4590 ment as hydrophobic as, for example, the pocket in chymotrypsin into which aromatic or hydrophobic side chains bind. Accordingly, one will anticipate some molecular reorganization as the hydrophobic pocket opens with concomitant entry of a group from the small molecule which is binding. F o r hydrophilic regions of the active sites of enzyme, these comments may not apply, for, in these situations, water may occupy sites in the absence of substrates and binding may occur by displacement of water; thus hydrophilic regions of a n active site may have geometries that differ little as a function of the presence or absence of substrate. Even after being bound to the enzyme, certain portions of the small molecule may still retain considerable steric mobility, as, for example, does the trifluoromethyl group of N-trifluoroacetyl-D-tryptophan with a T~ 10-lo sec representing possibly only rotational freedom about the C 3axis.
k!l)26 some of the ketone to imine. The fact that kh has been determined makes it convenient to define a corrected the range 0.024-0.029 M-' sec-I is calculntetl f o r tlic derate constant (k,,,,) as k6[(CD3)2CO],/[(CD3),CO]deuteration of dilute acetone-& by hydroxide ions i n water at 25". Considering the effect of acetone cont.3 I ) C'i. E . S. Lewis ; i n d J . 1~. stant determined by Jones. It therefore seems cloubtl'ui (31) J . Hinc, F. C. Kokcsh, I
