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Light signature

Laser beams differential between normal and altered forms of the "bad" cholesterol

From the meeting of a biochemist and a physicist there sprung the development of a rapid test that, should it prove to be efficient and of easy application, could well perfect  cholesterol examinations, altering the interpretation of the levels of fat in the blood and even avoiding unnecessary treatments. By using light, by way of a technique called sweep Z, it has been possible to distinguish the normal structure of a form of cholesterol named LDL (low density lipoprotein) from the damaging and altered structure: whilst the healthy LDL appears like a perfect sphere, the other reminds one of a squashed sphere.

The biochemist Magnus Gidlund and the physicist Antônio Martins Figueiredo Neto, both from the University of Sao Paulo (USP), who were responsible for the original application of this technique, met each other at the end of 2003 during the planning of a course for science teachers. Dr. Gidlund related his experiences with LDL to Dr. Figueiredo and the limitations of the biochemistry techniques used for analyzing this lipoprotein. Better know as the “bad” cholesterol, LDL is not in reality totally unwanted. And, in truth, it is essential to life as it enters into the composition of hormones and of the membranes of cells.

The problem comes about when these lipoproteins lose atomic size particles that have a negative charge (electrons) – or undergo oxidation as the physicists and chemists say – and its structure becomes slightly deformed. The oxidized form of  the low density lipoprotein – undetected in the traditional blood examinations – causes lesions on the internal walls of the blood vessels. As a consequence, this molecule starts an inflammation that little by little increases the thickness of the wall of the veins and the arteries and obstructs the passage of blood, raising the risk of a stroke or a vascular accident. This reason explains the interest of doctors, biochemists, and clearly all of us, in understanding if the level of LDL in the blood is normal or if a fraction of it has been oxidized.

Double layer
The solution materialized when Dr. Gidlund described for the physicist the structure of LDL. One is dealing with a sphere of fat formed by two layers with different physical and chemical properties: the nucleus of LDL repels the molecules of water and is classified as hydrophobic, whilst its surface attracts water molecules – it is hydrophilic. Dr. Figueiredo recognized within this explanation a surprising proximity to his own work: on the molecules of some liquid crystals also with a hydrophilic region and a hydrophobic region, although they grouped themselves into a structure that was a little different, appearing more like an American football.

Dr. Figueiredo and one of the members of his team, the physicist Sergio Gomez, set out to analyze the LDL using a technique called sweep Z, routinely used to measure a property associated with the divergence of light – the non-linear index of refraction – of materials such as liquid crystals, used in computer screens. The physicists helped themselves to the same working principal: instead of liquid crystals they stored a fine layer of LDL between thin glass sheets. Afterwards they submitted the LDL sample to laser light of low power. On passing through the LDL layer, the light deviated, in the same way as the suns rays that incident on a swimming poll. By calculating this deviation the physicists mounted a graph – an optical signature – of each material studied.

The strategy worked. Drs. Gidlund and Figueiredo had observed a major difference between the refractive index of normal LDL and that of oxidized LDL. “Sweep Z is one of the simplest techniques in physics used in the study of complex fluids and allows for making these measurements in around half an hour”, explained Dr. Figueiredo. The results were published in December of 2004 in an article in Chemistry and Physics of Lipids, written in collaboration with Paulo Boschcov from the Federal University of Sao Paulo, Rozane Turchiello from the Chemistry Institute of USP, and Maria Cristina Jurado from the Biomedical Sciences Institute of USP.
Drs. Gidlund and Figueiredo are now verifying if there exist variations in the optical signature of LDL in people with the profiles of different cholesterol levels. As well they are working on the identification of the optical signature of the “good” cholesterol – the HDL or the high density lipoprotein, which eliminates the fats in the blood. “If the results prove the efficiency in the differentiation of normal and oxidized LDL”, stated Dr. Figueiredo, “this test could complement the traditional blood examination.”