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The binding of a number of structurally related xanthine compounds by bovine serum albumin was investigated in an attempt t o elucidate some of the structural specificities of the interaction. The study included caffeine, theophylline, 8-nitrotheophylline, 8-chlorotheophylline, theophylline-7-acetic acid, 8-methoxycaffeine, 8-ethoxycaffeine, 8-chlorocaffeine, 8-methylcaffeine, theobromine, S-nitrotheobromine, uric acid, 1,3,7-trimethyluric acid, uracil, and 1,3-dimethyluracil. Under the conditions employed, interactions between the protein and uric acid, uracil, and dimethyluracil could not be detected. T h e remaining compounds were bound to varying degrees and evidence was obtained to indicate that a single site on the protein was responsible for the major portion of the binding. Anionic species were generally found to possess much stronger interactive tendencies than those which had no formal charge.

The purpose of this study was to determine the sedimentation velocity and free diffusion behavior of humic acid extracted from Miami clay with neutral sodium pyrophosphate solution. In addition the organic colloids extracted from several soils of the Miami catena were analyzed using the eletrophoretic technique. The electrophoresis examination indicated that the organic colloids extracted from the clay fractions of a Brookston, Crosby, and Miami soil were similar. In all cases it appeared that the extract consisted mainly of dark colored humic acid, with two additional colloids present in very small quantities. The sedimentation velocity and free diffusion study showed that the humic colloid isolated from Miami clay was polydisperse with respect to particle size. The following data were obtained for the colloid dissolved in a water solution: Sedimentation constant s20 2.8 × 10‐13 cm/sec/dyne Diffusion constant D20 3.9 × 10‐7 sq cm/sec Partial specific volume V 0.67 Frictional coefficient f/f0 2.3 Weight‐average molecular weight Mw 53,000

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