Abstract

MOIST EARTH SMELLING GEOSMIN AS A TERPENE BICYCLIC ALCOHOL

Prof. Dr. Dhrubo Jyoti Sen*

ABSTRACT

Do you know the smell of the air before or after it rains? It isn't the water that you smell, but a mixture of other chemicals. The odor you smell before rain comes from ozone, a form of oxygen which is produced by lightning, and ionized gases in the atmosphere. The name given to the characteristic odour of rain after it rains, especially following a dry spell, is petrichor. The word petrichor comes from the from Greek, petros, meaning ‘stone’ + ichor, the fluid flowing in the veins of the gods in Greek mythology. Petrichor is caused primarily by a molecule called geosmin. Geosmin (meaning earth smell in Greek) is produced by Streptomyces, a Gram–positive type of Actinobacteria. The chemical is released by the bacteria when they die. It is a bicyclic alcohol with the chemical formula C12H22O. Humans are very sensitive to geosmin and can detect it at levels as low as 5 parts per trillion. Geosmin is found in beets and also freshwater fish, such as catfish and carp, where it concentrates in fatty skin and dark muscle tissues. Cooking these foods together with an acidic ingredient renders the geosmin odorless. Common ingredients you can use include vinegar and citrus juices. Geosmin isn't the only molecule that you smell after it rains. It has been analyzed air from rain storms and found ozone, geosmin and also aromatic plant oils. During dry spells, some plants release the oil, which is absorbed into clay and soil around the plant. The purpose of the oil is to slow seed germination and growth, since it would be unlikely for the seedlings to prosper with insufficient water. When a raindrop hits a porous surface it traps tiny pockets of air. These bubbles then speed upward, like bubbles in a glass of champagne, before breaking the drop's surface and releasing microscopic particles, called aerosols, into the air. The researchers think these aerosols carry the rainlike aroma. Scientists have long observed that raindrops can trap and release aerosols when they fall on water, but this is the first time they've observed the process happening on soil.

Keywords: Geosmin, Argosmin, Petrichor, Decalol, Decalene, Germacradienol, Farnesyl diphosphate, Terpene, Bicyclic alcohol, E1 mechanism.