The â??long tailâ?? of the protein tumbling correlation function: observation by 1 H NMR relaxometry in a wide frequency and concentration range
Abstract
Inter-protein interactions in solution affect the auto-correlation function of Brownian tumbling not only in terms of a simple increase of the correlation time, they also lead to the appearance of a weak slow component (â??long tailâ??) of the correlation function due to a slowly changing local anisotropy of the microenvironment. The conventional protocol of correlation time estimation from the relaxation rate ratio
R 1/
R 2 assumes a single-component tumbling correlation function, and thus can provide incorrect results as soon as the â??long tailâ?? is of relevance. This effect, however, has been underestimated in many instances. In this work we present a detailed systematic study of the tumbling correlation function of two proteins, lysozyme and bovine serum albumin, at different concentrations and temperatures using proton field-cycling relaxometry combined with
R 1Ï? and
R 2 measurements. Unlike high-field NMR relaxation methods, these techniques enable a detailed study of dynamics on a time scale longer than the normal protein tumbling correlation time and, thus, a reliable estimate of the parameters of the â??long tailâ??. In this work we analyze the concentration dependence of the intensity and correlation time of the slow component and perform simulations of high-field 15N NMR relaxation data demonstrating the importance of taking the â??long tailâ?? in the analysis into account.
Source: Journal of Biomolecular NMR