There is no secondary standard of length established in the region of 10-10m. In the field of X-ray spectroscopy, x-unit, introduced by Siegbahn, was used as a relative scale of length. The x-unit was determined by the grating constant of calcite crystal. Since variation was observed among the lattice spacings of calcite crystals, wavelengths of certain X-ray lines, mainly CuKα1and MoKα1, were employed as empirical working standard. Discrepancy was found in the commonly used wavelength values of CuKα1 and MoKα1. Bearden introduced a new secondary standard A based on the peak wavelength of WKα1 line ( (WKα1) =0. 209 010 0A) . The absolute value of A, however, has no greater accuracy than the order of ppm. With X-ray/optical interferometer, Deslattes and co-workers have measured the grating constant of silicon α0in terms of optical wavelength (a0 (25°C) =0.543106 61 nm (±0.25 ppm) ) . They measured the densities and isotopic abundances for highly perfect silicon crystals. Finally they have determined the Avogadro constant NA=6. 022 0943×1023mol-1. This new NAreduced the uncertainty of previous direct measurements to one-thirtieth.
Macrocyclic antibiotics can bind alkali metal ions forming 1: 1 complexes, and act as cation-carriers (ionophores) passing through biological and artificial membranes. Molecular structures in crystals and conformational states in solutions are described for valinomycin and nactins, both in free and complexed states. Valinomycin molecule (oval shaped) possesses an approximate symmetry of 1 and six intramolecular H-bonds. On complexation, two H-bonds of 13-membered-ring are disrupted and new ones are formed to complete complex molecule of pseudo C3 symmetry with minor conformational change. In polar solvent, only three H-bonds are observed for K+-valinomycin complex, indicating that the molecule is widely opened to release K+ as ion-pump. In crystals, molecular shape of nonactin (‘torus’, C2 and an approximate S4 symmetry, less stable conformation) is markedly different from that of tetranactin (‘propeller’, C2 symmetry, stable), but they exhibit similar IR and NMR spectra in solutions which indicate molecular symmetry 4. Interconversion of the conformation of free and complexed nactins may be readily achieved by intramolecular motion. Energy differences in intra-ligand-steric-interaction among the complexes of nactins with alkali metal ions are rather small in comparison with those of ion-dipole interactions and hydration energies of cations.