MAXIMUM LIKELIHOOD DENSITY MODIFICATION BY PATTERN RECOGNITION OF STRUCTURAL MOTIFS

摘要

An electron density for a crystallographic structure having protein regions and solvent regions is improved by maximizing the log likelihood of a set of structures factors {Fh} using a local log-likelihood function: iLL/i(&rgr;(x,{Fh})) = 1n[p(&rgr;(x| iPROT/i)pPROT(x) + p(&rgr;(x)]| iSOLV/i)pSOLV(x) + p(&rgr;(x)| iH/i)pH(x)], where |PROT(x) is the probability that x is in the protein regions, p(&rgr;(x)|iPROT/i) is the conditional probability for &rgr;(x) given that x is in the protein region, and pSOLV(x) and p(&rgr;(x)|SOLV) are the corresponding quantities for the solvent region, pH(x) refers to the probability that there is a structural motif at a known location, with a known orientation, in the vicinity of the point x; andp (&rgr;(x)|iH/i) is the probability distribution for electron density at this point given that the structural motif actually is present. One appropriate structural motif is a helical structure within the crystallofraphic structure.