Gas-phase FT-IR-spectra of natural amino acids

AbstractWe present the first gas-phase Fourier transform infrared (FT-IR) spectra of uncapped (natural) amino acids using a fast thermalheating technique up to around 570 K. The experimental spectra of phenylalanine, proline, valine, leucine and isoleucine in therange 900–3700 cm 1 show no sign of decomposition and are in good agreement with ab initio calculated frequencies and intensitiesat the B3LYP/6-31G(d,p) level, if non-hydrogen bonded structures are assumed at the high temperatures of the experiment. 2005 Elsevier B.V. All rights reserved. 1. IntroductionGas-phaseinfraredspectraarefreefromsolventinter-actionsandcanbecompareddirectlywithabinitiocalcu-lations of harmonic and anharmonic vibrationalfrequencies [1,2]. In addition, gas-phase spectra can becomparedwithspectrainliquidphase,solutionsandma-trixisolation,inordertoassestheinteractionwiththesol-vent and neighboring molecules [3,4]. So far, gas-phasespectra are available only for molecules with a sufficienthigh vapor pressure or thermal stability. Especially bio-molecules, like amino acids and peptides, have very lowvapor pressures and are well known for their thermalinstability.Indeed,allnaturalaminoacidsdecomposebe-foreoruponsublimationtosomeextent[5],thereforepre-venting easy measurements of gas-phase infrared spectrainstatic,thermallyheatedgascells.Somegas-phasespec-tra ofamino acids were measured using thermally heatednozzles [6–8] or laser desorption techniques [9–11]. Sinceboth sources yield only very low number densities, sensi-tivedetectionschemeslikeREMPI[6,7,9],IR-CRLAS[8]or microwave spectroscopy [10,11] are required.However,onlythreenaturalaminoacids,namelyphenyl-alanine, tryptophane and tyrosine, have chromophoreswithlowlyingelectronicstatesandaresuitedforREMPIdetection in combination with IR-UV double resonanceexperiments.In earlier experiments, we could show that we can ob-tain gas-phase FT-IR spectra of solid samples of adenine[12], thymine [13], cytosine [14] and benzotriazol [15] byrapid heating to about 250–300 C. Thermodynamicalpropertiesoftheketo/enolequilibriumcouldbeobtainedfrom measurements at different temperatures [15]. In thiswork,wepresentaslightlymodified,butnonetheless,verysimplerapidheatingset-up,whichcanbeappliedtother-mally very unstable molecules, too. We report the firstgas-phase FT-IR spectra of five different natural aminoacids: valine (Val), proline (Pro), phenylalanine (Phe),iso-leucine (Ile), and leucine (Leu). The spectra are com-pared to density functional calculations at the B3LYP/6-31G(d,p) level of theory, underlining once again thegood quality of density functional methods in describingthe harmonic vibrations of monomeric species.2. ExperimentalThe measurements were performed in a simple heatedgas cell (see Fig. 1). A stainless steel tube, 9 cm long and3 cm in diameter, is sealed at both ends with NaCl