Cross-linking site-directed

Ideally, one wishes to determine the location of the cross-linking sites within each partner of the complex. However, detailed structural information on both partners is rarely available. For example, although sequences of many tRNA species are known and a crystallographic structural model for this nucleic acid is available, the corresponding information for synthetases has not been obtained. In this case, attention is directed at the location in the tRNA of the cross-linking sites. From this, a picture can be drawn of the way in which the enzyme molecule lies across the nucleic acid. 

While there is no direct correlation between Vf and the magnitude of the saturation weight gain M, the latter is highly affected by resin polarity. Nevertheless, for cross-linked polymers, polarity and free volume are inter-related because both are concentrated about cross-linking sites (Soles and Yee 2000). 

Due to the retractive forces in stretched mbber, the aldehyde and zwitterion fragments are separated at the molecular-relaxation rate. Therefore, the ozonides and peroxides form at sites remote from the initial cleavage, and underlying mbber chains are exposed to ozone. These unstable ozonides and polymeric peroxides cleave to a variety of oxygenated products, such as acids, esters, ketones, and aldehydes, and also expose new mbber chains to the effects of ozone. The net result is that when mbber chains are cleaved, they retract in the direction of the stress and expose underlying unsaturation. Continuation of this process results in the formation of the characteristic ozone cracks. It should be noted that in the case of butadiene mbbers a small amount of cross-linking occurs during ozonation. This is considered to be due to the reaction between the biradical of the carbonyl oxide and the double bonds of the butadiene mbber [47]. 

Radioiodinated derivatives have been prepared to define more closely the target site of a-conotoxins on the acetylcholine receptor (R. Myers, unpublished data). In membrane preparations from Torpedo electroplax, photoactivatable azidosalicylate derivatives of a-conotoxin GIA preferentially label the p and 7 subunits of the acetylcholine receptor. However, when the photoactivatable derivative is cross-linked to detergent solubilized acetylcholine receptor (AChR), only the 7 subunit is labeled. Since snake a-neurotoxins mainly bind to the a subunits of AChR and a-conotoxins compete directly with a-bungarotoxin, the cross-linking results above are both intriguing and problematic. 

The reversibility of DNA cross-linking by the QMPs illustrated in Fig. 9.4 has not been examined directly but is presumably controlled as before by the leaving group strength of the DNA nucleophiles and the electronic characteristics of the QM intermediates. Unless both possible QMs form simultaneously, the cross-linking agents still remain bound to DNA at one site as the other is transiently released during... 

Hunke, S., Mourez, M., Jehanno, M., Dassa, E. and Schneider, E. (2000). ATP modulates subunit-subunit interactions in an ATP-binding cassette transporter (MalFGK2) determined by site-directed chemical cross-linking, J. Biol Chem., 275, 15 526-15 534. 

Polyphosphazene-based PEMs are potentially attractive materials for both hydrogen/air and direct methanol fuel cells because of their reported chemical and thermal stability and due to the ease of chemically attaching various side chains for ion exchange sites and polymer cross-linking onto the — P=N— polymer backbone. Polyphosphazenes were explored originally for use as elastomers and later as solvent-free solid polymer electrolytes in lithium batteries, and subsequently for proton exchange membranes. 

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