Icosahedral hosts

Fig. 9.13 An icosahedral host, (a) X-ray crystal struture of the rhino-virus, a spherical virus linked to the common cold, (b) a schematic representation of the rhinovirus displaying triangulation.

Phase Trigonal prismatic void in 2c Metal host lattice atoms in 12k -f- 6h(l) -1- 6h(2) Icosahedral center in 2a Octahedral void in 6g ... 

Phase Octahedral void I in 16c Octahedral void 11 in 8a Icosahedral center in 16d Metal host lattice atoms in 48f + 32e... 

Hawthorne and co-workers have also produced a series of macrocyclic Lewis acid hosts called mercuracarborands (156, 157, and 158) (Fig. 84) with structures incorporating electron-withdrawing icosahedral carboranes and electrophilic mercury centers. They were synthesized by a kinetic halide ion template effect that afforded tetrameric cycles or cyclic trimers in the presence or absence of halide ion templates, respectively.163 These complexes, which can bind a variety of electron-rich guests, are ideal for catalytic and ion-sensing applications, as well as for the assembly of supramolecular architectures. 

Bacteriophage T7 Bacteriophage T7 and its close relative T3 are relatively small DNA viruses that infect Escherichia coli. (Some strains of Shigella and Pasteurella are also hosts for phage T7.) The virus particle has an icosahedral head and a very small tail. The virus particle is fairly complex, with S different proteins in the head and 3-6 different proteins in the tail. One tail protein, the tail fiber protein, is the means by which the virus particle attaches to the bacterial cell surface. Only female cells of Escherichia coli can be infected with T7 male cells can be infected but the multiplication process is terminated during the latent period. 

At the present time, there is no accepted chelating agent which can be used against common influenza viruses in humans. A virus has a core of either DNA or RNA and a protective coat of many identical protein units. All viruses are either rods or spheres, that is the protein coats are cylindrical shells having helical symmetry or spherical shells having icosahedral symmetry. Viruses reproduce inside living cells, where each viral nucleic acid directs the synthesis of about 1000 fresh viruses. These are then released and the host cell may die. 

An excellent example where a capsid virus has been given a new supramolecular application can be found in the work of Nolte who took an icosahedral capsid virus, cowpea chloritic mottle virus (CCMV) and used it as a nanoreactor for polymer synthesis [30], Natural CCMV spontaneously assembles in acidic aqueous solution and disassembles in basic solution. The capsid contains pores open at pH 5 to release RNA into the host. Once the RNA leaves, the empty capsule is left. The Nolte group was able to assemble the subunits around polystyrene sulfonate with a mass of 9.9 kDa but the resulting structure had a different morphology to the natural system. Indeed, capsules formed around polymers with masses between 2 and 85 kDa but not around those with masses above 100 kDa. This raised the question of the potential for polymers to form within a capsid but to test the possibility a mixture of botanical, biological and chemical approaches was needed. 

Parvoviridae is a family of small icosahedral, nonenveloped animal viruses that contain a linear ssDNA genome of approximately 5000 bases with short unique terminal palindromic sequences that fold back on themselves to form hairpin duplexes (van Regenmortel et al, 2000). They are further classified into two subfamilies, Parvcwirinae and Densovirinae, and various genera depending on host specificity, strand specificity of the genomic ssDNA, and on whether helper viruses are required for productive infection (dependoviruses). Members of the Parvoviridae... 

The alphaviruses are a group of 26 icosahedral, positive-sense RNA viruses primarily transmitted by mosquitoes [64]. These 700-A-diameter viruses are some of the simplest of the membrane-enveloped viruses, and members of this group cause serious tropical diseases with characteristic symptoms such as myositis, fever, rash, encephalitis, and polyarthritis [65]. The structures of two different alphavirus-Fab complexes have been determined by cryo-TEM Ross River virus (RR) and Sindbis virus (SIN) [66]. The amino acid sequences of the RR and SIN virus structural and nonstructural proteins are 49 and 64% identical, respectively [67]. The viral RNA genome and 240 copies of the capsid protein form the nucleocapsid core [68-73], and the El and E2 glycoproteins form heterodimers that associate as 80 trimeric spikes on the viral surface. Native SIN and RR lack the E3 glycoprotein because it disassociates from the spike complex after its display on the plasma membrane surface [74, 75]. El has a putative fusion domain that may facilitate host membrane penetration [76, 77]. E2 contains most of the neutralizing epitopes and is also probably involved in host cell recognition [78-80]. 

Hepatitis B virus (HBV) is a member of the Hepadnaviridae family and of the genus Orthohepadnavirus [15]. HBV causes chronic, acute, and fulminate hepatitis and is still a major health issue, with hundreds of millions of individuals infected despite the development of a number of efficacious vaccines [120]. HBV first assembles the capsid around the RNA pregenome and reverse transcriptase. On assembly, the pregenome is retrotranscribed [121] and the nucleocapsid is enveloped by portions of the host cellular membrane and viral glycoprotein. There are two sizes of HBV, composed of 90 or 120 capsid protein dimers in a T=3 or T=4 icosahedral arrangement, respectively [122, 123]. 

---