Hydrogen bond acidic structures

In anhydrous phosphoric(V) acid, tetrahedral PO4 groups are connected by hydrogen bonds, a structure which can be represented... 

A crystallographic scale of acidity has been developed. Measuring the mean C—H O distances in crystal structures correlated well with conventional P a(DMSO) values. An ab initio study was able to correlate ring strain in strained hydrocarbons with hydrogen-bond acidity. ... 

The formation of a stable hydrogen-bonded ring structure as in poly(itaconic acid) and in poly(maleic acid) has also been shown to affect hydration states (Muto, Komatsu Nakagawa, 1973 Muto, 1974). 

The principal factors affecting solvent-ion interactions can be classified as ion-dipole, Lewis acid-base, hydrogen-bonding, solvent structural, and steric. The solvent obviously plays a major part in these interactions. Therefore, to interpret trends in conductance data, bulk solvent properties such as viscosity and dielectric constant should be considered. Table 1 lists selected physical properties for a number of organic solvents. 

All cyclosporines (A,B,C,. .. U,V,W), are oligopeptides containing 11 amino acids closed in a cyclic form. All of these are known amino acids except the first, which sometimes has not been isolated from natural sources. All of them are L-amino acids except for the amino acids in positions 3 and 8. Because of all of the hydrogen bonds, the structure of cyclosporine is quite rigid. Cyclosporines (A,B,C,. .. U,V,W) only differ in the second amino acid. 

The effect of hydrogen bonds on the physical properties of crystals is shown in a striking way by oxalic acid. This substance exists in two anhydrous crystal forms. 1 One of these, the a form, contains layers of molecules held together by hydrogen bonds, the structure of a layer... 

Fig. 11. A view of the ribbon-like hydrogen-bonded super-structure of the 6-hydroxy- ra ,S -3-hexenoic acid 107 in the crystal.

The life that we know also uses proteins for the majority of structural and catalytic functions. Proteins are particularly suited for these functions because of the structural properties of polymers of amino acids. The polyamide backbone of proteins is neutral, unlike that of nucleic acids. Further, the backbone has a repeating dipole able to make hydrogen bonds. These structural features are exploited as proteins fold into globular structures, as they promote the formation of stable secondary structures such as alpha helices and beta sheets. 

In this chapter, we discuss the hydrogen bonding in structures where water is the sole or majority molecular species present. These are structures which are determined wholly or primarily by the hydrogen-bonding characteristics of the water molecules. They demonstrate the consequences of the dual hydrogen-bond donor-acceptor functionality, which, when combined with the cooperative and flip-flop dynamic properties of the hydroxyl groups, are essential for the formation of the hydration shells around the proteins and nucleic acids, and help to maintain their three-dimensional structures. 

The partially protonated cyclen and cyclam and their derivatives are present in aqueous solution in some stable conformations stabilized by intramolecular hydrogen bonds. It leads, for example, to broad signals in NMR at room temperature and intermediate pH. Such hydrogen bond-stabilized structures are sometimes rather stable as it was proved, for example, in the case of l,8-bis(methylphosphonic acid) cyclam derivatives where solution structure is probably the same as found in the solid state <2000CCC1289>. These closed structures are opened after full protonation. 

HMPA (hexamethylphosphoramide) OP(NMe2)3 4 does not deboronate the C2B10H12 isomers at room temperature but acts as an hydrogen bond acceptor for the acidic hydrogens at the carbon atoms of these clusters to yield 1 1 carboranerHMPA adducts on crystallization.6 These C-H--0 hydrogen bonded supramolecular structures provide the first definitive X-ray structures of the unsubstituted carboranes. 

Strong absorption, and by its exact frequency gives much information about structure. For (hydrogen-bonded) acids, the C—O band is at about 1700 cm . ... 

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