Hydrogen separation classification

Figure 1.15 Classification of protein structure according to Levitt and Chothia.15 The protein is viewed from such an angle that most segments of secondary structure are seen end on. Each strand of a j8 sheet is represented by a square. The front end of each a helix is represented by a circle. The segments that are close in space are close together in the diagram. The segments are connected by bold or thin arrows (from the N- to the C-terminus) to indicate whether the connection is at the near or the far end. The approximate scale is diameter of a helix = 5 A j8 strand = 5 X 4 A separation of helixes = 10 A separation of hydrogen-bonded j8 strand = 5 A separation of nonhydrogen-bonded strands = 10 A.

The chemist is accustomed to think of the chemical bond from the valence-bond approach of Pauling (7)05), for this approach enables construction of simple models with which to develop a chemical intuition for a variety of complex materials. However, this approach is necessarily qualitative in character so that at best it can serve only as a useful device for the correlation and classification of materials. Therefore the theoretical context for the present discussion is the Hund (290)-Mulliken (4f>7) molecular-orbital approach. Nevertheless an important restriction to the application of this approach must be emphasized at the start viz. an apparently sharp breakdown of the collective-electron assumption for interatomic separations greater than some critical distance, R(. In order to illustrate the theoretical basis for this breakdown, several calculations will be considered, the first being those for the hydrogen molecule. 

BROENSTED. After verifying the structure of atoms and ions by different models of nucleus and shell, hydrogen ions were classified as protons which do not exist freely and which connect with water molecules forming hydronium ions H30 + (aq). Based on this classification, Broensted and Lowery separately developed their own acid-base definition relating to protons in 1923. This definition proved independent of the aqueous solution and continued to expand... 

Chiral stationary phases that are currently available can be classified into those containing cavities (cellulose derivatives, cyclodextrins, synthetic polymers, crown ethers, and chiral imprinted gels), affinity phases (bovine serum albumin, human serum albumin, a-glycoprotein, enzymes), multiple hydrogen-bond phases, Ti-donor and Ti-acceptor phases, and chiral ligand exchange phases. This classification scheme was used in a review that gave numerous pharmaceutical examples of separation by... 

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