Hairpins

Eaton W A, Munoz V, Thompson P A, Flenry E R and Flofrichter J 1998 Kinetics and dynamics of loops, i helices, [i-hairpins, and fast-folding proteins Acc. Chem. Res. 31 745-53... 

The specific nature of the reentry loops is not the point of this illustration. The sketch shows both hairpin turns and longer loops. Problem 7 at the end of the chapter examines the actual nature of the reentry loop. 

ColEl Regulation by RNA Hairpins. Rephcation of the E. coli plasmid ColEl is regulated by two short RNA molecules and a protein in a system that provides an example of the unique stmcmral elements accessible to RNA molecules. Multidimensional heteronuclear nmr spectroscopy has been used to characterize the complex formed between the two RNAs (25). Each of the RNA molecules fold back on the other to form a pair of hairpin... 

Use of D-amino acids in the synthesis of a hairpin loop portion from the CD4 receptor provides a stable CD4 receptor mimic, which blocks experimental allergic encephalomyelitis (144). This synthetic constmct is not simply the mirror image or enantiomer of the CD4 hairpin loop, but rather an aH-D-constmct in the reverse sequence, thus providing stereochemicaHy similar side-chain projections of the now inverted backbone (Fig. 11). This peptide mimetic, unlike its aH-L amino acid counterpart, is resistant to en2yme degradation. As one would expect, the aH-D amino acid CD4 hairpin loop, synthesi2ed in the natural direction, the enantiomer of the natural constmct, is inactive. 

Fig. 11. Use of D-amino acids in the synthesis of a hairpin loop portion from the CD4 receptor (a) all L-Ser—Lys—Ala tripeptide constmcted in the natural...

Fig. 5. Thermal vaporization sources (a) hairpin (b) spiral (c) basket (d) boat and (e) canoe, which ate all resistively heated sources and (f) focused...

U-Tube Heat Excbajiger (Fig. 11-36J) The tube bundle consists of a stationaiy tube sheet, U tubes (or hairpin tubes), baffles or support plates, and appropriate tie rods and spacers. The tube bundle can be removed from the heat-exchanger shell. A tube-side header (stationary head) and a shell with integr shell cover, which is welded to the shell, are provided. Each tube is free to expand or contract without any limitation being placed upon it by the other tubes. 

The hairpin heat exchanger, unhke the removable bundle TEMA styles, is designed for bundle insertion and removal from the return end rather than the tubesheet end. This is accomplished by means of removable spht rings which shde into grooves machined around the outside of each tubesheet and lock the tubesheets to the external closure flanges. This provides a distinct advantage in maintenance since bundle removal... 

TABLE 11-15 Double-Pipe Hairpin Section Data... 

The best known use of the hairpin is its operation in true counter-current flow which yields the most efficient design for processes that have a close temperature approach or temperature cross. However, maintaining countercurrent flow in a tubular heat exchanger usually implies one tube pass for each shell pass. As recently as 30 years ago, the lack of inexpensive, multiple-tube pass capability often diluted me advantages gained from countercurrent flow. 

Surface Condensers Surface condensers (indirect-contact condensers) are used extensively in the chemical-process industiy. They are employed in the air-poUution-equipment industry for recoveiy, control, and/or removal of trace impurities or contaminants. In the surface type, coolant does not contact the vapor condensate. There are various types of surface condensers including the shell-and-tube, fin-fan, finned-hairpin, finned-tube-section, ana tubular. The use of surface condensers has several advantages. Salable condensate can be recovered. If water is used for coolant, it can be reused, or the condenser may be air-cooled when water is not available. Also, surface condensers require less water and produce 10 to 20 times less condensate. Their disadvantage is that they are usually more expensive and require more maintenance than the contac t type. 

In the constant-strain method, the specimen is stretched or bent to a fixed position at the start of the test. The most common shape of the specimens used for constant-strain testing is the U-beud, hairpin, or horseshoe type. A bolt is placed through holes in the legs of the specimen, and it is loaded by tightening a nut on the bolt. In some cases, the stress may be reduced during the test as a result of creep. In the constant-load test the specimen is supported horizontally at each end... 

To date, RNA calculations have been performed on a variety of systems of different topologies including helical duplexes, hairpin loops, and single strands from tRNA, rRNA, and ribozymes. In a simulation of an RNA tetraloop of the GRNA type, which is very common and known to be remarkably stable, it was found that without imposing any external infonnation the simulation found the right confonnation even when it started from the wrong one [72]. Studies have used Ewald summation methods to handle the... 

Figure 2.8 Adjacent antiparallel P strands are joined by hairpin loops. Such loops are frequently short and do not have regular secondary structure. Nevertheless, many loop regions in different proteins have similar structures, (a) Histogram showing the frequency of hairpin loops of different lengths in 62 different proteins, (b) The two most frequently occurring two-residue hairpin loops Type I turn to the left and Type II turn to the right. Bonds within the hairpin loop are green, [(a) Adapted from B.L. Sibanda and J.M. Thornton, Nature 316 170-174, 1985.]...

The hairpin (i motif occurs frequently in protein structures... 

Figure 2.14 shows examples of both cases, an isolated ribbon and a p sheet. The isolated ribbon is illustrated by the structure of bovine trypsin inhibitor (Figure 2.14a), a small, very stable polypeptide of 58 amino acids that inhibits the activity of the digestive protease trypsin. The structure has been determined to 1.0 A resolution in the laboratory of Robert Huber in Munich, Germany, and the folding pathway of this protein is discussed in Chapter 6. Hairpin motifs as parts of a p sheet are exemplified by the structure of a snake venom, erabutoxin (Figure 2.14b), which binds to and inhibits... 

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