Best Naturals L-theanine (Suntheanine®) is the premier, patented form of the amino acid L-theanine. Suntheanine® is the purest for of L-theanine on the market today. L-theanine is an amino acid derived from green tea that is responsible for the relaxation-inducing effect of tea consumption. Suntheanine® has been shown to promote an alert state of relaxation.