NOVELTY - Method for producing ultra-pure, biobased gamma -butyrolactone product, involves (a) combining a genetically engineered biomass comprising polyhydroxyalkanoate polymer, a solvent and optionally a catalyst, (b) mixing the biomass and solvent together while optionally applying heat, (c) separating the organic and aqueous phases of the biomass and solvent mixture, (d) removing the solvent from the biomass, and (e) converting the biomass comprising polyhydroxyalkanoate to a biobased chemical product.    USE - The method is useful for producing ultra-pure, biobased gamma -butyrolactone product which is used in pharmaceutical composition (claimed).    ADVANTAGE - The method produces reduced volume of side products which helps to produce visually colorless biochemical product having low detectable odor, and eliminates the necessity of waste treatment.    DETAILED DESCRIPTION - Method for producing ultra-pure, biobased gamma -butyrolactone product, involves (a) combining a genetically engineered biomass comprising polyhydroxyalkanoate polymer, a solvent and optionally a catalyst, (b) mixing the biomass and solvent together while optionally applying heat, (c) separating the organic and aqueous phases of the biomass and solvent mixture, (d) removing the solvent from the biomass, and (e) converting the biomass comprising polyhydroxyalkanoate to a biobased chemical product, where the biobased chemical in the product is greater than 95% and the product does not comprise acetamide, n-methyl pyrrolidone or n-ethylpyrrolidone and fatty acids. INDEPENDENT CLAIMS are included for the following:    (1) method for producing ultra-pure, biobased product, involves performing the above-mentioned steps, where the product is a biobased gamma -butyrolactone product when the biomass comprises a poly-4-hydroxybutyrate, a biobased crotonic acid product when the biomass comprises a poly-3-hydroxybutyrate, a biobased acrylic acid product when the biomass comprises a poly-3-hydroxypropionate, or a biobased delta -valerolactone when the biomass comprises a poly- 5-hydroxyvalerate and the product does not comprise acetamide, n-methyl pyrrolidone or n-ethyl pyrrolidone and fatty acids;    (2) biobased gamma -butyrolactone product produced by the above-mentioned method (M1);    (3) biobased crotonic acid product, biobased acrylic acid product or biobased delta -valerolactone product produced by the above-mentioned method (M2);    (4) poly-4-hydroxybutyrate biomass produced from renewable resources which is suitable as a feedstock for producing the gamma -butyrolactone product, where the level of poly-4-hydroxybutyrate in the biomass is greater than 50 wt.% of the biomass;    (5) gamma -hydroxybutyrate is partially or wholly deuterated or fluorinated; and    (6) pharmaceutical composition comprising a sodium salt of gamma -hydroxybutyrate and one or more carriers.