Head-to-tail manner

Some details of the chain-initiation step have been elucidated. With an oxygen radical-initiator such as the /-butoxyl radical, both double bond addition and hydrogen abstraction are observed. Hydrogen abstraction is observed at the ester alkyl group of methyl acrylate. Double bond addition occurs in both a head-to-head and a head-to-tail manner (80). 

The physical properties of any polyisoprene depend not only on the microstmctural features but also on macro features such as molecular weight, crystallinity, linearity or branching of the polymer chains, and degree of cross-linking. For a polymer to be capable of crystallization, it must have long sequences where the stmcture is completely stereoregular. These stereoregular sequences must be linear stmctures composed exclusively of 1,4-, 1,2-, or 3,4-isoprene units. If the units are 1,4- then they must be either all cis or all trans. If 1,2- or 3,4- units are involved, they must be either syndiotactic or isotactic. In all cases, the monomer units must be linked in the head-to-tail manner (85). 

The polymerization of vinyl chloride monomer, in common with other vinyl monomers, proceeds by a free-radical mechanism involving the usual steps of initiation, propagation, and termination. Poly(vinyl chloride) is formed in a regular head-to-tail manner Eq. (1) [3-6]. 

The formation of a cyclic tetrapyrrole—ie, a porphyrin—occurs by condensation of four molecules of PEG (Figure 32-6). These four molecules condense in a head-to-tail manner to form a linear tetrapyrrole, hy-... 

Analogous behavior was followed by the phenyl-substituted silene 156. The initially formed silene 157 underwent 1,3-methyl migration to give the silene 158, which then dimerized in a head-to-tail manner to yield three different stereoisomeric dimers 159, two of which were characterized by crystal structures. Again, the exchange of trimethylsilyl and trimethylsi-loxy groups at the ends of the Si=C bond occurred, followed by 1,3-methyl silicon-to-silicon rearrangements. The steps are summarized in Eq. (54). 

Another hypothesis was provided by Mikio Shimitso (1982) on the basis of studies of steric effects in molecular models. It had been noted years previously that the fourth nucleotide at the 3 end of the tRNA molecules (referred to as the discrimination base) might have a recognition function. In the case of certain amino acids (i.e., their tRNA-amino acid complexes) this base pair, in combination with the anticodon of the tRNA molecule, can select the amino acid corresponding to the tRNA species in question this is done on the basis of the stereochemical properties of the molecule. Since the anticodon of a tRNA molecule and the fourth nucleotide of the acceptor stem are far apart in space, two tRNA molecules must complex in a head-to-tail manner. The pocket thus formed can then fit specifically to the corresponding amino acid. 

The crystal structures of (EDT-TTFBr2)2MX4 and (EDO-TTFBr2)2MX4 are quite similar, although the space group symmetry is different in these two systems. However, this difference comes only from the conformation of terminal six-membered rings of the donor molecules, which plays no important role in the physical properties of the present salts. The donor molecules are stacked in a head-to-tail manner to form quasi-one-dimensional columns as shown in Fig. 6a. 

Chrysanthemic acid (1) consists of ten carbons, suggesting that it is a monoterpene. The cyclopropane ring of the acid moiety is a feature of pyrethrins. Rivera et al. isolated chrysanthemyl pyrophosphate synthase (CPPase or alternatively referred to as chrysanthemyl diphosphate synthase) underlying the formation of chrysanthemyl pyrophosphate (16) containing a cyclopropane ring from two molecules of dimethylallyl pyrophosphate (15) (DMAPP) and the gene thereof [21]. They found that the reaction involves the cF-2-3 cyclopropanation of DMAPPs in a non-head-to-tail manner. 

That microtubules exhibit an intrinsic polarity (i.e., with protofilaments running parallel and tubulin protomers regularly arranged in a head-to-tail manner) is obvious from experiments other than those involving image reconstruction. This intrinsic polarity was observed by Rosenbaum and Child (1976) and Witman (1975), who demonstrated biased tubulin addition to microtubules in vitro. This biased addition is a consequence of the nature of tubule-tubulin interactions at the two ends of the microtubule ... 

Geraniol and nerol are cis-trans isomers. In the rarely occurring lavandulol, the isoprene units are not coupled in the normal head-to-tail manner. 

The monomer units in I are linked tail to tail and not in the normal head-to-tail manner. Therefore I is not comparable with a normal dimer. In substance II a dimer lies to the right of the tail-to-tail linkage. In the copolymerization of a-methylstyrene with, for example, methyl methacrylate, the addition of one molecule of a-methylstyrene corresponds to substance I,... 

Lambert and co-workers50 also synthesized a pair of cyclic octapeptides 82 and 83 and studied their propensity to form nanotubular aggregates (Fig. 5). A linear peptide was initially synthesized on SynPhase Crowns with a Rink amide handle by standard Fmoc chemistry. The first residue used was aspartic acid protected as the ct-allyl ester. The support-bound linear peptide was cyclized in a head-to-tail manner by deprotection of... 

That the chalcone units combined in a head-to-tail manner was conclusively decided by showing that the thiolane could be dehydrogenated by dichlorodicyanoquinone in boiling chlorobenzene to a dibenzoyldiphenylthiophene, 8, whose 13C NMR revealed the presence of two different benzoyl groups (18). It followed that each of the two possible modes of head-to-nead fusion of chalcone units in a thiolane could be rejected. 

Spom, Roberts and Goodman81 have recently provided a broad overview of the field of the retinoids, based on the IUPAC-IUB Joint commission definition that Retinoids are a class of compounds consisting of four isoprenoid units joined in a head-to-tail manner. (The work was updated in 1994) Whereas there is only a handful of naturally occurring retinoids using this definition, more than a thousand man-made retinoids are known. The IUPAC-IUB bulletin recommended that the term, retinal, not be used as a chemical designation but be reserved as an adjective referring to the retina. They suggested that retinaldehyde be used instead in scientific literature as the name of chemical related to vision. 

Fig. 17. An incremental approach to the design of a four-helix bundle protein (Ho and DeGrado, 1987). (a) The sequence of a peptide is first optimized for forming a very stable tetramer of a helices. The stability of the tetramer can be assessed from the dissociation constant for the cooperative monomer-to-tetramer equilibrium, (b) Two optimized helical sequences are then connected in a head-to-tail manner by a single loop. The loop sequence can be optimized by evaluating a series of alternate designs, (c) Finally, the entire four-helix bundle structure can be constructed from four optimized helices and three optimized loops.

Terpenoids are widespread natural products that are formed from C5 isoprene units leading to their characteristic branched chain structure. Terpenoids are divided into families on the basis of the number of isoprene units from which they are formed. Thus there are monoterpenoids (Cio), sesquiterpenoids (C15) diterpenoids (C20), sesterterpenoids (C25), triterpenoids (C30) and carotenoids (C40). The isoprene units are normally linked together in a head-to-tail manner. However, the C30 triterpenoids and C40 carotenoids are formed by the dimerization of two Ci5 and C20 units, respectively. Hence, in these cases the central isoprene units are linked in a head-to-head manner. The presence of tertiary centres in the isoprenoid backbone of the terpenoids facilitates skeletal rearrangements in the biosynthesis of these natural products. As a consequence, on first inspection some structures appear not to obey the isoprene rule. 

The commercially available Big Blue mouse (mouse lineage Al) and rat contains 40 and 15-20 copies in a head-to-tail manner in chromosome 4, i.e., 80 and 30-40 copies per genome in homozygous strain,... 

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