ELECTROSTATIC PROPERTIES OF BETA-CYTIDINE AND CYTOSINE MONOHYDRATE FROM BRAGG-DIFFRACTION

The charge-density distribution in the crystal structure of the nucleo side beta-cytidine at 123 K has been from X-ray diffraction data (Ag K alpha, lambda = 0.5608 Angstrom) using all 7233 reflections with sin theta/lambda less than or equal to 1.14 Angstrom(-1). Maps of electros tatic potential for the cytosine base in cytidine are similar to those derived from previous charge-density studies of cytosine monohydrate and 1-beta-D-arabinosylcytosine, after taking chemical differences int o account. These results were obtained from pseudoatom multipole refin ements including kappa as a variable to describe the expansion or cont raction of the spherical valence shell for each atom type. A new struc ture refinement of this type has also been carried out for cytosine mo nohydrate. A survey of kappa values for hydrogen indicates that this i s a variable which is not well determined experimentally. Variations i n kappa(H) are relayed into the population parameters obtained for oth er pseudoatoms and can have a small but significant effect on molecula r properties, such as the dipole moment. Assuming an average theoretic al value of kappa(H) = 1.24, the molecular dipole moments calculated f rom the monopole and dipole pseudoatom population parameters are 17(4) and 8.2(15) Debye for cytidine nucleoside and cytosine in the monohyd rate, respectively. Systems of atomic point charges are presented for cytidine, cytosine and water. These are generally satisfactory in repr oducing the experimentally determined molecular electrostatic potentia ls and dipole moments.