Change in roundtable topic|
Slight change for Sunday. While the event is same time and place (10:30, Sunday at Pellissipi) our discussant will be Professor Robert Compton, Departments of Physics and Chemistry, University of Tennessee.
Dr. Compton topic is Chirality: One of the Facts of Life.
The word chiral is of Greek origin (kheir, meaning hand) and was introduced by Lord Kelvin to refer to any object whose mirror image cannot be superimposed upon itself. The right and left hand are non-superimposible mirror images and hence chiral. Chirality is present in nature at many levels from hands and feet to twining plants, sea shells, and the molecules of life. Many biological molecules exist in only one of these mirror image forms (e.g., amino acids which make up proteins in all of life forms exist only in the so-called L configuration). The right-handed spiraling DNA super-molecule contains the genetic instructions used in the development and functioning of all known living organisms. The sugar molecules making up the backbone of DNA are also of one specific handedness. The definition of chirality can be extended to moving objects such as spinning electrons, photons and moving Galaxies. We speak of right- or left-handed helicity when a moving object is "spinning" as a right-hand or left-hand screw, respectively. The weak force existing at short distances inside the nucleus can detect a difference between a left- or right-handed electron. This observation in 1956 is known in physics as the "fall of parity". As a result of this symmetry breaking, beta decay results in mainly "left-handed" electrons. Following the "big-bang" creation of the Universe, spiral galaxies were produce which are observed from the Earth to exist in either a Z or S shape. Since they are receding from our Galaxy they are observed to be chiral by earthly humans. Interestingly, it is found that there are approximately equal numbers of such galaxies as one would expect from a point singularity (the big bang). Pasteur first demonstrated that chiral molecules exhibit optical activity (a difference in interaction for right- or left- circularly polarized light) and that the materials (molecules) that sustain life are present in only one of the mirror image forms. Any theory on the origins of life will have to account for this specific homochirality. I will briefly discuss some of the explanations proffered to explain homochirality including the improbable, but possible, influence of the weak force.
More information on Chirality can be found in the review article "The Chirality of Biomolecules" by R.N. Compton and Richard M. Pagni in Advances in Atomic, Molecular and Optical Physics, Vol. 48, 219-261(2002).
Posted by dbuck
May 10, 2010