Calculation of the infrared spectra of proteins.

The CHARMM22 force field with associated partial charges is used to calculate the infrared spectra of a number of small proteins and some larger biothreat proteins. The calculated high-frequency region, from about 2,500 to 3,500 cm(-1), is dominated by stretching modes of hydrogen bonded to other atoms, and is very similar in all proteins. There is a peak at 3,430 cm(-1) whose intensity is predicted by these calculations to be a direct measure of arginine content. The calculated low-frequency THz region, up to 300 cm(-1), is also very similar in all the proteins and just reflects the vibrational density of states in agreement with experimental results. Calculations show that the intermediate-frequency region between 500 and 1,200 cm(-1) shows the greatest difference between individual proteins and is also the least affected by water absorption. However, to match experimental measurements in the amide region, it was necessary to reduce the hydrogen partial charges.

Calculated infrared spectra of nerve agents and simulants.

Since organophosphorus nerve agents are among the most toxic known chemical warfare agents, it is desirable to have a way to distinguish between one and another. Infrared spectroscopy is a common tool for identifying molecules. Given the difficulty in handling these chemicals, calculated IR spectra can be useful. Calculated IR spectra are presented for G agents, V agents, and simulants. Quantum chemistry calculations were performed using the Gaussian 03 package at the B3LYP/6-31+G(d,p) level of theory. The most prominent IR lines are due to vibrations of P-O-C and P=O groups within the molecules. It should be possible to distinguish between the G-series and V-series agents using IR spectroscopy, but unique identification of individual chemical agents is unlikely.