Both the tocopherols and tocotrienols occur in α (alpha), β (beta), γ (gamma) and δ (delta) forms, as determined by the number and position of methyl groups on the chromanol ring. All eight of these vitamers feature a chromane double ring, with a hydroxyl group that can donate a hydrogen atom to reduce free radicals, and a hydrophobic side chain which allows for penetration into biological membranes. Of the many different forms of vitamin E, gamma-tocopherol (γ-tocopherol) is the most common form found in the North American diet, but alpha-tocopherol (α-tocopherol) is the most biologically active. Palm oil is a source of tocotrienols.