High Resolution Fourier Transform Infrared Spectroscopy Studies of Multidimensional H Transfer in Tropolone/Tropolonoid Molecules-a Bridge to the Behavior in Biological Systems
EMSL Project ID
25617
Abstract
In the year 2000 high resolution infrared spectroscopy experiments conducted in EMSL at PNNL were initiated on the nonrigid gaseous tropolone molecule (2-hydroxy-2,4,6-cycloheptatrien-1-one). On the basis of results obtained from this work it is proposed to continue the experiments on a similar schedule during the next three years. At first glance the connection between the EMSL science theme "Biological Interactions and Dynamics", with its strong bias towards the behavior of proteins in cells, and the FTIR spectroscopy of a 15-atom nonrigid molecule in the gas phase is opaque. However, the intramolecular multidimensional dynamics that occur in proteins, and in proteinaceous molecules such as enzymes, are all about the multidimensional interactions between an atom and its neighborhood of atoms in a molecule. As shown below, the IR spectroscopy of tropolone evaluates the interactions arising between all of its 15 atoms—due to the extraordinary twist that numerous spectral doublets, arising through quantum mechanical tunneling, are observed for this unique nonrigid molecule. Its size qualifies tropolone as a bridge molecule with its intramolecular dynamics between those in small and in truly large molecules. Further, tunneling doublets occur for its 2H, 13C, and 18O isotopomers, and for its simple chemical derivatives (e.g. a ring C-H replaced by C-X with X = -OH, -NH2, -Cl, -Br, etc.). The motivation for this proposal is to obtain and understand this important body of data and its implications for large biological systems (especially H transfer enzymes). The feature element in the tautomerization reaction of tropolone is the simple H transfer reaction OH•••O  O•••HO. This planar molecule has two equivalent minimum energy tautomer configurations separated by a saddle-point barrier on a multidimensional energy surface. This allows the tautomerization reaction to occur by quantum mechanical tunneling with energy level splittings readily detectable by FTIR spectroscopy. Because the energy level splittings depend strongly on the rotation-contortion-vibration state, they furnish vital probes of the intramolecular multidimensional dynamics and potential energy couplings that occur in tropolone. The readily obtained OH/OD and 18O/16O isotopomers have shown trenchant isotope effects on the state-specific tunneling splittings of tropolone(OH). Observations on additional isotopomers and on simple chemical derivatives of tropolone will multiply the data base. The thorough experimental testing of detailed theoretical-computational analyses1 being developed for applications to tunneling molecules will help grow the general scientific understanding of intramolecular multidimensional dynamical behavior. The four volume study2 from Wiley-VCH, Hydrogen Transfer Reactions edited by Hynes, Klinman, Limbach, and Schowen, has just been published. Two of the volumes are devoted to biological aspects including proton-coupled electron transfer, enzymatic H transfer coupled to protein dynamics, nuclear tunneling and coupled motions. These representative topics seem to be particularly amenable to the introduction of atom level intramolecular dynamics transferable from the study of tropolones. The monograph begins with the chapter entitled Coherent Proton Tunneling in Hydrogen Bonds of Isolated Molecules: Malonaldehyde and Tropolone, by R. L. Redington.
Project Details
Project type
Large-Scale EMSL Research
Start Date
2007-06-01
End Date
2010-09-30
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members
Related Publications
Craig NC, and RL Sams. 2008. "An Investigation of the Rotamers of Butadiene by High-Resolution Infrared Spectroscopy." Journal of Physical Chemistry A 112(49):12637-12646. doi:10.1021/jp807677y