Oligomer specificity

Comparative analyses of the number of favorable interactions in various arrangements have been made to distinguish the register and oligomer specificity of coiled coils, such as tropomyosin (McLachlan and Stewart,... 

B. van der Waals Forces, Steric Constraints, and Oligomer Specification... 86... 

Characterization of Protein Conformation in an Oligomer-Specific Fashion... 

Formaldehyde—Alcohol Solutions. These solutions are blends of concentrated aqueous formaldehyde, the alcohol, and the hemiacetal. Methanol decreases the average molecular weight of formaldehyde oligomers by formation of lower molecular weight hemiacetals. These solutions are used to produce urea and melamine resins the alcohol can act as the resin solvent and as a reactant. The low water content can improve reactivity and reduce waste disposal and losses. Typical specifications for commercially available products are shown in Table 7 (117). 

Direct production of select MDCHA isomer mixtures has been accompHshed usiag mthenium dioxide (30), mthenium oa alumiaa (31), alkah-moderated mthenium (32) and rhodium (33). Specific isomer mixtures are commercially available from an improved 5—7 MPa (700—1000 psi) medium pressure process tolerant of oligomer-containing MDA feeds (34). Dimethylenetri(cyclohexylamine) (8) [25131 -42-4] is a coproduct. 

The use of separation techniques, such as gel permeation and high pressure Hquid chromatography interfaced with sensitive, silicon-specific aas or ICP detectors, has been particularly advantageous for the analysis of siUcones in environmental extracts (469,483—486). Supercritical fluid chromatography coupled with various detection devices is effective for the separation of siUcone oligomers that have molecular weights less than 3000 Da. Time-of-flight secondary ion mass spectrometry (TOF-sims) is appHcable up to 10,000 Da (487). 

Prior to the development of analytical techniques to quantify specific methylene urea oligomers, methylene urea polymer distributions were characterized by physical (solubihty) methods. Products were separated into three fractions (3). 

The most widely used molecular weight characterization method has been GPC, which separates compounds based on hydrodynamic volume. State-of-the-art GPC instruments are equipped with a concentration detector (e.g., differential refractometer, UV, and/or IR) in combination with viscosity or light scattering. A viscosity detector provides in-line solution viscosity data at each elution volume, which in combination with a concentration measurement can be converted to specific viscosity. Since the polymer concentration at each elution volume is quite dilute, the specific viscosity is considered a reasonable approximation for the dilute solution s intrinsic viscosity. The plot of log[r]]M versus elution volume (where [) ] is the intrinsic viscosity) provides a universal calibration curve from which absolute molecular weights of a variety of polymers can be obtained. Unfortunately, many reported analyses for phenolic oligomers and resins are simply based on polystyrene standards and only provide relative molecular weights instead of absolute numbers. 

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