Introduction
Temperature is a fundamental physical parameter that can be defined
thermodynamically as the degree of kinetic energy of the constituents of
the matter. Macroscopically, temperature is generally associated with
heat and its measurement has been commonly used to monitor the proper
operation of electrical, mechanical, chemical, and biological systems.
Luminescence thermometers rely on the change of the luminescence
intensity, bandwidth, peak wavelength, or lifetime, with the
temperature. Luminescence thermometers are particularly useful for fast
readout where adverse conditions are found. For example, temperature
monitoring in electric power transformer networks, where quick rise of
oil temperature and the presence of high magnetic fields limits the use
of conventional thermocouples. As we moved towards miniaturization,
nanoscale luminescence thermometers have been developed for use in, for
example, studies of temperature gradients in electronic integrated
circuitry, microfluidic devices, and intracellular
structures.1 Recently, studies of thermogenesis in
organelles such as mitochondria, plasmatic reticulum, centrosome, and
nucleus using luminescence-based thermometers have been
reviewed.2 Intracellular metabolic processes are
diverse and very complex with the regulation of the biochemical
reactions being carried out mainly by enzymatic activity. Understanding
the mechanisms behind heat production in cells on a molecular level is a
challenge. In this scenario, luminescence nanothermometers may be a
suitable tool to probe temperature gradients, especially in proteins.
Protein in a cell: a thermodynamic approach
Proteins participate in different cellular functions such as catalysis,
structure, and transport.3 Proteins are synthesized on
ribosomes and the polypeptide linear chain folds spontaneously into a
three-dimensional structure known as the native state. The amino acid
sequence that is characteristic of each type of protein is responsible
for the folding dynamics and the morphology of the three-dimensional
folded state. Proteins may be unfolded by an external perturbation
(physical, chemical, or mechanical agent) but they generally
spontaneously return to the folded native state in physiological
environments. Protein folding may be viewed as a thermodynamic process.
The entire folding process is very complex with numerous possible
stochastic thermodynamic pathways and meta-stable intermediate states
that leads to a minimum Gibbs free energy equilibrium state, according
to the folding energy funnel hypothesis. Figure 1 shows the energy
landscape of a protein during folding. The width of the dominant funnel
represents the configurational entropy while the depth represents the
free energy of an individual configuration averaged over the solvent
alone. After the partially ordered molten globule is reached, another
appropriate reaction coordinate to describe the folding process is Q,
the fraction of native contacts. In the upper region of the funnel,
helical order is established and the protein becomes
compact.4