Currently, i am designing a drug using fragment based drug design approach. After i removed my natural substrate( a 3 rings compound) from protein crystal, i decided to use one of the original ring as a part of the fragment. When i screened for this original ring using 'de novo receptor', the outcome is 'no fragment is found'. Does anyone knows why this happen? It is the ring of original substrate, it should recognize its own binding mode,right? Or do i need to make some adjustment in parameter? what sort of adjustment that i can make?