1. Johnson Research Foundation and Department of Biochemistry and Biophysics, University of Pennsylvania, Philadelphia, Pennsylvania, USA
2. Present address: Division of Medicinal Chemistry and Natural Products, School of Pharmacy, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-7360, USA.

Correspondence to: Correspondence and requests for materials should be addressed to A.J.W. (e-mail: Email: wand@mail.med.upenn.edu). The relaxation data has been deposited in the BMRB under accession number 4970.

Abstract

Internal motion is central to protein folding¹, to protein stability through the resulting residual entropy², and to protein function^{1, 3, 4, 5, 6, 7}. Despite its importance, the precise nature of the internal motions of protein macromolecules remains a mystery. Here we report a survey of the temperature dependence of the fast dynamics of methyl-bearing side chains in a calmodulin–peptide complex using site-specific deuterium NMR relaxation methods. The amplitudes of motion had a markedly heterogeneous spectrum and segregated into three largely distinct classes. Other proteins studied at single temperatures tend to segregate similarly. Furthermore, a large variability in the degree of thermal activation of the dynamics in the calmodulin complex indicates a heterogeneous distribution of residual entropy and hence reveals the microscopic origins of heat capacity in proteins. These observations also point to an unexpected explanation for the low-temperature 'glass transition' of proteins. It is this transition that has been ascribed to the creation of protein motional modes that are responsible for biological activity^{5, 6, 7}.