Quantum chemical calculations are a powerful tool for understanding the strength and stability of non-covalent interactions in biological systems. These calculations are based on the principles of quantum mechanics and can provide detailed information about the electronic structure, energetics, and geometries of molecular systems. By modeling the interactions between molecules at the quantum level, researchers can gain insights into the factors that govern the stability and strength of non-covalent interactions such as hydrogen bonding, van der Waals forces, and - stacking.Here are some specific examples from published literature where quantum chemical calculations have been used to investigate non-covalent interactions in biomolecules:1. Hydrogen bonding in proteins: In a study by Kryachko and Remacle 2005 , quantum chemical calculations were used to investigate the strength and directionality of hydrogen bonds in proteins. They found that the strength of hydrogen bonds depends on the specific amino acid residues involved and the local environment, which can help explain the stability of protein structures.Reference: Kryachko, E. S., & Remacle, F. 2005 . Hydrogen bonding in proteins as studied by quantum chemistry. Journal of Computational Chemistry, 26 1 , 82-93.2. Van der Waals forces in nucleic acids: In a study by Sponer et al. 2010 , quantum chemical calculations were used to investigate the role of van der Waals forces in the stability of DNA and RNA base pairs. They found that these forces play a significant role in stabilizing the base pairs, particularly in non-canonical structures such as G-quadruplexes.Reference: Sponer, J., Jurecka, P., & Hobza, P. 2010 . Accurate interaction energies of hydrogen-bonded nucleic acid base pairs. Journal of the American Chemical Society, 132 15 , 5405-5412.3. - stacking in carbohydrates: In a study by Gmez et al. 2011 , quantum chemical calculations were used to investigate the role of - stacking interactions in the stability of carbohydrate structures. They found that these interactions play a significant role in stabilizing the conformations of certain carbohydrates, such as cellobiose.Reference: Gmez, H., Mendoza, S., & Lemus, R. 2011 . - stacking interactions in carbohydrates: A quantum chemical study. Journal of Molecular Structure: THEOCHEM, 960 1-3 , 35-40.These examples demonstrate the utility of quantum chemical calculations in providing a detailed understanding of the factors that govern the strength and stability of non-covalent interactions in biological systems. By gaining insights into these interactions, researchers can better understand the structure, function, and dynamics of biomolecules such as proteins, nucleic acids, and carbohydrates.