The molecular weight of the antibiotic saramycetin was determined as 2 100 by two ultracentrifugal methods: (a) approach to sedimentation equilibrium and (6) combination of the zero concentration diffusion constant (Dm = 34 X lop7 set.-') with the zero concentration sedimentation constant (Szo, w = 1.2 X cm. sec.-l). At concentrations above 5 mg./ml. saramycetin associates, forming aggregates with a molecular weight of 5 5,000.
HE MOLECULAR weight of the antifungal agent T saramycetin had been previously reported by Baudet and Cherbuliez (1) t o be 14,000 daltons on the basis of sedimentation and diffusion constant measurements. Evidence will be presented in this paper that saramycetin is capable of molecular association and that the apparent molecular weight is dependent on polarity of solvent and solute concentration.
EXPERIMENTAL
Molecular weights were measured by the Archibald (2) approach to sedimentation equilibrium method. One to 0.1% solutions, in 12-mm. interference cells, were centrifuged in an An-D rotor at 22". Concentration gradients were formed at speeds ranging from 8766 t o 42,040 r.p.m. using the schlieren optical system with a phase bar angle of 75". The concentration gradient was evaluatcd using Engelberg's modification (3), at the meniscus and cell bottom from photographs recorded on Kodak metallographic plates magnified tenfold on a Nikon magnifier. Total solute conceiitration was measured with a synthetic boundary centerpiece in an interference cell centrifuged at 29,500 r.p.m.
Sedimentation coefficients for saramycetin solutions with concentrations from 10 mg./ml. to 1.25 mg./ml. were measured in a 4", 12-mm. cell centrifuged in an An-D rotor at 42,040 r.p.m. For solutions with concentrations of 1.25 mg./ml. t o 0.313 mg./ml., the sedimentation coefficients were evaluated from schlieren peaks produced with the aid of a 12-mm. synthetic boundary centerpiece in an interference cell, rotated at speeds t o 42,040 r.p.m.
Diffusion constants were evaluated in the analytical ultracentrifuge (4), at 22O, using a synthetic boundary centerpiece and the schlieren optical system. Solute concentrations were from 10 mg./ ml. t o 0.313 mg./ml. The first image was recorded at two-thirds of the final speed of 42,040 r.p.m. or 29,500 r.p.m. This first image was considcred t o be recorded at zero time for the diffusion constant calculations.
Partial specific volume was determined at 20' in a 1-nil. pycnometcr.
RESULTS AND DISCUSSION
As the concentration of saramycetin is diminished from a concentration of 10 mg./ml. to 0.313 mg./ nil. in 0.2 M NaC1-0.02 M phosphate buffer, pH 6.85, the sedimentation coefficient (s) first increases and then decreases (Fig. 1). Two S values at infinite dilution can be calculated by extrapolating