Infrared absorption spectroscopy of diacetylene ions trapped in solid argon.

The C4H2+ diacetylene radical cation has been generated in a pulsed jet electrical discharge through both a diacetylene/argon mixture and an acetylene/argon mixture. The product mixture was trapped on a 12 K cryostat window and studied via Fourier transform infrared absorption spectroscopy. The diacetylene cation was also produced via low energy electron bombardment of an effusive C4H2/Ar beam. Two new infrared bands at 3201.6 and 1827.9 cm(-1) have been identified as vibrations of the diacetylene cation, viz the v4 (sigmau) (C-H stretching) and the v5(sigmau) (C[triple bond]C stretching) modes, respectively. Geometry optimization and harmonic frequency calculations, carried out at various spin unrestricted levels (B3LYP, CCSD(T)) for spin doublet structures, indicate that, in its electronic ground state X2pi(g), the C4H2+ cation is linear. Three additional new bands at 2957.5, 1693.8 and 594.5 cm(-1) have been tentatively assigned to the C-H stretching, C[triple bond]C stretching and C[triple bond]C-H (in-plane) bending modes, respectively, of the nonlinear diacetylene anion (C4H2-, X2B(u)).

Hydrogen bonding in human manganese superoxide dismutase containing 3-fluorotyrosine.

Incorporation of 3-fluorotyrosine and site-specific mutagenesis has been utilized with Fourier transform infrared (FTIR) spectroscopy and x-ray crystallography to elucidate active-site structure and the role of an active-site residue Tyr34 in human manganese superoxide dismutase (MnSOD). Calculated harmonic frequencies at the B3LYP/6-31G** level of theory for L-tyrosine and its 3-fluorine substituted analog are compared to experimental frequencies for vibrational mode assignments. Each of the nine tyrosine residues in each of the four subunits of the homotetramer of human MnSOD was replaced with 3-fluorotyrosine. The crystal structures of the unfluorinated and fluorinated wild-type MnSOD are nearly superimposable with the root mean-square deviation for 198 alpha-carbon atoms at 0.3 A. The FTIR data show distinct vibrational modes arising from 3-fluorotyrosine in MnSOD. Comparison of spectra for wild-type and Y34F MnSOD showed that the phenolic hydroxyl of Tyr34 is hydrogen bonded, acting as a proton donor in the active site. Comparison with crystal structures demonstrates that the hydroxyl of Tyr34 is a hydrogen bond donor to an adjacent water molecule; this confirms the participation of Tyr34 in a network of residues and water molecules that extends from the active site to the adjacent subunit.