DFT Study on an Unnatural Amino Acid: 4- Hydroxyproline

4-hydroxy-l-proline is formed by hydroxylation of proline, an amino acid found in protein, whose inhibition results in hair problems in humans, causing scurvy disease. In this chapter, we discuss the DFT study on cis and trans conformers of 4- hydroxy-l-proline, i.e., CHLP and THLP using the B3LYP/6-31+G(d,p)level. The equilibrium structures of both conformers are obtained to analyze their vibrational properties. We have also discussed the results of an in-depth study on cis-4-hydroxy-d-proline (CHDP). The scan of potential energy surface provides the global minimum structure of CHDP along with its possible conformers. HOMO, LUMO, and MESP surfaces as well as charge distribution, are used to explain the chemical reactivity. The equilibrium geometry of CHDP dimer has been obtained and intermolecular interactions are explored by the NBO analyses. Vibrational analysis has been carried out on CHLP, THLP, and CHDP (monomer and dimer). A complete assignment to vibrational modes has been presented based on their potential energy distribution. The calculated frequencies, after proper scaling, are compared with experimental FT-IR frequencies recorded by KBr disc and Nujol mull techniques. Several electronic as well as thermodynamic parameters have also been reported.

Keywords: B3LYP, Charge distribution, DFT, Dimer, Electronic parameter, FTIR, H-bond, HOMO-LUMO, Hydroxyproline, Inter-molecular interaction, MESP, NBO, Proline, Thermodynamic parameter, Vibrational spectra.