conformational restriction

Conformational restriction means stiffening a floppy molecule so it naturally holds the exact shape its target prefers. A flexible molecule is like a length of string that can flop into countless shapes, only one of which fits the binding pocket. By building in rigidity, you pre-shape the string into the right form before it ever reaches the lock.

Flexible molecules pay an entropic penalty on binding: they must give up their freedom to wiggle in order to settle into the single active conformation, and that loss of freedom costs binding energy. If you lock the molecule into that productive shape ahead of time, for instance by adding a ring or a rigid bridge, it loses less freedom on binding and so binds more tightly. As a bonus, rigidifying often improves selectivity, because the frozen shape may fit the target but not its more flexible-tolerant relatives.

The catch is precision: you must lock in the right conformation. Rigidify into the wrong shape and you can abolish activity entirely. Restriction can also reduce solubility and make synthesis harder, so it is a sharp tool that rewards knowing the bound geometry, ideally from a structure.