Blocking mechanisms

In the water-blocking mechanism large volumes of invaded liquid may be retained by low permeability or low-pressure formations. The blocking may occur for an oil wet and a water wet sandstones. 

The emulsion blocking mechanism involves formation of emulsion in the pores either by self-emulsification of water-based filtrate with the crude oil, or oil filtrate from an oil-based fluid emulsifying formation water. The emulsions are viscous and can block the pores. The remedial design is to prevent emulsification either by eliminating oil from completion fluid or by the use of demulsifiers. 

Tsaparlis, G. (1994). Blocking mechanisms in problem solving from the Pascual-Leone s M-space perspective. In H-J Schmidt (Ed.), Problem solving and misconceptions in chemistry and physics (pp. 211-226). Dortmund International Council of Association for Science Education. 

The ion-channel blocking mechanism has been widely tested and found to be important in both pharmacology and physiology. Examples are the block of nerve and cardiac sodium channels by local anesthetics, or block of NMDA receptor channels by Mg2+ and the anesthetic ketamine. The channel-block mechanism was first used quantitatively to describe block of the squid axon K+ current by tetraethylammonium (TEA) ions. The effects of channel blockers on synaptic potentials and synaptic currents were investigated, particularly at the neuromuscular junction, and the development of the single-channel recording technique allowed channel blockages to be observed directly for the first time. 

This is an important result. The simple open-channel block mechanism predicts that the total open time per burst is the same as the mean open time in the absence of blocker (Neher, 1983), even though openings are now chopped up by channel blockages. In fact, for channels that give bursts of openings in control recordings, the total open time per burst is constant in the presence or absence ... 

Hille, B., Ionic Channels of Excitable Membranes, 2nd ed., Sinauer, Sunderland, MD, 1992 (see chap. 7, Endplate channels and kinetics chap. 15, Channel-block mechanisms). 

Uojtanowicz, A.K., Krilov, Z., Langlinais, J.P. "Study on the Effect of Pore Blocking Mechanisms on Formation Damage," SPE paper 16233, SPE Production Operations Symposium, Oklahoma City, OK, March 8-10, 1987. 

Kubetzko S, Sarkar CA, Pliickthun A (2005) Protein PEGylation decreases observed target association rates via a dual blocking mechanism. Mol Pharmacol 68 1439-1454... 

H. Allen, O. Hill, N.I. Hunt, and A.M. Bond, The transient nature of the diffusion controlled component of the electrochemistry of cytochrome c at bare gold electrodes an explanation based on a self-blocking mechanism. J. Electroanal. Chem. 436, 17-25 (1997). 

Fig. 3. The mucosal block mechanism for the intestinal regulation and control of iron uptake into the blood...

In the case of branched polysaccharides, incorporation of side chains may take place through different mechanisms. In one of them, the assembly of the main chain is independent of the presence of side chains, and their incorporation into a polymeric molecule occurs as a modification of an initially formed, linear polysaccharide. Another situation is possible when incorporation of monosaccharide residues present in side chains is a necessary condition for elongation of the main chain, either through the monomeric or the block mechanism that is, intermediate formation of a linear, polysaccharide chain does not occur. Both mechanisms of incorporation of side chains were demonstrated to take place. 

These polysaccharides were the first examples where the block mechanism of polysaccharide-chain assembly was established, and the main features of the mechanism were first demonstrated in just this single case (for reviews, see Refs. 282-284). 

Another example of O-specific polysaccharides that are assembled through the block mechanism are the polymers from Salmonella serogroups C2 and C3. Their polysaccharide chains (18) are composed97 of linear, tetrasaccharide repeating-units having abequosyl and D-glucosyl branches,... 

It may be possible that the block mechanism of chain assembly also operates in the biosynthesis of colanic acid (33), the extracellular polysaccharide of E. coli.346 347... 

It seems possible that a block mechanism also operates in the assembly of the chain of polysaccharide 42, present in the cell wall of Bacillus cereus.124 In this case, formation of the polymer from a-GlcNAc-pp-Bpr, UDP-ManNAc, UDP-GalNAc, and UDP-Glc was observed.74... 

Formation of the three-dimensional network of bacterial peptidoglycan includes synthesis both of glycosidic and peptide linkages, but only the former type of reaction is discussed in this Section. The assembly of the carbohydrate chains of a peptidoglycan has been shown to occur through a block mechanism. The initial reaction consists in transfer of N-acetyl-muramyl-pentapeptide phosphate from the corresponding UDP derivative... 

It seemed of interest to check whether the type of biosynthetic mechanism involved may be related to the structure of the polymeric chain formed. In the block mechanism of chain assembly, formation of polyprenyl glycosyl diphosphates through transfer of glycosyl phosphate is a necessary step of the process. Thus far, only UDP-activated monosaccharides have been found to participate in this reaction (see Section 11,3). Consequently, the presence of a monosaccharide residue of this group in the main chain of a polymer may be regarded as a necessary condition for assembly of the chain through the block mechanism. 

The block mechanism of chain assembly is characteristic for polymeric chains of the UGT type (see Salmonella O-specific polysaccharides 10-12 and 18) and the UG type (see capsular polysaccharides 25, 27, and 33), with UDP-activated sugars serving as initiators of chain growth. It seems rather safe to suggest that the biosynthesis of other polymers of these types occurs through a block mechanism as well. 

---