Three chains

Positive-ion electrospray mass spectrum of human hemoglobin (a) as initially obtained with all the measured masses, and (b) after calculation of true mass, as in Figure 8.3. The spectrum transforms into two main peaks representing the main alpha and beta chains of hemoglobin with accurate masses as given. This transformation is fnlly automated. The letters A, B, C refer to the three chains of hemoglobin. Thus, A13 means the alpha chain with 13 protons added. 

Figure 14.2 Models of a collagen-like peptide with a mutation Gly to Ala in the middle of the peptide (orange). Each polypeptide chain is folded into a polyproline type II helix and three chains form a superhelix similar to part of the collagen molecule. The alanine side chain is accommodated inside the superhelix causing a slight change in the twist of the individual chains, (a) Space-filling model, (b) Ribbon diagram. Compare with Figure 14.1c for the change caused by the alanine substitution. (Adapted from J. Bella et al.. Science 266 75-81, 1994.)...

An even more complicated nomenclature problem arises with the closely related all-oxygen cryptands. These compounds do not utilize nitrogen as the three-chain junction. Most examples of this class of compounds have utilized pentaerythritol or glycerol as the junction. This naturally imparts a somewhat lower flexibility to the molecule than would be present in the nitrogen-containing cases. Structures of two such molecules are illustrated below. 

PTC was coupled with (Pro-Ala-Gly)n, n = 12. The covalently bridged three-chain compound could be separated by gel chromatography from the respective two chains and the single chain in low yield, PTC-[(Pro-Ala-Gly-)12]3 shows a strong fold (Figs. 20,21) in comparison to the dimer or the monomer. 

As a further illustration, consider the oxidation of various substrates by S2Oj. Three chain mechanisms that are considered feasible are labeled Schemes C, D, and... 

Collagen forms a triple helix, where three chains of connected amino acids form weak hydrogen bonds between the double-bonded oxygen atoms and the hydrogen atoms attached to the adjacent chain s nitrogens. The three chains then twist together like three cords in a rope. 

A trivalent cation, for example AF, has the potential to link three chains. Sterically, this is improbable Mehrotra Bohra (1983) assert that simple aluminium tricarboxylates are not known in solution. Nevertheless we consider it probable that a small proportion of AF ions link three chains, in which all three charged ligands are COO". More probable molecular structures would contain one or two F" ions with a single F", an [A1F(H20)3] unit could bridge two polyanion chains, while an [A1F2(H20)2] would have no crosslinking ability. 

The three possible termination reactions have been written as combination reactions when, in fact, disproportionation reactions may also occur. Under a given set of experimental conditions, only one of the three chain breaking steps (4a), (4b), or (4c) can be expected to be important. 

These Monte Carlo distributions can be used in the standard three-chain model for rubber-like elasticity to generate, for example, stress-strain isotherms [5]. Non-Gaussian effects can cause large increases in modulus at high... 

Monte Carlo computer simulations were also carried out on filled networks [50,61-63] in an attempt to obtain a better molecular interpretation of how such dispersed fillers reinforce elastomeric materials. The approach taken enabled estimation of the effect of the excluded volume of the filler particles on the network chains and on the elastic properties of the networks. In the first step, distribution functions for the end-to-end vectors of the chains were obtained by applying Monte Carlo methods to rotational isomeric state representations of the chains [64], Conformations of chains that overlapped with any filler particle during the simulation were rejected. The resulting perturbed distributions were then used in the three-chain elasticity model [16] to obtain the desired stress-strain isotherms in elongation. 

Fig. 10 Co-operative twist deformation of a bundle with three chains. Reproduced with permission from [13]. Copyright 1998 Am Chem Soc... 

Most known //-solenoids are monomers. Recently, however, several triple-stranded (TS) //-solenoids have been described in which the three chains wind around a common axis. The three monomers are related by... 

Fig. 5. The triple //-spiral in the adenovirus fiber shaft. (A) The triple //-spiral domain alone. Shown are amino acids 319-392 of each of the three chains, forming four triple /1-spiral repeats (van Raaij et al, 1999b). (B) Structure of a chimeric adenovirus fiber shaft-fibritin foldon domain protein (Papanikolopoulou et al, 2004b).

The remaining three antiparallel /3 structures form a miscellaneous category (see Fig. 84). Lactate dehydrogenase d2 and gene 5 protein each has several two-stranded antiparallel j8 ribbons, but they do not coalesce into any readily described overall pattern. The N-terminal domain of tomato bushy stunt virus protein has a unique /3 structure in which equivalent pieces of chain from three different subunits wrap around a 3-fold axis to form what has been called a /3 annulus (Harrison et ah, 1978). Each of the three chains contributes a short strand segment to each of three three-stranded, interlocking /3 sheets. This domain provides one of the subunit contacts that hold the virus... 

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