Behavior of the thermal diffusivity of native and oxidized human low-density lipoprotein solutions studied by the Z-scan technique

Abstract

2012Behavior of the thermal diffusivity of native and oxidized human low-density lipoprotein solutions studied by the Z-scan technique JOURNAL OF BIOMEDICAL OPTICS, BELLINGHAM, v. 17, n. 10, supl. Abstract. Modifications in low-density lipoprotein (LDL) have emerged as a major pathogenic factor of atherosclerosis, which is the main cause of morbidity and mortality in the western world. Measurements of the heat diffusivity of human LDL solutions in their native and in vitro oxidized states are presented by using the Z-Scan (ZS) technique. Other complementary techniques were used to obtain the physical parameters necessary to interpret the optical results, e.g., pycnometry, refractometry, calorimetry, and spectrophotometry, and to understand the oxidation phase of LDL particles. To determine the sample's thermal diffusivity using the thermal lens model, an iterative one-parameter fitting method is proposed which takes into account several characteristic ZS timedependent and the position-dependent transmittance measurements. Results show that the thermal diffusivity increases as a function of the LDL oxidation degree, which can be explained by the increase of the hydroperoxides production due to the oxidation process. The oxidation products go from one LDL to another, disseminating the oxidation process and caring the heat across the sample. This phenomenon leads to a quick thermal homogenization of the sample, avoiding the formation of the thermal lens in highly oxidized LDL solutions.

Effect of Oxidation on the Structure of Human Low- and High-Density Lipoproteins

“…7 b). A similar behavior was observed with LDL particles (32,41). LDL samples stop presenting a nonlinear optical response (q ¼ 0) at~90 min of in vitro oxidation with copper, whereas HDL samples presented a nonlinear optical response (under the same experimental conditions) even after 18 h of in vitro oxidation time.…”

“…Since all lipoproteins present similarities in their overall structure, it is possible that the mechanism of thermal lens formation is also similar. The ZS experiments with HDL samples showed that the decrease of the q parameter as a function of oxidation time occurs at a lower rate than observed for the LDL samples (32).Although LDL loses the nonlinear optical response signal altogether after <2 h of in vitro oxidation, HDL presents this signal for over 18 h. It has been shown previously in the case of LDL experiments that the behavior of the parameter q is inversely proportional to the sample's thermal diffusivity (32). The thermal lens is formed because the sample absorbs heat from the Gaussian laser beam, which diffuses radially across it, imposing a temperature gradient.…”

“…If the heat propagates slowly, it creates a variation of the refractive index through the radial direction, which gives rise to the lens formation. However, the more oxidized the LDL sample, the greater is the thermal diffusivity, and for samples oxidized for >90 min, the thermal lens does not form in the timescale of the experiment (milliseconds) (32). Recent studies have shown that the increase of thermal diffusivity is partially associated with the production of hydroperoxides during the oxidation process (32).…”

“…However, the more oxidized the LDL sample, the greater is the thermal diffusivity, and for samples oxidized for >90 min, the thermal lens does not form in the timescale of the experiment (milliseconds) (32). Recent studies have shown that the increase of thermal diffusivity is partially associated with the production of hydroperoxides during the oxidation process (32). From this picture, one can conclude that the changes in the ZS signal are related to a global modification in the sample as a function of the oxidation process, namely, to structural modifications and chemical reactions occurring in the solution, which form hydroperoxides.…”

“…The data were analyzed using the Thermal Lens model (31). More details about the setup, data acquisition, and data treatment can be found in a previous work (32).…”