS /l-solenoids, while the term /1-solenoid, if not otherwise qualified, will apply to the predominant group of single-stranded /l-solenoids. 

The duplex is a right-handed double helix with 10 bases per turn. The diameter of the helix is 20 A (2 nm) and the pitch is 34 A (3.4 nm). The sugar-phosphate backbone is on the outside of the helix, and the two antiparallel chains are connected by the hydrogen-bonded bases. The DNA in prokaryotes and eukaryotes is generally found in the duplex form, although there are some single-stranded DNA viruses. DNA is a very robust molecule in comparison with many proteins. The simple double-helical secondary structure is readily reassembled after denaturation, unlike the complex tertiary protein structures that can denature... 

This K is considerably smaller than that of the single-strand polymer at neutral pH, and suggests that the molecules have a rather thick structure. Adopting the interrupted-helix model, we may apply Eq. (97) as in the case of poly-L-proline. Noting that the unit translation distance b0 is 1.70 A for the double-stranded Watson-Crick helix (263 ) [i. e., a distance of 3.40 A for two residues, one in each chain], and taking Mu = 328 for poly(adenylic acid), we obtain r = 22 residues per helical... 

---