Noncovalent bonds ionic interactions

The self-assembly of small molecules into complex material based on a multitude of different noncovalent interactions, such as hydrogen bonding, ionic interactions, hydrophobicity and metal coordination, has been established over the last century [2, 30, 38, 44-49]. In this context, the real power of self-assembly becomes evident when not only multiple noncovalent interactions but also multiple levels of self-assembly occur within the same small molecule system. These multiple tiers of self-organizational hierarchy can yield highly complex structures with sec-... 

Noncovalent bonds (i.e., hydrogen bonds, ionic interactions, van der Waals forces, and hydrophobic interactions) play important roles in determining the physical and chemical properties of water. They also have a significant effect on the structure and function of biomolecules. 

Figure 1 Example of target molecule (L-histidinyl-L-cysteine-amide) containing a variety of functional elements. The molecule may potentially form several noncovalent bonds (hydrogen bonds, ionic interactions, metal coordinations), as well as reversible covalent bonds (imines, disulfides).

The molecular recognition effect at the basis of the specific interaction between the imprinted stationary phase and the template is based on several kinds of noncovalent bond—ionic, hydrogen bond, charge transfer, dipole-dipole—and its strength depends both on the nature of the imprinted binding sites and on the composition of the mobile phase. 

Helix-sense bias can be induced for optically inactive, dynamically racemic helical polymers through specific noncovalent bonding interactions such as hydrogen bonding, ionic interaction, and inclusion interaction. Such chiral induction has been best explored for polyphenylacetylene derivatives. Single-handed helicity can be induced for designed, achiral polyphenylacetylene derivatives, 168-179,... 

Physical and Chemical Integrity of Proteins. The primary sequence of proteins and peptides is comprised of L-amino acids linked together by covalent amide bonds. Substituent group polarity and/or charge is a critical determinant of secondary and tertiary structure and stability. Secondary structures (a-helices and P-sheets) arise from hydrophobic, ionic, and Van der Waals interactions that fold the primary amino acid chain upon itself. Most therapeutic proteins exhibit tertiary structure vital to functionality and are held together by covalent and noncovalent bonding of secondary structures (Figure 5.2). 

Many proteins consist of a single polypeptide chain, and are defined as monomeric proteins. However, others may consist of two or more polypeptide chains that may be structurally identical or totally unrelated. The arrangement of these polypeptide subunits is called the quaternary structure of the protein. [Note If there are two subunits, the protein is called dimeric , if three subunits trimeric , and, if several subunits, multimeric. ] Subunits are held together by noncovalent interactions (for example, hydrogen bonds, ionic bonds, and hydrophobic interactions). Subunits may either function independently of each other, or may work cooperatively, as in hemoglobin, in which the binding of oxygen to... 

Larger proteins often contain more than one polypeptide chain. These multi-subunit proteins have a more complex shape, but are still formed from the same forces that twist and fold the local polypeptide. The unique three-dimensional interaction between different polypeptides in multi-subunit proteins is called the quaternary structure. Subunits may be held together by noncovalent contacts, such as hydrophobic or ionic interactions, or by covalent disulfide bonds formed from the cysteine residue of one polypeptide chain being cross-linked to a cysteine sulfhydryl of another chain (Fig. 15). 

Collectively, the direct thrombin inhibitors are prototypically represented by hirudin, the antithrombotic molecule found in the saliva of the medicinal leech (Hirudo medicinalis), This protein is a 65 amino acid molecule that forms a highly stable but noncovalent complex with thrombin (7). With two domains, the NH2-terminal core domain and the COOH-terminal tail, the hirudin molecule inhibits the catalytic site and the anion-binding exosite in a two-step process. The first step is an ionic interaction that leads to a rearrangement of the thrombin-hirudin complex to form a tighter bond that is stoi-chiometrically I I and irreversible. The apolar-binding site may also be involved in hirudin binding. This complex and... 

While technically simpler than the covalent approach, the self-assembly approach creates more heterogeneous sites and also requires templates with specific functional groups.8 Since sol-gel chemistry is aqueous based, H-bonding interactions are significantly weaker compared to the conventional organic polymerization methods. Often, hydrophobic effects and ionic interactions are utilized. A number of other examples of the noncovalent approach to imprinting in sol-gel-derived materials are provided in recent reviews.5 17 In the sections below, the focus will be on some of the newer aspects of small molecule imprinting in silica that involve the use of chiral templates... 

The groups of Reek and Meijer have also applied the noncovalent approach for catalyst anchoring to a dendrimer support [62], Phosphine functionalized ligands were attached to the periphery of polypropylene imine) dendrimers via combined ionic interactions and H-bonding using a specific binding motif that is complementary to that of the support (Fig. 6). 

Fig. 4 The first supramolecular dendritic catalyst in which 32 phosphine ligands are noncovalently anchored to the functionalized dendrimer by hydrogen bonds and ionic interactions...

Noncovalent interactions are weak inter- or intramolecular interactions that result from a combination of electrostatic interactions (ionic), hydrogen bonding, hydrophobic interactions (stacking or intercalation), and van der Waals interactions (dipole-dipole or induced dipole-induced dipole). Complexes formed by these types of interactions are usually fragile. This property is often essential to their biological function, which depends on the equilibria between the associated and free forms of these molecules. 

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