Hydrogen bonding domains

The vibrational dynamics of water solnbilized in lecithin-reversed micelles appears to be practically indistingnishable from those in bulk water i.e., in the micellar core an extensive hydrogen bonded domain is realized, similar, at least from the vibrational point of view, to that occurring in pure water [58], On the other hand, the reorientational dynamics of the water domain are strongly affected, due to water nanoconfmement and interfacial effects [105,106],... 

One of the problems associated with the use of heterocycles is that the intrinsic concentration of protonic charge carriers can only be moderately increased through acid doping (also see Section 4.3). This is particularly the case when the dynamics within the hydrogen-bonded domain is highly constrained through immobilization (especially in fully polymeric systems), which is probably the direct consequence of a reduced dielectric constant. This also leads to a further increase of the Haven ratio... 

Figure 5a. An example of a partial energy map, the local relaxed map for the S4 family of conformations. Contours are indicated at 4, 6, and 8 kcal/mol above the global SI minimum, which does not appear on this map. The dashed lines surround the different inter-residue hydrogen bond domains (with a cutoff criterion of 2.05A for the O. .. H distance), with the tic marks on the d hes pointing toward the region where the given hydrogen bond is allowed.

Hydrogen-bonding Domains in Metal Complexes and the Role of Metals in Hydrogen Bonding... 

The Goward group studied the proton dynamics of thin (160 nm) and ultrathin (10 nm) NAFION films using variable temperature and variable humidity sohd-state MAS NMR spectroscopy, and showed that T2 remained constant within error for both film thickness and under aU humidity and temperature conditions, consistent with fast exchange within the hydrogen-bonded domains. The chemical shifts measured at low humid-... 

A microwave pulse from a tunable oscillator is injected into the cavity by an anteima, and creates a coherent superposition of rotational states. In the absence of collisions, this superposition emits a free-mduction decay signal, which is detected with an anteima-coupled microwave mixer similar to those used in molecular astrophysics. The data are collected in the time domain and Fourier transfomied to yield the spectrum whose bandwidth is detemimed by the quality factor of the cavity. Hence, such instruments are called Fourier transfomi microwave (FTMW) spectrometers (or Flygare-Balle spectrometers, after the inventors). FTMW instruments are extraordinarily sensitive, and can be used to examine a wide range of stable molecules as well as highly transient or reactive species such as hydrogen-bonded or refractory clusters [29, 30]. 

The simulation trajectory shown in Fig. 8b provides an explanation of how the force profile in Fig. 8a arises. During extension from 0 to 10 A the two /9-sheets slid away from each other, each maintaining a stable structure and its intra-sheet backbone hydrogen bonds. As the extension of the domain reached 14 A, the structure within each sheet began to break in one sheet, strands A and G slid peist each other, while in the other sheet, strands A and B slid past each other. The A -G and A-B backbone hydrogen bonds broke nearly simultaneously, producing the large initial force peak seen in Fig. 8a. 

The simulation (Lu et al., 1998) suggested how Ig domains achieve their chief design requirement of bursting one by one when subjected to external forces. At small extensions, the hydrogen bonds between strands A and B and between strands A and G prevent significant extension of a domain, i.e.. 

Proteins are biopolymers formed by one or more continuous chains of covalently linked amino acids. Hydrogen bonds between non-adjacent amino acids stabilize the so-called elements of secondary structure, a-helices and / —sheets. A number of secondary structure elements then assemble to form a compact unit with a specific fold, a so-called domain. Experience has shown that a number of folds seem to be preferred, maybe because they are especially suited to perform biological protein function. A complete protein may consist of one or more domains. 

Fig. 5. Protein folding. The unfolded polypeptide chain coUapses and assembles to form simple stmctural motifs such as -sheets and a-hehces by nucleation-condensation mechanisms involving the formation of hydrogen bonds and van der Waal s interactions. Small proteins (eg, chymotrypsin inhibitor 2) attain their final (tertiary) stmcture in this way. Larger proteins and multiple protein assembhes aggregate by recognition and docking of multiple domains (eg, -barrels, a-helix bundles), often displaying positive cooperativity. Many noncovalent interactions, including hydrogen bonding, van der Waal s and electrostatic interactions, and the hydrophobic effect are exploited to create the final, compact protein assembly. Further stmctural...

Chain extenders are usually low molecular weight symmetrical diols or diamines. Chain extenders react with isocyanates in the same way as polyols do, but because they are low molecular weight, a high concentration of hydrogen-bonded molecules can associate and phase out of the polyol to form plastic-like domains called hard segments . Hard segments will be discussed in Section 4. Some of the more common diol and diamine chain extenders are shown in Table 3. 

Zinc salt of maleated EPDM rubber in the presence of stearic acid and zinc stearate behaves as a thermoplastic elastomer, which can be reinforced by the incorporation of precipitated silica filler. It is believed that besides the dispersive type of forces operative in the interaction between the backbone chains and the filler particles, the ionic domains in the polymer interact strongly with the polar sites on the filler surface through formation of hydrogen bonded structures. 

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