Hydrocarboxyl Radical as a Product of α-Alanine Ultraviolet Photolysis

Abstract

UV photodissociation of α-alanine was studied by parahydrogen matrix isolation infrared spectroscopy. The temporal behavior of Fourier transform infrared spectra revealed that UV irradiation at 213 nm yielded the HOCO radical as a direct photoproduct from the S 2 excited state. The concentration of HOCO quickly approached a steady state due to secondary photodissociation of HOCO to produce CO 2 + H or CO + OH. On the other hand, no photoproducts were detected by S 1 excitation at 266 nm. Irradiation of fully deuterated α-alanine at 213 nm yielded ∼2 times more cis-DOCO radicals than the lower energy isomer trans-DOCO, indicating that the conformation of the hydroxyl group is fairly well-preserved upon photodissociation of α-alanine. The present study suggests that HOCO may be a good tracer species in the search for amino acids in interstellar space.

Ultraviolet Photodissociation of Proteinogenic Amino Acids

“…This is consistent with the photochemical behavior of simple carboxylic acids, where one of the primary pathways is HOCO dissociation from irradiation near 200 nm. − Although HOCO + is produced from photoionization of amino acids, this was the first observation of direct HOCO radical production . In addition, the counterproduct amino radical is not stable and immediately forms the imine, E / Z ethanimine, by self-elimination of a H atom.…”

“…The transitions at 1649.9 and 1652.2 cm −1 in the postirradiation spectrum of leucine are in close proximity to the C�N stretching frequencies of methanimine and ethanimine (Figure 5) and suggest an imine photoproduct. 24,64 The geometry and anharmonic vibrations of 3-methylbutan-1-imine ((CH 3 ) 2 CHCH 2 CHNH), the imine resulting in dissociation of the most stable conformer of leucine, 33 are shown in the SI (Table S3). The calculated frequencies of 3-methylbutan-1imine align well with the observed transitions in the postirradiation spectrum of leucine (Table S4).…”

“…Proteinogenic amino acids found on meteorites exhibit an enantiomeric imbalance. − Asymmetric photodissociation or synthesis by circularly polarized light in the interstellar medium (ISM) has been discussed as one of the possible origins of this enantiomeric imbalance and has been the focus of numerous studies over the past few decades. − Most photolysis studies so far reported, however, have been using solutions or solids, where amino acids are found in their zwitterionic form . Only a handful of studies have investigated the photochemistry of neutral amino acids below the ionization threshold, − although amino acids in ISM are likely to be nonionic in the gas phase . The concentrations of gas-phase molecules in the ISM are dictated by a balance of gas-phase production, gas-phase destruction, desorption from grain surfaces, and accretion to grain surfaces .…”

“…The photochemistry of α-alanine was recently characterized in solid p H 2 . Excitation to the S 2 state of alanine using 213 nm light produced neutral HOCO radicals and E / Z ethanimine (CH 3 CHNH) + H .…”

“…The photochemistry of α-alanine was recently characterized in solid p H 2 . Excitation to the S 2 state of alanine using 213 nm light produced neutral HOCO radicals and E / Z ethanimine (CH 3 CHNH) + H . Excitation of the most stable conformer of α-alanine by 213 nm light resulted in the following dissociation scheme: normala normall normala normaln normali normaln normale → italich ν c i s normal‐H normalO normalC normalO * + normalC normalH 3 normalC normalH normalN normalH 2 * c i s normal‐H normalO normalC normalO * → c i s normal‐H normalO normalC normalO → normalt normalu normaln normaln normale normall normali normaln normalg t r a n s normal‐H normalO normalC normalO c i s normal‐H normalO normalC normalO * → normalC normalO 2 + normalH t r a n s normal‐H normalO normalC normalO → normalh ν normalC normalO + normalO normalH , ( C O 2 + …”