The structures of cationized arginine complexes [Arg + M]+, (M = H, Li, Na, K, Rb, Cs, and Ag) and protonated arginine methyl ester [ArgOMe + H]+ have been investigated in the gas phase using calculations and infrared multiple-photon dissociation spectroscopy between 800 and 1900 cm-1 in a Fourier transform ion cyclotron resonance mass spectrometer. The structure of arginine in these complexes depends on the identity of the cation, adopting either a zwitterionic form (in salt-bridge complexes) or a non-zwitterionic form (in charge-solvated complexes). A diagnostic band above 1700 cm-1, assigned to the carbonyl stretch, is observed for [ArgOMe + H]+ and [Arg + M]+, (M = H, Li, and Ag), clearly indicating that Arg in these complexes is non-zwitterionic. In contrast, for the larger alkali-metal cations (K+, Rb+, and Cs+) the measured IR-action spectra indicate that arginine is a zwitterion in these complexes. The measured spectrum for [Arg + Na]+ indicates that it exists predominantly as a salt bridge with zwitterionic Arg; however, a small contribution from a second conformer (most likely a charge-solvated conformer) is also observed. While the silver cation lies between Li+ and Na+ in metal-ligand bond distance, it binds as strongly or even more strongly to oxygen-containing and nitrogen-containing ligands than the smaller Li+. The measured IR-action spectrum of [Arg + Ag]+ clearly indicates only the existence of non-zwitterionic Arg, demonstrating the importance of binding energy in conformational selection. The conformational landscapes of the Arg-cation species have been extensively investigated using a combination of conformational searching and electronic structure theory calculations [MP2/6-311++G(2d,2p)//B3LYP/6-31+G(d,p)]. Computed conformations indicate that Ag+ is di-coordinated to Arg, with the Ag+ chelated by both the N-terminal nitrogen and Neta of the side chain but lacks the strong M+-carbonyl oxygen interaction that is present in the tri-coordinate Li+ and Na+ charge-solvation complexes. Experiment and theory show good agreement; for each ion species investigated, the global-minimum conformer provides a very good match to the measured IR-action spectrum.

The gas-phase structures of protonated and alkali-metal-cationized lysine (Lys) and epsilon-N-methyllysine (Lys(Me)) are investigated using infrared multiple photon dissociation (IRMPD) spectroscopy utilizing light generated by a free electron laser, in conjunction with ab initio calculations. IRMPD spectra of Lys.Li(+) and Lys.Na(+) are similar, but the spectrum for Lys.K(+) is different, indicating that the structure of lysine in these complexes depends on the metal ion size. The carbonyl stretch of a carboxylic acid group is clearly observed in each of these spectra, indicating that lysine is nonzwitterionic in these complexes. A detailed comparison of these spectra to those calculated for candidate low-energy structures indicates that the bonding motif for the metal ion changes from tricoordinated for Li and Na to dicoordinated for K, clearly revealing the increased importance of hydrogen-bonding relative to metal ion solvation with increasing metal ion size. Spectra for Lys(Me).M(+) show that Lys(Me), an analogue of lysine whose side chain contains a secondary amine, is nonzwitterionic with Li and zwitterionic with K and both forms are present for Na. The proton affinity of Lys(Me) is 16 kJ/mol higher than that of Lys; the higher proton affinity of a secondary amine can result in its preferential protonation and stabilization of the zwitterionic form.

The mechanism of fluorescence and fluorescence quenching of the green fluorescent protein (GFP) is not well-understood. To gain insight into the effect of the surrounding protein on the chromophore buried at its center, the intrinsic electronic absorption and deactivation pathways of a gaseous model chromophore, p-hydroxybenzylidene-2,3-dimethylimidazolone (HBDI) were investigated. No fluorescence from photoactivated gaseous HBDI(-) was detected in the range 480-1100 nm, in line with the ultrafast rate of internal conversion of HBDI(-) in solution. Two different gas-phase deactivation pathways were found: photofragmentation and electron photodetachment. Electronic action spectra for each deactivation pathway were constructed by monitoring the disappearance of HBDI(-) and appearance of product ions as a function of excitation wavelength. The action spectra measured for each pathway are distinct, with electron photodetachment being strongly favored at higher photon energies. The combined (total) gas-phase action spectrum has a band origin at 482.5 nm (23340 cm(-1)) and covers a broad spectral range, 390-510 nm. This extended gas-phase action spectrum exhibits vibronic activity that matches well with the results of previous cold condensed-phase experiments and high-level in vacuo computations, with features evident at +550, +1500, and +2800 cm(-1) with respect to the band origin.