Infrared Multiple Photon Dissociation Spectroscopy of Cationized Asparagine: Effects of Metal Cation Size on Gas-Phase Conformation

Abstract

Gas-phase structures of cationized asparagine (Asn) including complexes with Li(+), Na(+), K(+), Rb(+), Cs(+), and Ba(2+), as well as protonated Asn, are examined by infrared multiple photon dissociation (IRMPD) action spectroscopy utilizing light generated by a free electron laser. Experimental spectra for the alkali metal cation complexes exhibit systematic trends, whereas spectra for Ba(2+)(Asn) and H(+)(Asn) are more distinct. To identify the structures formed experimentally, measured IRMPD spectra are compared to spectra calculated at a B3LYP/6-311+G(d,p) level with several effective core potentials and basis sets evaluated for the heavy metal systems. The dominant conformation ascertained for complexes with the smaller metal cations, Li(+)(Asn) and Na(+)(Asn), is a charge-solvated, tridentate [N,CO,CO] structure that binds the metal cation with the amine group of the amino acid backbone and to the carbonyl oxygen atoms of the backbone and amino acid side chain. For the larger alkali metal cation complexes, K(+)(Asn), Rb(+)(Asn), and Cs(+)(Asn), an additional charge-solvated, tridentate [COOH,CO] structure that binds the metal cation with the two oxygen atoms of the backbone carboxylic acid group and the carbonyl oxygen atom of the Asn side chain may also be present. The Ba(2+)(Asn) spectrum is characteristic of a single charge-solvated [N,CO,CO] conformation, in contrast to Gly, Trp, Arg, Gln, Pro, Ser, Val, and Glu, which all take on a zwitterionic structure when complexed to Ba(2+). In no case do the cationized Asn complexes show definitive evidence of forming a zwitterionic structure in the complexes studied here. For H(+)(Asn), a mixture of two [N,CO] structures, which differ only in the orientation the side chain and are calculated to be nearly identical in energy, explains the experimental spectrum well.

Thermodynamics and mechanism of protonated asparagine decomposition

“…The next two lowest energy excited conformers, [N,CO]ttgt and tttt, are calculated to comprise only 0.9%-3.7% and 0.4%-0.7% of the total population. Confirmation of these computational results comes from an infrared multiphoton dissociation (IRMPD) action spectroscopy study that investigates the gas-phase conformations of cationized asparagine complexes [56]. The IRMPD spectrum of H ϩ (Asn) is fully accounted for by a combination of the [N,CO]tgtt and [N,CO]tcgt structures both being populated.…”

IR spectroscopy of cationized aliphatic amino acids: Stability of charge-solvated structure increases with metal cation size

“…The IRMPD spectra were recorded over the wavelength range 1000-1850 cm −1 to detect the characteristic CO stretching mode, which is very indicative for the presence of either SB or CS structures. For SB structures, which contain the anionic carboxylate moiety, this vibration is expected in the region between 1650 and 1700 cm −1 , whereas for CS structures, where the carboxylic acid carbonyl is maintained, it should lie between 1700 and 1800 cm −1 [29][30][31][32]54,56,75,76].…”

“…Amino acids complexed with alkali [28][29][30][31][32][33][34] and alkaline earth metal cations [35][36][37][38][39][40], as well as cationized peptides [39,[41][42][43][44] and proteins [45,46] have been investigated. Many of these photodissociation experiments were conducted in the infrared wavelength range using light generated by a tunable free electron laser (FEL) [47], which is typically scanned between 5 and 20 m in these experiments [18,[25][26][27][28][29][30][31][32][34][35][36][37][38][39][40][41][42][43][44][45].…”

Conformational analysis of glutamic acid: a density functional approach using implicit continuum solvent model

“…It is known that in solid state or in solution, amine group of amino acids get protonated, carboxyl group gets deprotonated, and the amino acid becomes zwitterionic [1,[30][31][32]. In an isolated system, all natural amino acids are canonical [33,34] and zwitterionic form can be stabilized by water molecules and metal ions [6,[30][31][32][33][34][35][36][37][38][39][40][41].…”

“…Metal ion size-charge [6,34,[40][41][42] or amount of water molecules [31,[35][36][37][38][39] are also an important issue in zwitterion stability. Glutamic acid has two carboxylic groups of which one is on C α and the other one is on C γ .…”