Introduction

Terpineols are naturally occurring unsaturated monocyclic mono-terpenoid alcohols and can be found in flowers such as narcissus and freesia, in herbs, such as marjoram, oregano, rosemary and in lemon peel oil. Reports on the level of terpenoids in oils occasionally vary considerably. One wonders how much this is due to the variations in the plants and the variations in the isolation process as terpineols could also be an artifact. In addition, terpineols are interesting because of their wide range of biological properties[1].

The common isomers of terpineols

There are five common isomers of terpineols: alpha- (α-T), beta- (β-T), gamma- (γ-T), delta- (δ-T) and terpinen-4-ol (T-4-ol).

α- and β-Terpineol occur in optically active forms and as a racemate. Both α-T and T-4-ol are the most important commercial products, and they occur in a large number of essential oils. On the other hand, β-, γ-, and δ-terpineols do not occur very often in nature. Terpineols, especially the most commonly used compounds as α-T and T-4-on, exert a wide range of biological actions on humans, animals, and plants. They are not only popular fragrance ingredients used in perfumes, cosmetics, and household cleaning products but also used to flavour foods and beverages. They also possess various important biological and medicinal properties.

α-Terpineol

α-Terpineol (α-TOH) is classified as a tertiary alcohol monoterpenoid. Due to its floral, lilac-type, and sweet odour, α-TOH is one of the most common ingredients used in the food industry as a flavouring agent, among other extensive applications. As a result, its annual worldwide demand is ~330 tons. Besides this, recent studies have been highlighting some novel biological functionalities for this monoterpenoid, such as antitumor, antidepressant, anticonvulsant, anti-inflammatory, antidiarrheal, antibacterial and antifungal. α-TOH is recognized as a thermolabile and volatile molecule due to its low molecular weight (<300?g?mol?1) and being moisture- and air-sensitive[2].

Biological activity

α-Terpineol, which has strong antifungal and insecticidal activity, was not shown to cause oxidative stress; however, researchers declared that α-terpineol has high antioxidant and antinociceptive potential. Terpinolene, a sedative, was reported to affect antioxidant metabolism, followed by dramatic increments in total oxidative stress levels and mortality rates in human peripheral lymphocytes. Terpinolene was also shown to downregulate Akt1 expression in K562 cells and to reduce cell proliferation even at reduced (50 mg/L) concentrations. Besides, researchers reported that terpinolene exposure caused significant alterations in cytoplasmic ROS production, lipid peroxidation, mitochondrial potential, and antioxidant enzyme levels. In addition, IC50 and LD50 values for terpinolene were between 50 and 350 mg/L, which were calculated in a variety of model organisms, including yeast, rodents, human cancer cells, and plant pests, while IC50 and LD50 values for α-terpineol were higher (1500–3000 mg/L) than those calculated for terpinolene.

[1] Christina Khaleel, G. Buchbauer, N. Tabanca. “α-Terpineol, a natural monoterpene: A review of its biological properties.” Open Chemistry 16 1 (2018): 349–361.

[2] Lorena de Oliveira Felipe. “Comprehensive study of α-terpineol-loaded oil-in-water (O/W) nanoemulsion: interfacial property, formulation, physical and chemical stability.” NPJ Science of Food (2021): 1–12.