Backbone modifications

For nanofibers and nanotubes, backbone modification refers to selective chemical transformation of one or more of the core blocks. In our preparation of nanotubes from the PI-PCEMA-PtBA and PS-PCEMA-PtBA triblock nanofibers, the core PI and PtBA blocks were sculpted away either fully or partially. These reactions are examples of nanofiber backbone modification. One can also consider inorganic reactions carried out inside one of the block copolymer phases to be a kind of backbone modification. These reactions lead to the formation of interesting polymer-inorganic hybrid nanostructures. This topic will be reviewed in the following paragraphs. 

More recently we derivatized nanotubes to prepare water-dispersible tetra-block/Pd hybrid catalytic nanofibers [53] and triblock/Pd/Ni superparamag-netic nanofibers [83]. To prepare the former, nanotubes were prepared from PIi85-PtBAi5-P(CEMA67%-HEMA33%)85-PGMA245. The hydroxyl groups of the precursor PHEMA block was not fully cinnamated in this tetrablock because the HEMA imits facilitated the transport of Pd and Ni across the 

The complexed Pd(II) was then reduced by NaBH4 to Pd. The left panel of Fig. 17 is a TEM image for such nanotubes containing 4.0wt.% reduced Pd nanoparticles. 

The Pd-loaded nanofibers were dispersible in water as well as in water-based electroless plating solutions. Thus, the Pd nanoparticles could be used as a catalyst for the further electroless deposition of other metals. We, for example, plated P d into the tubular core onto the initially formed Pd nanoparticles via electroless plating to yield essentially continuous Pd nanowires. The right image in Fig. 17 shows a TEM image of such hybrid nanofibers after the incorporation of Pd to a total of 18.4 wt.%. In fact, we could time the final 

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One often encountered but usually undesirable consequence of backbone modification is the introduction of a chiral center at the phosphoms. Although chiral syntheses are being explored, all commonly used synthetic methods yield a mixture of diastereomers having either R or configuration. 

In an oligonucleotide having n backbone linkages, this gives rise to 2 isomers that vary in their mechanism of action, cellular transport, as well as pharmacokinetics (53). Some of the most frequently encountered backbone modifications are Hsted in Table 1. 

Fig. 1 Chemical structures of backbone modifications used in therapeutic nucleic acid analogs. Shown are the unmodified DNA/RNA chemical structures in addition to a selection of first (PS), second (OMe, MOE), and third generation (PNA, LNA, MF) nucleic acid modifications...

In view of these constraints, we recently suggested a different strategy for the improvement of the material properties of synthetic poly (amino acids) (12). Our approach is based on the replacement of the peptide bonds in the backbone of synthetic poly(amino acids) by a variety of "nonamide" Linkages. "Backbone modification," as opposed to "side chain modification," represents a fundamentally different approach that has not yet been explored in detail and that can potentially be used to prepare a whole family of structurally new polymers. 

Our interest in the synthesis of poly (amino acids) with modified backbones is based on the hypothesis that the replacement of conventional peptide bonds by nonamide linkages within the poIy(amino acid) backbone can significantly alter the physical, chemical, and biological properties of the resulting polymer. Preliminary results (see below) point to the possibility that the backbone modification of poly(amino acids) circumvents many of the limitations of conventional poly(amino acids) as biomaterials. It seems that backbone-modified poly (amino acids) tend to retain the nontoxicity and good biocompatibility often associated with conventional poly (amino acids)... 

Poly (iminocarbonates) are little known polymers that, in a formal sense, are derived from polycarbonates by the replacement of the carbonyl oxygen by an imino group (Fig. 5). This backbone modification dramatically increases the hydrolytic lability of the backbone, without appreciably affecting the physicomechanical properties of the polymer the mechanical strength and toughness of thin,... 

Spatola, A. F., Peptide backbone modifications, in Chemistry and Biochemistry of Amino Acids. Peptides, and Proteins (B. Weinstein, ed.), Marcel Dekker, New York, 1983, pp. 268-357. 

Kohn, J., and Danger, R., Backbone modification of synthetic poly-o-L-amino acids, in Peptides (G. R. Marshall, ed)., Escom, Leiden, Netherlands, 1988, pp. 658-660. 

Example 56 the Isis Pharmaceutical group in their extensive investigations of antisense oligonucleotides as therapeutics has described the synthesis of 3 -C-methylene nucleoside phosphonoamidites for the new backbone modification of oligonucleotides [90]. This paper gives good insight into tricoordinate phosphorus and related H-phosphonate chemistry in the service of nucleotide synthesis. 

M. Zanda, Trifluoromethyl group An effective xenobiotic function for peptide backbone modification. New J. Chem. 28 (2004) 1401-1411. http //www.nibib.nih.gov/in Vol. 2006. http //imaging.cancer.gov/in Vol. 2007. http //www.molecularimaging.org. http //www.ismrm.org/. http //interactive.snm.org/in Vol. 2007. 

The discovery of yet other nonhydrolyzable amide bond isosteres has particularly impacted the design of protease inhibitors, and these include hydroxymethylene or FfCF OH)], 12 hydroxyethylene or T fCF OFQCFy and T fCFkCHiOH)], 13 and 14, respectively dihydroxyethylene or ( [ )], 15, hydroxyethylamine or 4 [CH(0H)CH2N], 16, dihydroxyethylene 17 and C2-symmetric hydroxymethylene 18. In the specific case of aspartyl protease inhibitor design (see below) such backbone modifications have been extremely effective, as they may represent transition state mimics or bioisosteres of the hypothetical tetrahedral intermediate (e.g., xF[C(OH)2NH] for this class of proteolytic enzymes. 

Several other types of backbone modification have also been proposed, which produce nuclease-resistant oligos. Of these, a-oligos have been extensively studied. In a-oligos the base is transposed from the natural P-orientation to the unnatural a-orientation to form a parallel duplex with target sequence. This parallel duplex is nuclease-resistant, but does not elicit RNase H activity (Cazenave et al., 1989). These modifications have generated limited interest and application in antisense research. 

Figure 7.14 Development routes for antisense drugs. Examples of (a) a section of the backbone of a deoxyribonucleic chain, (b) backbone modifications, (c) sugar residue modifications and (d) base modifications...

In recent times peptide analogs have also been produced by a modification of the CO—NH bond (peptide backbone modification),9) ... 

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