198 Topics in Current Chemistry Editorial Board: A. de Meijere KN ...

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Directional Aspects of Intermolecular Interactions 13 c Fig. 5 c. Stereoview of the surroundings of C-S-C (filled bonds) in mesolanthionine dihydrochloride [111]. Note Cl – in C-S-C plane, N + perpendicular to it Electrophiles such as metal cations or hydrogen bond donors, with at least a partial positive charge, approach in a direction that is from 50 to 90° from the CSC plane. Frontier orbital theory [19] would suggest that these electrophiles (electron acceptors) are interacting with the highest occupied molecular orbital of the sulfur atom. This is a lone-pair orbital nearly perpendicular to the C-S-C plane (an sp hybrid orbital, while the S-C bonds are pure 3p orbitals). On the other hand, nucleophiles (electron donors), such as negatively charged groups or ions, approach the sulfur within 30° of the C-S-C plane, and tend to lie along the extension of one of the C-S bonds, the direction predicted for the lowest unoccupied molecular orbital (able to accept electrons). Therefore electrophilic attack of divalent sulfur would be expected to occur in a direction perpendicular to the sulfide plane, while nucleophilic attack would be expected to occur in the direction of an S-C bond. This study was then extended to S◊◊◊S contacts. An analysis of crystal structures showed that if one of the sulfur atoms acts as an electrophile, it approaches the other which thus has nucleophilic characteristics [20]. Thus the mutual arrangement of the two nonbonded XSY groups is such that one sulfur atom (S 1, the electrophile) approaches another sulfur atom (S 2, the nucleophile) in the X 2S 2Y 2 plane in such a way that S 2 lies in a direction perpendicular to the X 1S 1Y 1 plane (Fig. 6). An analysis by Chakrabarti [21] of protein structures in the PDB showed that metal ions approach the sulfur of methionine at about 38(5)° from the perpendicular to the C-S-C group. This is similar to values in the range found, as just described, for small-molecule crystal structures in which the metal ion is presumed to interact with a sulfur lone-pair orbital [18]. It was also found that metal ions approach cysteine residues such that the M◊◊◊S-C-C torsion angle is ± 90° or 180°, and that the conformation of the cysteine side chain is generally affected by the metal ion. The metal ions that readily bind to sulfur in proteins are copper, iron, mercury, and zinc. The geometry of binding of metal ions to methionine or cysteine did not appear to depend on the identity of the individual metal. In a similar analysis of the surroundings of C-Cl, C-Br, and C-I groups in crystal structures it was found that electrophiles approach the carbon-halogen bonds at an angle of about 100° (nearly perpendicularly), while nucleophiles
14 J.P. Glusker a c Fig. 6 a – c. Directional preferences of binding of disulfide (-S-S- groups): a diagram of the interactions (X 1,X 2,Y 1,Y 2 = any atom); b chemical formula of 4,5-bis(methylsulfanyl)- 1,2-dithiole-3-thione; c stereoview of the S◊◊◊S interactions in 4,5-bis(methylsulfanyl)-1,2dithiole-3-thione [112] approach at about 165° (almost directly) [22, 23]. This is analogous to the situation around sulfur, just described. It was found that C-Cl◊◊◊O contact distances are shorter in the C-Cl bond direction than perpendicular to it. On the other hand, C-C◊◊◊H distances do not show this effect. Distances involving bromine or iodine show even greater asymmetry. This is described as “polar flattening”[24]. In addition, this anisotropy can lead to a quadrupole at the nucleus of the halogen atom, with lower electron density in the carbon-halogen direction. This will facilitate an attraction of electronegative oxygen atoms. b
Page 1 and 2: 198 Topics in Current Chemistry Edi
Page 3 and 4: Design of Organic Solids Volume Edi
Page 5 and 6: Volume Editors Prof. Dr. Edwin Webe
Page 7 and 8: VIII preface tion at an amazing lev
Page 9 and 10: Contents of Volume 196 Carbon Rich
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Page 13 and 14: 4 J.P. Glusker Perturbations to the
Page 15 and 16: 6 J.P. Glusker 2.1 Sources of Cryst
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Page 19 and 20: 10 J.P. Glusker The forces here hav
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64 A. Nangia · G.R. Desiraju [23]
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68 A. Nangia · G.R. Desiraju Fig.
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70 A. Nangia · G.R. Desiraju -NH 2
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94 A. Nangia · G.R. Desiraju 42. J
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132 Y. Aoyama 4 Catalysis . . . . .
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134 Y. Aoyama 2.2 Symmetry-Controll
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136 Y. Aoyama A trigonal centre is
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138 Y. Aoyama 18 19 Fig. 5. 2D net
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140 Y. Aoyama 24 ded (O-H◊◊◊O
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144 Y. Aoyama In the presence of a
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146 Y. Aoyama clusion compounds (se
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148 Y. Aoyama are intriguing guests
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150 Y. Aoyama The tuning or enginee
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152 Y. Aoyama Fig. 16. Binding isot
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154 Y. Aoyama The desorption of the
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158 Y. Aoyama 5 Concluding Remarks
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160 Y. Aoyama 4. Hölderich W, Hess
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Crystalline Polymorphism of Organic
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Author Index Volumes 151-198 Author
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Author Index Volumes 151 - 198 2 ]
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Springer and the environment At Spr
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198 Topics in Current Chemistry Editorial Board: A. de Meijere KN ...

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