Changes in total light scattering and absorption caused by changes in particle conformation—A test of theory

Abstract

Theoretical methods were recently proposed for relating optical and conformational changes of biological cells and large subcellular structures. Two types of conformational changes were treated: a simple particle swelling or shrinking in which the refractive-index increment varies inversely with particle volume, and changes in internal particle structure. The effects on two optical quantities were considered: suspension transmission as measured with a highly collimated optical system, and transmission as measured with a 4π steradian photometer (integrating sphere). In the first case, the beam is attenuated by total scattering, at all angles, plus absorption, while in the second attenuation is by absorption only. The present study tests these theoretical methods. Yeast cells and spinach chloroplasts were made to shrink by increasing medium osmolarity. Transmissions were measured with special spectrophotometers. Initial particle volume, refractive index, and absorption coefficient as well as changes in particle volume were independently measured and used to calculate theoretical changes in transmission. Some theoretical changes in transmission controlled by total scattering plus absorption were also calculated assuming that medium osmolarity influences internal particle structure as well as particle volume. Both theoretical models lead to qualitative agreement with experiment. However, the best agreement is obtained assuming simultaneous volume and structure changes. With suspensions of spinach chloroplasts, transmissions governed by absorption only were always found to increase as the particles shrink. The observed transmission changes quantitatively agree with theoretical predictions based on the observed volume changes.