The nutritional content of vitamin E is defined by α-tocopherol activity. The molecules that contribute α-tocopherol activity are four tocopherols and four tocotrienols, identified by the prefixes alpha- (α-), beta- (β-), gamma- (γ-), and delta- (δ-).[10] Natural tocopherols occur in the RRR-configuration only. The synthetic form contains eight different stereoisomers and is called 'all-rac'-α-tocopherol.[11] Water-soluble forms such as d-alpha-tocopheryl succinate are used as food additive.[citation needed]