Comparison of two highly refined structures of bovine pancreatic trypsin inhibitor

The high resolution structures of bovine pancreatic trypsin inhibitor refined in two distinct crystal forms have been compared. One of the structures was a result of new least-squares X-ray refinement of data from crystal form I, while the other was the joint X-ray/neutron structure of crystal form II. After superposition, the molecules show an overall root-mean-squares deviation of 0.40 A for the atoms in the main chain, while the deviations for the side-chain atoms are 1.53 A. The latter number decreases to 0.61 A when those side-chains that adopted drastically different conformations are excluded from comparison. The discrepancy between atomic temperature factors in the two models was 6.7 A2, while their general trends are highly correlated. About half of the solvent molecules occupy similar positions in the two models, while the others are different. As expected, solvents with the lowest temperature factors are most likely to be common in the two crystal forms. While the two models are clearly similar, the differences are significantly larger than the errors inherent in the structure determination.