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The Subcommittee has attempted to elucidate the nature of problems encountered in the definition and use of statistical descriptors as applied to crystallography and to propose procedural improvements. The report contains (a) a dictionary of statistical terms established for use by experimentalists; (b) a description of the statistical basis for refinement procedures; (c) sections dealing with defects in the physical model used for refinement, and with the choice and significance of weighting schemes; and (d) recommendations, some of which may be readily implemented, whilst others may require a long-term effort to bring them into general use.

The formal refinement methods of least-squares adjustment or difference-map analysis give atomic positions in protein structures with standard deviations which are large compared with the standard deviations of accepted molecular dimensions. This paper describes a method of adjusting the Cartesian coordinates to obtain a properly weighted fit to both the positions from the refinement and the molecular parameters. The equations which have to be solved have many unknowns but few coefficients, and an effective iterative method can be used. The results of applications of the method to insulin are summarized.

The structure of histidine hydrochloride monohydrate has been refined by three-dimensional least-squares methods. The amino acid part of the molecule is in the zwitter ion form; the imidazole ring is positively charged. The distances in the imidazole ring, combined with some previous results, are satisfactorily interpreted with the valence bond theory, and imply a carbon-nitrogen double-bond distance of 1.265 /~. All available hydrogen atoms form hydrogen bonds.