Backbone cleavage

PVF is more thermally stable than other vinyl halide polymers. High molecular weight PVF is reported to degrade in an inert atmosphere, with concurrent HF loss and backbone cleavage occurring at about 450°C (71,72). In air, HF loss occurs at about 350°C, followed by backbone cleavage around 450°C. 

Fig. 3. Mechanisms for polymer degradation. The illustration is a schematic representation of three degradation mechanisms I, cleavage of cross-links II, hydrolysis, ionisa tion, or protonation of pendent groups III, backbone cleavage. Actual biodegradation may be a combination of these mechanisms.

The most important degradation mechanism of asparagine and glutamine residues is formation of an intermediate succinimidyl peptide (6.63) without direct backbone cleavage (Fig. 6.29, Pathway e). The reaction, which occurs only in neutral and alkaline media, begins with a nucleophilic attack of the C-neighboring N-atom at the carbonyl C-atom of the Asn side chain (slow step). The succinimide ring epimerizes easily and opens by hydrolysis (fast step), as shown in Fig. 6.27, to yield the iso-aspartyl peptide (6.64) and the aspartyl peptide (6.65) in a ratio of 3 1. 

The biological mechanism of action is helieved to involve the production of superoxides near the DNA strand, resulting in DNA backbone cleavage and cell apoptosis. The actual mechanism is not yet known, but is believed to proceed from the reduction of molecular oxygen into superoxide via an unusual auto-redox reaction on a hydroxyquinone moiety of the compound following. There is also some speculation the compound becomes activated into its reactive oxazolidine form. 

Partial Reduction of Multiple-Cystine Peptides and Peptide Backbone Cleavage by... 

Backbone cleavages may involve removal of the N-terminal formyl moiety from the methionine, the removal of one or more amino acids at either the N- or C-terminus of the chain, and cuts in the backbone at one or more sites, possibly with the removal of internal peptide sequences. 

Photodegradation. UV/Vis exposure can induce protein oxidation, aggregation, and backbone cleavage. For example, oxidation has been observed in the histidine residue of human growth hormone (hGH) exposed to photostability conditions (6.7 x 106 lux hours). The proposed oxidation mechanism and product is shown in Figure 136. 

PVF decomposes in air at temperatures above 350°C (682°F) by dehydrofluorination. At approximately 450°C (842°F), backbone cleavage occurs.9 PVF films discolor at high temperatures, but retain considerable strength after heat-aging at 217°C (423°F).12... 

As pointed out by Heller (2), polymer erosion can be controlled by the following three types of mechanisms (1) water-soluble polymers insolubilized by hydrolytically unstable cross-links (2) water-insoluble polymers solubilized by hydrolysis, ionization, or protonation of pendant groups (3) hydrophobic polymers solubilized by backbone cleavage to small water soluble molecules. These mechanisms represent extreme cases the actual erosion may occur by a combination of mechanisms. In addition to poly (lactic acid), poly (glycolic acid), and lactic/glycolic acid copolymers, other commonly used bioerodible/biodegradable polymers include polyorthoesters, polycaprolactone, polyaminoacids, polyanhydrides, and half esters of methyl vinyl ether-maleic anhydride copolymers (3). 

Alkali will open adenine and cytosine rings at positions which can ultimately lead to DNA backbone cleavage. Incubation with 1 N NaOH splits some deoxyadenosine residues between C8 and N9 and a lower proportion of deoxycytidine rings, probably between C2 and N3. Treatment of the product with piperdine gives fairly uniform cleavage at the modified bases. The ratio of adenine cytosine breaks occur with a frequency of about 5 1. 

Electron-capture dissociation (ECD) [89], where the multiple-charge ion captures an electron to produce cationic products. The electron capture results in a radical site in the ion, leading to more and different backbone cleavages in the protein. It has been argued that the fragmentation actually takes place at the electron-capture site, i.e., prior to internal energy redistribution. 

Figure 17.2 Nomenclatiue of backbone cleavages in a peptide, y- and c-ions should be armotated as y" and c", because of the proton rearrangement involved in their generation. For more details [103-104].

Other fragmentation techniques have been introduced [108]. Some of these, e.g., sustained off-resonance irradiation (SORl) and infrared multiphoton dissociation (IRMPD), provide similar fragmentation as in CID, i.e., preferential backbone cleavages at the peptide amide bond (b- and y-ions). Others like electron-capture dissociation (ECD) [109-110] induce different fragmentation reactions, i.e., the formation of c- and z -ions due to cleavage of N-C bonds. 

To this end, different ion fragmentation tools have been characterized with respect to phosphopeptide fragmentation, e.g., electron-capture dissociation (ECD) [31] and infrared multiphoton dissociation (IRMPD) [32]. An application of ECD in PTM analysis is the top-down protein characterization (Ch. 18.3.5) of carbonic anhydrase [33]. IRMPD is applied in the study on protein kinase C phosphorylation [30]. Both ECD and IRMPD were applied in a subsequent nano-ESI-FT-ICR-MS study on protein kinase A phosphorylation [34]. Combined ECD and IRMPD for multistage MS-MS in FT-ICR-MS was applied for phosphopeptide characterization [35]. ECD provides backbone cleavages (c- and z -ions) without H3PO4 loss, whereas in IRMPD the loss of H3PO4 is prominent and only a few backbone cleavages (b- andy-ions) are observed cf. Ch. 17.6.1). 

It is very likely that the pyrolysis of poly(2-vinylpyridine-co-styrene) takes place by a random scission mechanism characteristic for both styrene and poly(2-vinylpyridine). Both benzene and pyridine were detectable in the pyrogram, but their levels were extremely low and these compounds are not included in Table 6.2.8 (even as trace type compounds). The presence of benzene and pyridine may be the result of the elimination of these compounds at very low level from the polymer backbone, but more likely they are formed from the free radicals that are already generated from the backbone cleavage. 

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