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

52 J.P. Glusker
72, 104], but others do not. Their interactions are more varied and apparently
less specific. This is an exciting area of research and more results are being
published each month. There is still a need to establish unequivocally what the
recognition rules are for protein-nucleic acid interactions.
10
Incipient Chemical Reaction Pathways
Two interacting molecules may interact with each other. Crystal structure analyses
can give information on this in at least two ways. Inspection of coordinates
of crystal structures containing interacting groups may reveal the reaction
pathway between them [105]. The nucleophilic addition of a nitrogen atom to a
carbonyl group showed this (Fig. 41) [106]. The N◊◊◊C distance varied from 2.91
to 1.49 Å in different crystal structures in such a way that the nitrogen approaches
the carbon of the carbonyl group in a direction perpendicular to the plane
of the latter. Now that high intensity synchrotron radiation is available, it is
possible to follow chemical reactions in proteins, seeing how the molecules
interact with each other. This method, which involves taking instantaneous
X-ray diffraction photographs as a function of time, has been applied to glyco-
a
b
Fig. 41 a, b. Incipient chemical reaction (approach of N to a carbonyl group) [104]: a diagram
of the interaction involved; b geometry of the interaction. As d decreases in length, Dz increases
in height