Cavities alpha

Figure 2.9 Cation sites in LTA. Site I is centered on the six-rings and displaced into the alpha cavity. Site II is near the center of the eight-rings and close to the eight-ring planes. Site III is centered on the four-rings and displaced into the alpha cavity.

Figure 4.4 (a) View of the alpha (grc) cavity bicapped in the c-axis direction by wbc cavities forming pockets accessible though eight-rings on opposite sides of each alpha cavity. The three unique T-atoms in the structure are... 

Unusual behavior was reported" for the complexation of the ions IO4 and CIO4 with beta cyclodextrin. Rohrbach and coworkers found" that these ions show values of the forward rate-constant that approach the diffusion-controlled limit that is, 10 to 10 dm mol". s , whereas all of the other ions studied show values significantly less than this limit. In order to explain this observation, Rohrbach suggested" that, because of their larger radii, the CIO and IO4 ions may be too large to fit into the alpha cyclodextrin cavity, but, instead, may form a complex in which the ions straddle one of the entrances to the cavity. 

Some modifications to the cyclodextrin structure have also been found to improve their complexing ability. Casu and coworkers prepared 2,3,6-tri-O-methyl and 2,6-di-O-methyl derivatives of alpha and beta cyclodextrin. They observed that tri-O-methyl-alpha cyclodextrin shows an almost ten-fold increased stability of the complex with the guest, Methyl Orange, compared with the unmodified alpha cyclodextrin. A possible reason for this increase in stability is that the methyl groups are responsible for an extension of the hydrophobic cavity of the cyclodextrin. Other workers,however, observed a much smaller enhancement of stability of complexes on methylation of the cyclodextrin, and a decrease in stability has even been reportedfor the one host-two guests complex of tropaeolin with beta cyclodextrin. Thus, the effect of methylation on the stability of a complex varies with the guest species involved, and cannot be readily predicted. 

Figure 2. The effect of LSD-25 on the electroencephalogram of the normal unrestrained cat. Recording modes as indicated by connections and electrode position on cortex of cat brain in upper part of picture. Record 4 is derived from hypothalamus. A (control) animal is in quiet waking state. Note alphalike waves in three cortical leads. B records twenty minutes after LSD-25 (15 jug/kg) had been administered into peritoneal cavity, which leads to disappearance of alphas and to arousal pattern. Calibration in microvolts and seconds as indicated (From Bradley and Elkes, 1953). P. B. Bradley and T. Elkes, "The Effects of Some Drugs on the Electrical Activity of the Brain," Brain, Vol. 80, 1957. Reprinted by permission of the authors and the publisher.

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