Computational and Experimental Studies on Double H-Bonded Zwitterion Dimer Model for L-2-Aminoadipic Acid

A double H-bonded zwitterion dimer structure is proposed for L-2-aminoadipic acid. The intermolecular N–H···O and O–H···O bond interactions whose vibrational modes are observed as striking IR absorption spectral features were analyzed. From molecular dynamics simulation, radial distribution function analysis showed values of 1.57 Å for the N–H···O bond and 1.68 Å for the O–H···O bond. Similarly, the radius of gyration analysis showed average values of 1.48 nm for the N–H···O bond and 1.57 nm for the O–H···O bond. DFT calculations at the B3LYP/6-311++G(d,p) level with implicit water solvation yielded the most stable zwitterion dimer. DFT results showed optimized H···O distances of 1.764 Å and 1.774 Å, with vibrational frequency redshifts of 15% (N–H···O) and 10% (O–H···O) relative to free N–H/O–H stretching frequencies. From VCD spectrum analysis, the functional groups participating in the two H-bonds exhibit non-robust modes, signifying the presence of the H-bonds in the dimer. NBO analysis shows stabilization energies of 15.5 kcal/mol (N–H···O) and 14.1 kcal/mol (O–H···O). All analyses showed that the zwitterionic structure of L-2-aminoadipic acid strengthens the N–H···O bond more than the O–H···O bond. The ECD spectra showed a broad band at 205 nm experimentally and 219 nm computationally, showing close agreement. Mole fraction variations in water reduced the intensity of the band without shifting its position. This observation has been attributed to the strong zwitterion dimer structure not amenable to dissociation into its constituent monomer species, thereby indicating that there is apparently no influence of N–H···O and O–H···O bonds on the electronic circular dichroism band structure.