COMPLEX SYSTEMS AND BIOMEDICAL SCIENCES
Protein folding from heterogeneous unfolded state revealed by time-resolved X-ray solution scattering, T.W. Kim (a,b), S.J. Lee (a,b), J. Jo (a,b), J.G. Kim (a,b), H. Ki (a,b), C.W. Kim (a), K.H. Cho (a), J. Choi (b), J.H. Lee (c), M. Wulff (d),
Y.M. Rhee (a) and H. Ihee (a,b), Proc. Natl. Acad. Sci. USA 117, 14996-15005 (2020); https://doi.org/10.1073/ pnas.1913442117. (a) Korea Advanced Institute of Science and Technology, Daejeon (Republic of Korea)
(b) Institute for Basic Science, Daejeon (Republic of Korea) (c) Pohang Accelerator Laboratory, Pohang (Republic of Korea) (d) ESRF
 A. Sali et al., Nature 369, 248-251 (1994).  J.R. Telford et al., Acc. Chem. Res. 31, 755-763 (1998).  J. Sabelko et al., Proc. Natl. Acad. Sci. U.S.A. 96, 6031-6036 (1999).
VITRIFICATION KINETICS VERSUS MICROSCOPIC DYNAMICS
Understanding the vitrification process plays a central role in commercial applications of glassy materials. XPCS reveals that the conventional wisdom based on a one-to-one correlation between the vitrification kinetics and the α-relaxation does not hold in all conditions; rather these two aspects of glassy dynamics are decoupled at deep undercooling.
PRINCIPAL PUBLICATION AND AUTHORS
that the unfolded state is highly heterogeneous in terms of protein conformations. The combined results from the kinetic and structural analyses of X-ray scattering data reveal that the various protein conformations in the unfolded state take
multiple relaxation pathways toward the native folded conformation, resulting in a stretched exponential behaviour (Figure 49). This observation directly reveals protein folding from heterogeneous unfolded states and exemplifies the strength of viewing microscopic protein conformations.
This study is the first of its kind where the protein folding dynamics is investigated by viewing the conformational heterogeneity in relation to its kinetic behaviour. Thus, it should serve as a cornerstone for future studies of protein folding. One of the recent challenges in the fields of biochemistry and biophysics has been to obtain structural information from intrinsically disordered or unfolded proteins. In fact, understanding their biological roles should be founded on such conformational information. The TRXSS approach, combined with the systematic structural analysis at the conformational ensemble level, will make it possible to explore the structure-function relationship of such proteins and, potentially, their energetics at the molecular level.
Fig. 49: Cyt-c folding dynamics in a funnel-
like free-energy landscape. Upon the
photo-reduction of oxidised cyt-c, the
unfolded state with heterogeneous protein
conformations (red dots in the projected
plane) undergoes spontaneous folding
along the multiple parallel pathways
(white arrows) toward the native state with
a homogeneously populated folded
structure (blue dot in the projected plane).
In this work, a bulk metallic glass (BMG) based on an 18 k gold alloy was used to investigate the kinetics of vitrification of glass-forming systems. Contrary to polymeric and molecular systems, BMG-forming liquids do not show any reorientational or intramolecular motion that could influence their vitrification and atomic motions and, therefore, are ideal candidates for the study of these phenomena. The freezing process was studied in an unprecedentedly large range of cooling rates using a fast-scanning calorimeter (FSC) that allows extremely high
cooling rates, avoiding crystallisation during the cooling from the melting temperature down to the glass transition.
Conventional wisdom held that the temperature dependence of the rate at which a liquid freezes and forms a glass should be equal to the rate at which the α-relaxation process decreases as the temperature is decreased. The α-relaxation is well described by the intermediate scattering function, which monitors the temporal evolution of the normalised density density correlation