Crystallinity Degradation

Backes, D.W., Aboleneen, H.I., and Simpson, J.I., Quantitation of vancomycin and its crystalline degradation product (CDP-1) in human serum by high performance liquid chromatography, 7. Pharm. Biomed. Anal., 16, 1281, 1998. 

Ghassempour, A. et al., Crystalline degradation products of vancomycin as a new chiral stationary phase for liquid chromatography, Chromatographia, 61, 151, 2005. 

Appearance of crystalline degradation products Frequency of crystalline products occurrence unknown... 

Fluorescence polarization immunoassay determination of serum vancomycin concentrations can result in falsely high vancomycin concentrations in excess of 30-80% in patients with renal dysfunction. This is due to the formation of a non-toxic, non-microbiologically active pseudometabolite, the vancomycin crystalline degradation product (132). A report on a 48-year-old man underlies the significance of resulting underdosing and eventually suboptimal clinical response (133). 

The establishment of the molecular formula of the neomycins was difficult, since no simple salt of these bases could be obtained in crystalline form. The A-acetylated products are, however, crystalline and, eventually, crystalline degradation products were obtained, the analysis and yields of which afforded the correct elementary composition for neomycin. Both neomycin B and neomycin C have the molecular formula... 

Comprehensive and systematic investigations of the role of chain mobility for polyester degradation show that, for polyesters of similar crystallinity, degradation is controlled exclusively by polymer chain mobility, where mobility is mainly determined by the difference between ambient temperature and melting temperature [926]. 

Because of the high melt viscosity of polyolefins, normal spinning melt temperatures are 240—310°C, which is 80—150°C above the crystalline melting point. Because of the high melt temperatures used for polyolefin fiber spinning, thermal stabilizers such as substituted hindered phenols are added. In the presence of pigments, the melt temperature must be carefully controlled to prevent color degradation and to obtain uniform color dispersion. 

A second degradation process is oxidation, often photo-induced especially by exposure to light not filtered for uv. The radicals resulting from this reaction promote depolymerization of the cellulose, as well as yellowing and fa ding of paper and media. Aging causes paper to become more crystalline and fragile, and this can be exacerbated particularly if the paper is subjected to poor conditions. 

In some appHcations the high heat stabiHty of the micropowder can be utilized over a reasonably wide temperature range. A maximum service temperature is normally 260°C, provided the crystalline melting point is between 320 and 335°C. Exposure above 300°C leads to degradation and possible evolution of toxic decomposition products. 

Chemical conversion of compounds to intermediates of known absolute configuration is a method routinely used to determine absolute configuration (86). This is necessary because x-ray analysis is not always possible suitable crystals are required and deterrnination of the absolute configuration of many crystalline molecules caimot be done because of poor resolution. Such poor resolution is usually a function of either molecular instability or the complex nature of the molecule. For example, the relative configuration of the macroHde immunosuppressant FK-506 (105) (Fig. 8), which contains 14 stereocenters, was determined by x-ray crystallographic studies. However, the absolute configuration could only be elucidated by chemical degradation and isolation of L-pipecoUc acid (110) (80). 

Phosphonium salts are typically stable crystalline soHds that have high water solubiUty. Uses include biocides, flame retardants, the phase-transfer catalysts (98). Although their thermal stabiUty is quite high, tertiary phosphines can be obtained from pyrolysis of quaternary phosphonium haUdes. The hydroxides undergo thermal degradation to phosphine oxides as follows ... 

Typical stress—strain curves are shown in Figure 3 (181). The stress— strain curve has three regions. At low strains, below about 10%, these materials are considered to be essentially elastic. At strains up to 300%, orientation occurs which degrades the crystalline regions causing substantial permanent set. 

Starch is made thermoplastic at elevated temperatures ia the presence of water as a plasticizer, aHowiag melt processiag alone or ia blends with other thermoplastics (192—194). Good solvents such as water lower the melt-transition temperature of amylose, the crystalline component of starch, so that processiag can be done well below the decomposition—degradation temperature. 

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