The first crystal structure of a gramicidin complex with sodium: high-resolution study of a nonstoichiometric gramicidin D-NaI complex

The crystal structure of the nonstoichiometric complex of gramicidin D with NaI has been studied using synchrotron radiation at 100 K. The limiting resolution was 1.25 Å and the R factor was 16% for 19 883 observed reflections. The general architecture of the antiparallel two-stranded gramicidin dimers in the studied crystal was a right-handed antiparallel double-stranded form that closely resembles the structures of other right-handed species published to date. However, there were several surprising observations. In addition to the significantly different composition of linear gramicidins identified in the crystal structure, including the absence of the gramicidin C form, only two cationic sites were found in each of the two independent dimers (channels), which were partially occupied by sodium, compared with the seven sites found in the RbCl complex of gramicidin. The sum of the partial occupancies of Na⁺ was only 1.26 per two dimers and was confirmed by the similar content of iodine ions (1.21 ions distributed over seven sites), which was easily visible from their anomalous signal. Another surprising observation was the significant asymmetry of the distributions and occupancies of cations in the gramicidin dimers, which was in contrast to those observed in the high-resolution structures of the complexes of heavier alkali metals with gramicidin D, especially that of rubidium.