Negative thermal expansion of a disordered native protein

Abstract

Despite immense interest in thermal expansion of solid materials, negative thermal expansion in particular, expansivity of proteins near ambient temperature is minimally reported. One reason for the paucity of protein expansion studies could be the susceptibility of proteins to conformational changes often within subdenaturing temperatures, given that the expansivity phenomenon should include only vibrational fluctuations in atom positions in the absence of a conformational change. This study encounters a disordered plant protein called At PP16-1 ( Arabidopsis thaliana phloem protein type 16-1) which shows negative thermal expansion in the 292–309(±1) K range of temperature at no expense of conformational transition. Temperature dependence of NMR chemical shifts of backbone atoms, global tumbling time derived from 15 N relaxations, and volume compressibility measurements yield a negative linear expansion coefficient 〈α〉 ∼ −5.3 × 10 −3 K −1 averaged over a sizable set of hydrogen bonds, and a decrease in the root mean square volume fluctuation by ∼36%. A frequency spectrum of normal modes in which one or a fewer low frequency vibrational modes of large fluctuation amplitudes are excited at the lower end, and many high-frequency vibrational modes of lower fluctuation amplitudes are excited at the higher end of the temperature range can explain negative thermal expansion.