Selective synthesis of new sugars based on 8-hydroxyquinoline as corrosion inhibitors for mild steel in HCl solution-effect of the saturated hydrocarbon chain: Theoretical and experimental studies

Corrosion Inhibition Studies of 8-Hydroxyquinoline Derivatives for N80 Steel in a 1.0 M HCl Solution: Experimental, Computational Chemistry, and Quantitative Structure-Activity Relationship Studies

Twelve kinds of 8-hydroxyquinoline derivatives were synthesized and characterized. The weight loss method was used to evaluate their inhibition efficiencies (IEs) in a 1.0 M HCl solution at 333 K. The results showed that the alkyl chain length, heteroatoms (S, N, and O), and number of benzene rings significantly affect the IE. Herein, the IE of 5-[(dodecylthio)methyl]-8-quinolinol reached 98.71%. Meanwhile, the potentiodynamic polarization results indicated that all 8-hydroxyquinoline derivatives were mixed-type inhibitors. Electrochemical impedance spectroscopy results revealed that 8-hydroxyquinoline derivatives can increase polarization resistance, supporting their adsorption on the N80 steel surface. Moreover, according to density functional theory (DFT), the frontier orbital distribution and quantum chemical parameters (E_HOMO, E_LUMO, dipole moment μ, etc.) were calculated, and the results confirmed that the substituents of protonated 8-hydroxyquinoline derivatives significantly influenced the frontier orbital distribution. Molecular dynamics simulation illustrated that all protonated 8-hydroxyquinoline derivatives were adsorbed parallel to the Fe(110) surface, and the interaction energy (E_int) evidenced that the molecular size would affect their strength of adsorption on the Fe(110) surface. The linear and nonlinear quantitative structure-activity relationship models were established by linear regression (LR) methods and BP neural networks (NN), respectively. The LR model was established by using E_int and μ, and the coefficient of determination (R²) was 0.934. In addition, the nonlinear NN model was obtained according to IE and all parameters (DFT parameters and E_int). Then, the two calculation inhibition efficiencies (IE_cal) were obtained from the LR and NN models, and the R² values of the linear correlation between the IE_cal and the experimental IE were 0.940 and 0.951, respectively. In addition, the IE of the tested inhibitor was 51.86% and the IE_cal values predicted by the LR and NN models were 52.68% and 53.06%, respectively. Our results demonstrate that both the LR and NN models have good fits and predictive ability.

Synthesis of chimera oligopeptide including furanoid β-sugar amino acid derivatives with free OHs: mild but successful removal of the 1,2-O-isopropylidene from the building block

Complementary to hydrophobic five membered ring β-amino acids (e.g. ACPC), β-sugar amino acids (β-SAAs) have found increasing application as hydrophilic building blocks of foldamers and α/β chimeric peptides. Fmoc-protected β-SAAs [e.g. Fmoc-RibAFU(ip)-OH] are indeed useful Lego elements, ready to use for SPPS. The removal of 1,2-OH isopropylidene protecting group increasing the hydrophilicity of such SAA is presented here. We first used N3-RibAFU(ip)-OH model compound to optimize mild deprotection conditions. The formation of the 1,2-OH free product N3-RibAFU-OH and its methyl glycoside methyl ester, N3-RibAFU(Me)-OMe were monitored by RP-HPLC and found that either 50% TFA or 8 eqv. Amberlite IR-120 H+ resin in MeOH are optimal reagents for the effective deprotection. These conditions were then successfully applied for the synthesis of chimeric oligopeptide: -GG-X-GG- [X=RibAFU(ip)]. We found the established conditions to be effective and-at the same time-sufficiently mild to remove 1,2-O-isopropylidene protection and thus, it is proposed to be used in the synthesis of oligo- and polypeptides of complex sequence combination.