Rotational relaxation of macromolecules determined by dynamic light scattering. I. Tobacco mosaic virus

Abstract

Abstract The intensity autocorrelation function for the depolarized component of forward‐scattered light from a solution of large polymeric molecules is derived in terms of the correlation function for the amplitudes of the Y 2 ,± 1 (θ,ϕ) fluctuations in the anglar distribution of segments in the solution without any assumptions regarding the statistical properties of the scatterad light field. Effects arising from the use of polychromatic incident light and from the mixing of the scattered and polychromatic incident light beams are examined in detail. Apparatus for observing the depolarized forward‐scattered light, digitizing and storing the fluctuating phototube current at rates from 10 to 540,000 times per second, and computing the correlation functions directly in the time‐domain is described herein. Correlation functions were obtained for 0.05 mg/ml solution of tobacco mosaic virus at pH 9.1 and also at pH 6. The degree of association of the virus appears to be independent of pH, and the monomer relaxation times (corrected to 25°C) extracted from the data by a least‐squares procedure lie in the range 0.44–0.49 msec, also independent of pH. The absence of faster component in the correlation function between 6 μsec and 0.5 msec is used in conjunction with thermal fluctuation theory to infer a lower limit for the effective Young's modulaus of the rod, E ≤ 2.5 × 10 7 dynes/cm 2 .