Karen M. Siegrist

Terahertz spectra and normal mode analysis of the crystalline VA class dipeptide nanotubes.

Terahertz (THz) vibrational modes are characterized by nonlocal, collective molecular motions which are relevant to conformational changes and molecular functions in biological systems. We have investigated the THz spectra of a set of small bionanotubes which can serve as very simple models of membrane pores, and have examined the character of the THz modes which can impact transport processes. In this work, THz spectra of the crystalline VA class dipeptide nanotubes were calculated at both the harmonic and vibrational self-consistent field (VSCF) level using the CHARMM22 force field with periodic boundary conditions. Comparison of the calculated THz spectra against the experimental spectra revealed that the VSCF corrections generally improved the predictions in the low-frequency region. The improvements were especially manifested in the overall blue-shifts of the VSCF frequencies relative to the harmonic values, and blue shifts were attributed to the overall positive coupling strengths in all systems. Closer examination of the motions in the most significantly coupled normal mode pairs leads us to propose that, when two similar side-chain squeezing modes are coupled, the rapidly increased van der Waals interactions can lead to a stiffening of the effective potential, which in turn leads to the observed blue-shifts. However, we also noted that when the side-chain atoms become unphysically proximate and the van der Waals repulsion becomes too large, the VSCF calculations tend to deviate in the high frequency region and for the system of l-isoleucyl-l-valine. In addition, normal-mode analysis revealed a series of channel-breathing motions in all systems except l-valyl-l-alanine. We show that the inner products of the backbone vibrations between these channel-breathing motions divided the remaining VA class dipeptide systems into two subgroups. It is suggested that these modes may facilitate a pathway for the guest molecule absorption, substitution and removal in the VA class dipeptide nanotubes. Normal mode analysis also demonstrated that the THz motions may contribute to the pore permeability either directly by changing the pore size, or indirectly by affecting the solvent-host effective potentials.

Final Scientific/Technical Report

JHU/APL conducted solid propellant fire characterization tests in warm, humid, ambient conditions near sea level. Yttria and ceria surrogate materials were placed in the fires. The substrates simulating ground surfaces were concrete from a Kennedy Space Center launch pad, and steel covered with a protective ablative material representing a launch platform. In-situ instrumentation consisted of witness materials, thermocouples, air handlers, filters, and cascade impactors; remote instrumentation consisted of optical cameras and spectrometers. Test and analysis team members included the Naval Air Warfare Center Aircraft Division, Sandia National Laboratories (SNL), Alliant Techsystems, and the Johns Hopkins University. Test data were analyzed, reported, and delivered, including plume rise and transport captured on video. Derivation of the alumina particle size distributions formed the basis for condensing vapor and agglomeration estimates. Assessment of alumina mass in the plume, along with the surrogate fraction from filter forensics, provided an estimate of airborne surrogate mass. Technical interchange meetings were held with SNL and the Jet Propulsion Laboratory. Specifications for the fire environment were developed and delivered. A thermochemistry model that simultaneously provides the maximum temperature and heat flux was developed and delivered. An SPIE paper on 3D pyrometry of the fire was written and presented.