Exploring the relevance of gas-phase structures to biology: cold ion spectroscopy of the decapeptide neurokinin A.

Linking the intrinsic tertiary structures of biomolecules to their native geometries is a central prerequisite for making gas-phase studies directly relevant to biology. The isolation of molecules in the gas phase eliminates hydrophilic interactions with solvents, to some extent mimicking a hydrophobic environment. Intrinsic structures therefore may resemble native ones for peptides that in vivo reside in a hydrophobic environment (e.g., binding pockets of receptors). In this study, we investigate doubly protonated neurokinin A (NKA) using IR-UV double resonance cold ion spectroscopy and find only five conformers of this decapeptide in the gas phase. In contrast, NMR data show that in aqueous solutions, NKA exhibits high conformational heterogeneity, which reduces to a few well-defined structures in hydrophobic micelles. Do the gas-phase structures of NKA resemble these native structures? The IR spectra reported here allow the validation of future structural calculations that may answer this question.

Stark coefficients for highly excited rovibrational states of H2O.

Quantum beat spectroscopy is combined with triple-resonance vibrational overtone excitation to measure the Stark coefficients (SCs) of the water molecule for 28 rovibrational levels lying from 27,600 to 41,000 cm(-1). These data provide a stringent test for assessing the accuracy of the available potential energy surfaces (PESs) and dipole moment surfaces (DMSs) of this benchmark molecule in this energy region, which is inaccessible by direct absorption. SCs, calculated using the combination of a high accuracy, spectroscopically determined PES and a recent ab initio DMS, are within the 1% accuracy of available experimental data for levels below 25,000 cm(-1), and within 4.5% for coefficients associated with levels up to 35,000 cm(-1). However, the error in the computed coefficients is over 60% for the very high rovibrational states lying just below the lowest dissociation threshold, due, it seems, to lack of a high accuracy PES in this region. The comparative analysis suggests further steps, which may bring the theoretical predictions closer to the experimental accuracy.

Isotopically selective collisional vibrational energy transfer in CF3H.

The authors investigate here the mechanism of collisionally enhanced isotopic selectivity observed in infrared multiple photon dissociation (IRMPD) of vibrationally preexcited CF3H by Boyarkin et al. [J. Chem. Phys. 118, 93 (2003)]. For both the carbon-12 and carbon-13 isotopic species they measure the dependence of the IRMPD yield on the time delay between the preexcitation and the dissociation pulses at different dissociation frequencies as well as its dependence on the initial isotopic composition of the sample. The results reveal that the collisional increase in isotopic selectivity originates not only from that of IRMPD itself but also from the isotopic selectivity of vibrational energy transfer, with the latter making the major contribution under their experimental conditions. They suggest that the observed isotopic selectivity in collisional energy transfer arises from the difference in overlap between the absorption spectra of the nu5 mode in the 12CF3H acceptor molecule with emission spectra of the same mode in the two isotopically different donors. Understanding the origin of this collisional effect has important implications for optimization of laser isotope separation processes.

Collisionally assisted, highly selective laser isotope separation of carbon-13.

We have further developed our recently reported two-laser technique for highly selective molecular isotope separation of carbon-13 [Boyarkin, Kowalczyk, and Rizzo, J. Chem. Phys. 118, 93 (2003)] with the objective of increasing the yield. An essential feature of this approach in its original conception is the significant increase of isotopic selectivity that occurs through collisions during the time between the overtone preexcitation laser pulse and the multiphoton dissociation pulse. We demonstrate here that under certain conditions, this collisional enhancement of the selectivity works equally well when the two pulses are overlapped in time, allowing the overall isotopic selectivity of the process to remain high while achieving a significant increase in the absolute dissociation yield. We also find that proper shaping of the CO2 laser dissociation pulse makes the fluence required for dissociation sufficiently low to allow irradiation of a large reaction volume by unfocused laser beams. Together, these factors may make this laser isotope separation scheme competitive with existing separation methods.