We present results of improving the OPLS-AA force field for peptides by means of refitting the key Fourier torsional coefficients. The fitting technique combines using accurate ab initio data as the target, choosing an efficient fitting subspace of the whole potential-energy surface, and determining weights for each of the fitting points based on magnitudes of the potential-energy gradient. The average energy RMS deviation from the LMP2/cc-pVTZ(-f)//HF/6-31G** data is reduced by ca. 40% from 0.81 to 0.47 kcal/mol as a result of the fitting for the electrostatically uncharged dipeptides. Transferability of the parameters is demonstrated by using the same alanine dipeptide-fitted backbone torsional parameters for all of the other dipeptides (with the appropriate side-chain refitting) and the alanine tetrapeptide. Parameters of nonbonded interactions have also been refitted for the sulfur-containing dipeptides (cysteine and methionine), and the validity of the new Coulombic charges and the van der Waals σ's and 's is proved through reproducing gas-phase energies of complex formation heats of vaporization and densities of pure model liquids. Moreover, a novel approach to fitting torsional parameters for electrostatically charged molecular systems has been presented and successfully tested on five dipeptides with charged side chains.

The molecular orientation and/or packing for monolayers formed by adsorption of (4-mercaptophenyl)phthalimide (4-MPP), a thiol-terminated polyimide model compound, onto gold substrates, was determined using surface-enhanced Raman scattering (SERS), reflection-absorption infrared spectroscopy (RAIR), X-ray photoelectron spectroscopy (XPS), ellipsometry, and electrochemistry. SERS, RAIR, and XPS showed that 4-MPP molecules were chemisorbed onto the gold surface through the thiol groups to form an oriented monolayer. For adsorption times less than about 2 h, the ellipsometric thickness of the films increased as a function of time, but for longer times the ellipsometric thickness reached a limiting value of about 11.5 Å. For adsorption times greater than about 2 h, the surface density was about 5.2 × 10 -10 mol/cm 2 , corresponding to a molecular area of 32 Å 2 /molecule. Considering the size of the 4-MPP molecule compared to that of an alkanethiol molecule, it was concluded that the adsorbed molecules were highly packed on the gold surface. The orientation of adsorbed 4-MPP molecules was determined quantitatively using infrared spectroscopy. It was found that the molecules were oriented with a vertical configuration in which the molecular axes tilted away from the surface normal by about 21°. There was no preferred rotation angle for the imide ring in 4-MPP. The thickness and orientation determined by infrared spectroscopy were in excellent agreement with results obtained from molecular dynamics, which are presented in the following paper.