Solution conformation of septamycin and its sodium salt

The interpretation of the 300 MHz 1H-nmr spectra of septamycin (1) and its sodium salt (1+) allow one to extract most of the parameters revealing their resemblant conformations in solution. The backbone forms a pseudocyclic structure closed by head-to-tail hydrogen bonding between the carboxylate fragment and the OH-14. In 1+, the sodium ion is trapped in a central hole by coordinating around it six to seven oxygen atoms (including COO-). The external lipophilic zone of the molecule keeps the sodium ion away from the surroundings. The dangling sugar-like fragment does not participate in the metal binding. It was not possible to detect any water molecule participating in the cyclization of septamycin-free acid, nor in septamycin-Na+, as was found in an X-ray study for the p-bromophenacyl ester.