Learning the Hydrophobic, Hydrophilic, and Aromatic Character of Amino Acids from Thermal Relaxation and Interfacial Thermal Conductance

In this study, the thermal relaxation of the 20 naturally occurring amino acids in water and in the protein lysozyme is investigated using transient nonequilibrium molecular dynamics simulations. By modeling the thermal relaxation process, the relaxation times of the amino acids in water occurs over a time scale covering 2-5 ps. For the hydrophobic amino acids, the relaxation time is controlled by the size of the hydrocarbon side chain, while for hydrophilic amino acids, the number of hydrogen bonds does not significantly affect the time scales of the heat dissipation. Our results show that the interfacial thermal conductance at the amino acid-water interface is in the range of 40-80 MW m^-2 K^-1. Hydrophobic and aromatic amino acids tend to have a lower interfacial thermal conductance. Notably, we show that amino acids can be correlated with their thermal relaxation times and molar masses, into simply connected phases with the same hydrophilicity, hydrophobicity, and aromaticity. The thermal relaxation slows down by a factor of up to five in the protein relative to that in water. In the case of the hydrophobic amino acids in the protein lysozyme, the slow down in the thermal relaxation relative to that in water appears to be controlled primarily by the size of the side chain.