Spectroscopy of biological molecules at very low temperatures: Theoretical studies

There has been remarkable progress in recent years in experimental studies of high-resolution vibrational spectroscopy of biological molecules. Such studies generally require low-temperature conditions. The experiments carry in principle a wealth of information on the potential energy surfaces, and the structures and other properties of the molecules. The quantitative interpretation of the spectroscopic experiments is a challenge to theory, since an anharmonic level of treatment is often required, and since the number of vibrational degrees of freedom is large for the systems of interest. This chapter presents an overview of recent progress in theoretical calculations of vibrational spectroscopy of small biological molecules, and examines the lessons of comparison with experiment. The emphasis is on anharmonic calculations with the vibrational selfconsistent field (VSCF) method, using directly potential energy surfaces from ab initio methods. Based on comparison with experiments several issues are examined including the role of anharmonic effects; the merits of different types of potential energy surfaces; the sensitivity of the spectra to specific conformers; and the interaction between biological molecules and a selected number of water molecules. Several outstanding challenges and important directions for future progress are discussed.