Linear dichroism spectroscopy of gas phase biological molecules embedded in superfluid helium droplets

Abstract

This article presents the current status of gas phase linear dichroism (LD) spectroscopy, including the theoretical background, the experimental technique, and a few examples in the UV/VIS and IR. Orientation and alignment of gas phase samples are achieved using a DC electric field. To reach the necessary degree of alignment, biological molecules vaporized from a heated oven need to be embedded in superfluid helium droplets. Excitation under different polarization directions of the light source relative to the alignment field can then be used to derive the direction of the transition dipole, or the size of the permanent dipole, or both. For biological molecules that have no resonance lines or too many resonance lines, LD offers an additional parameter for spectroscopic assignment and tautomeric and conformational identification. The direction of the vibrational transition dipole is proven more reliable for vibrational and tautomeric assignment than the energy or frequency information, which is often problematic because of its sensitivity to basis sets and calculation methods. Several examples of vibrational LD of nucleic acid bases will be discussed. On the other hand, if a chromophore with a known electronic transition dipole is attached to a biological molecule, as demonstrated in the case of tryptamine, the permanent dipole determined from LD is then representative of the molecular conformation. This method of conformational determination does not rely on detailed spectroscopic assignment, thus it is applicable to molecules that do not have resolvable vibronic bands. However, its application is currently limited to the availability of an effective chromophore, and the search for such a chromophore is an on-going effort.

Keywords:

• polarization spectroscopy
• linear dichroism
• superfluid helium droplets
• field induced orientation
• molecular alignment

Acknowledgements

The authors are appreciative of the earlier effort in constructing the experimental apparatus by Dr Chengyin Wu, a former postdoctoral fellow in our group. Discussions with Prof. Andrej Vilesov and help from Mikhail Slipchenko from University of Southern California on many aspects of the experiment are also appreciated. Encouragements from Professor Alkwin Slenczka, Professor J. Peter Toennies, and Professor Dudley Herschbach are highly appreciated. Our talented machinist, Ted Hinke, has made this work possible, with his many ingenious designs and modifications of the experimental apparatus. We are also deeply indebted to Professor Hua Guo and Dr Dingguo Xu, who have provided us with the desperately needed high level calculation results for the Pc compounds. This work is supported by the National Science Foundation, Division of Chemistry. Acknowledgment is made to the Donors of The Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research.