THE LOW-TEMPERATURE CHARGE DENSITY OF dl-HISTIDINE: A COMPARISON OF SERIAL AND IMAGING PLATE DATA COLLECTION. Michael Carducci, Robert Bolotovsky and Philip Coppens (Chemistry Department, State University of New York at Buffalo, Buffalo, NY 14260-3000 USA).

In response to the long (and often difficult) data collection requirements of accurate low temperature intensities for charge density analysis, many laboratories are turning from serial scintillation counters to area detectors. For comparison of these two techniques we present data sets of dl-histidine collected at 110K on a CAD4 with sealed tube source and scintillation counter (1) and on a modified Huber 4-circle goniometer with rotating anode source and Fuji imaging plates. The imaging plate system allows collection, in days, of data sets which required a month or more to be collected serially, but possessing different strengths and limitations compared to "standard" serial data.

Imaging plate data was processed using HIPPO (2) and modified versions of the Blessing software (3) also used to process the serial data. Each data set was subject to several identical refinements using the XD multipolar refinement package. In each case, the same positional and thermal parameters were obtained within 2 sigma and reasonable molecular geometries and rigid bond tests were obtained. In kappa and multipolar refinements, the charges and multipolar populations were found to be dependent on the refinement model and the treatment of discordant data during processing. We compare the differences between data sets with the observed charge densities of dl-histidine.

References

1. N. Li, Ph.D. Thesis, University at Buffalo (1989).

2. R. Bolotovsky, M. A. White, A. Darovsky and P. Coppens, J. Appl. Cryst. 28, 86 (1995).

3. R. H. Blessing, Cryst. Rev. 1, 3 (1987).

Support of this work by the National Science Foundation (CHE9317770) is gratefully acknowledged.