Elucidating the stability of bolaamphiphilic polypeptide nanosheets using atomistic molecular dynamics.

Atomistic molecular dynamics was employed to characterize the stability of nanosheets formed by bolaamphiphilic polypeptides. Two different nanosheets (based on RFL4FR and EFL4FE peptide sequences) were simulated to quantify the impact of the bolaamphiphilic nature of the peptides on the structure and energetics of the formed nanostructures. Our results corroborate the structural results obtained experimentally, indicating consistent values for the separation between the peptide planes as well as for nanosheet thickness. Energy analysis indicates that in general the stability of the nanosheets is dominated by electrostatic interactions and nanosheet-water environment interactions contribute considerably to stability. In general, the nanosheets were found to be very stable especially the EFL4FE system that presents a greater energy of interaction between the components of the system. PMF calculations indicate that the free energy required to remove a peptide from the nanosheet is greater than 250 kJ mol-1 reaching the highest value of 310 kJ mol-1 for the extraction of the peptide in the EFL4FE nanosheet.

Isotropic magnetic shielding constants of retinal derivatives in aprotic and protic solvents.

We investigate the nuclear isotropic shielding constants σ((13)C) and σ((17)O) of isomers of retinoic acid and retinal in gas-phase and in chloroform, acetonitrile, methanol, and water solutions via Monte Carlo simulation and quantum mechanics calculations using the GIAO-B3LYP∕6-311++G(2d,2p) approach. Electronic solute polarization effects due to protic and aprotic solvents are included iteratively and play an important role in the quantitative determination of oxygen shielding constants. Our MP2∕6-31G+(d) results show substantial increases of the dipole moment of both retinal derivatives in solution as compared with the gas-phase results (between 22% and 26% in chloroform and between 55% and 99% in water). For the oxygen atoms the influence of the solute polarization is mild for σ((17)O) of hydroxyl group, even in protic solvents, but it is particularly important for σ((17)O) of carbonyl group. For the latter, there is a sizable increase in the magnitude with increasing solvent polarity. For the carbon atoms, the solvent effects on the σ((13)C) values are in general small, being more appreciable in carbon atoms of the polyene chain than in the carbon atoms of the ß-ionone ring and methyl groups. The results also show that isomeric changes on the backbones of the polyene chains have marked influence on the (13)C chemical shifts of carbon atoms near to the structural distortions, in good agreement with the experimental results measured in solution.