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