Investigation of Interactions between Sodium Dodecyl Sulfate and L-Tryptophan Through Densimetry and Computer Modeling

The densities of solutions of L-tryptophan (Trp) in water and sodium dodecyl sulfate (SDS) in Trp aqueous solutions are measured in a wide range of temperatures (293.15, 298.15, 303.15, 308.15, and 313.15) K using a DMA 5000 М density meter (Anton Paar).The apparent molar volumes ( $${{V}_{{\varphi ,{\text{SDS}}}}}$$ ) of sodium dodecyl sulfate in the 0.0010–0.0199 mol kg−1 range of concentrations and its limiting apparent molar volumes at infinite dilution ( $$V_{{\varphi ,{\text{SDS}}}}^{^\circ }$$ ) in solutions containing the amino acid (at a fixed concentration of 0.01 mol kg–1) are determined. A rise in the first critical micelle concentration from m = 0.0080 for SDS solutions in water to m = 0.0099 for SDS solutions with Trp is observed. Values of derivatives ( $$\partial V_{{\varphi ,{\text{SDS}}}}^{^\circ }$$ /∂T)p and ( $${{\partial }^{2}}V_{{\varphi ,{\text{SDS}}}}^{^\circ }$$ /∂T  2)p, and the partial molar volumes of SDS transfer from water to amino acid aqueous solutions, are calculated. Results are discussed by considering different types of intermolecular interaction in the given solutions. Quantum-chemical DFT/B97D modeling of the complexes between SDS and L-tryptophan zwitterion is done using a combination of basis set 6-311++G(2d,2p) and Grimme’s functional hybrid exchange correlation with a dispersion correction. The polarizable continuum model (PCM) is used to determine the structure and energies of formation of the SDS…Trp complexes with allowance for hydration effects.