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An expression is derived for the rotational entropy of nonrigid polyatomic molecules with zero potential and apart from nuclear spin and symmetry number corrections. Previous empirical formulae are derived as special simplified cases of the general formula. Applications are made to several important types of molecular models.

This study was undertaken in order to compare the metabolic behavior of plasma cholesterol de-rived from the diet with that of cholesterol synthesized in vAvo from acetate (1). Cholesterol labeled with either isotopic carbon or hydrogen was administered orally to human subjects and the incorporation into plasma cholesterol was followed over an extended period. In certain instances, both endogenous and exogenous cholesterol metabolism were studied simultaneously by the use of appropriately labeled acetate and cholesterol. These techniques were also applied to an examination of the behavior of plasma cholesterol in four patients with hypercholesterolemia.

When a mammalian cell strain was incubated with 5-iododeoxyuridine and 5-bromodeoxyuridine, DNA thymine was partially replaced by the halogen-containing pyrimidines. The extent of incorporation of the unnatural bases increased when amethopterin and hypoxanthine were added to the medium. It is thus evident that the replacement of DNA thymine by selected structural analogs, a phenomenon previously reported for bacterial systems, is applicable to cells of higher organisms.

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