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The v4 fundamental band of CF3 Br-79 and CF3 Br-81, present in natural isotopic abundance, was investigated in the 8.3-m region by high-resolution infrared spectroscopic techniques. Tuneable diode laser spectra were recorded in the ranges 1202.5-1205.0 cm-1, 1208.0-1210.1 cm(-1) and 1212.5-1214.5 cm(-1). The tuneable diode laser spectra were obtained at the reduced temperature of 200 K and in a free-jet expansion. The latter technique was used to reduce spectral congestion, achieving a rotational temperature of about 50 K, with a resolution up to 0.0008 cm(-1). A Fourier transforminfrared spectrumcovering the entire spectral region of the v(4) band, between 1190 and 1220 cm(-1), was recorded at 298 K with a resolution of 0.004 cm(-1). The experimental wavenumbers from the different spectroscopic techniques were combined to accomplish the complete ro-vibrational analysis of v(4). In total, 4651 transitions were assigned to CF3 Br-79, 4047 to CF3 Br-81, with J(max)'' = K-max '' = 80; of these, 3171 for CF3 Br-79 and 2755 for CF3 Br-81 are from diode laser measurements. The data of each isotopologue were analysed using the model Hamiltonian for a degenerate vibrational state of a molecule of C3v symmetry. The v4 band of both the isotopologues resulted essentially unperturbed, but the Delta l = Delta k = +/- 2 l-resonance was found to be active within the v(4) = 1 state. Precise values of the vibrational energy and of the ro-vibrational parameters of v(4) = 1 for CF3 Br-79 and CF3 Br-81 were obtained. The bromine isotopic splitting amounts to 6.9 x 10(-3) cm(-1). In addition, the equilibrium geometry and the harmonic force field were calculated ab initio using the large-size basis set def2-QZVP in conjunction to the PBE0 functional.

We have studied carbon monoxide (CO) migration and binding in the nonsymbiotic hemoglobins AHb1 and AHb2 of Arabidopsis thaliana using Fourier transform infrared (FTIR) spectroscopy combined with temperature derivative spectroscopy (TDS) at cryogenic temperatures. Both proteins have similar amino acid sequences but display pronounced differences in ligand binding properties, at both physiological and cryogenic temperatures. Near neutral pH, the distal HisE7 side chain is close to the heme-bound ligand in the majority of AHb1-CO molecules, as indicated by a low CO stretching frequency at 1921 cm(-1). In this fraction, two CO docking sites can be populated, the primary site B and the secondary site C. When the pH is lowered, a high-frequency stretching band at similar to 1964 cm(-1) at the expense of the low-frequency band, indicating that HisE7 protonates and, concomitantly, moves away from the bound ligand. Geminate rebinding barriers are markedly different for the two conformations, and docking site C is not accessible in the low-pH conformation. Rebinding of NO ligands was observed only from site B of AHb1, regardless of conformation. In AHb2, the HisE7 side chain is removed from the bound ligand; rebinding barriers are low, and CO molecules can populate only primary docking site B. These results are interpreted in terms of differences in the active site structures and physiological functions.

Progress in Nuclear Magnetic Resonance Spectroscopy

C-13 shieldings, delta(C-13) O-17 shieldings, delta(O-17), and O-18 isotope effects on C-13 shieldings, (1)Delta(13)C(O-18/O-16), Of carbon monoxide (99.7% C-13, 0.9% O-17, and 11.8% O-18 enriched) in a variety of solvents and of the Fe-C-O unit of several carbonmonoxy hemoprotein models with varying polar and steric effects of the distal organic superstructure and constraints of the proximal side are reported. This enables, first, comparisons with hemoproteins, C-O vibrational frequencies, nu(C-O), and X-ray structural data to be made; second, to investigate whether polarizable CO is an adequate model for distal ligand effects in carbonmonoxy heme proteins and synthetic model compounds; third, to investigate the effect of electronic perturbation within the heme pocket and pocket deformation on delta(C-13), delta(O-17), (1)Delta(13)C(O-18/O-16), and nu(C-O). A variety of solvents with varying dielectric constants and solvation abilities appears to have negligible effect on delta(O-17), delta(C-13), and (1)Delta(13)C(O-18/O-16) and little direct effect on nu(CO) of dissolved carbon monoxide. On the contrary, C-13 and O-17 shieldings of several carbonmonoxy hemoprotein models vary widely and an excellent correlation was found between the infrared C-O vibrational frequencies and C-13 shieldings and a reasonable correlation with O-18 isotope effects on C-13 shieldings. The C-13 shieldings of heme models cover a 4.0 ppm range which is extended to 7.0 ppm when several HbCO and MbCO species at different pHs are included. The latter were found to obey a similar linear delta(C-13) vs nu(C-O) relationship. nu(C-O), delta(C-13), and (1)Delta(13)C(O-18/O-16) parameters of heme,model compounds reflect similar interaction which is primarily the modulation of rr back-bonding from Fe d(pi) to CO pi* orbital by the distal pocket polar interactions. Our results suggest that, contrary to earlier claims, polarizable carbon monoxide is not an adequate model for distal ligand effects in carbonmonoxy hemoproteins and synthetic model compounds. Very probably this is caused by the large effect of the electric field on the back-bonding and the large polarizability of the Jr: subsystem of the Fe-C-O unit. The O-17 shieldings of heme models cover a range of 17 ppm which is extended to 24 ppm when selected heme proteins are included. The lack of correlation between delta(C-13) and delta(O-17) suggests that the two probes do not reflect a similar type of electronic and structural perturbation. delta(O-17) is not primarily influenced by the local distal field interactions and does not correlate with any single structural property of the Fe-C-O unit; however, atropisomerism and deformation of the porphyrin geometry appear to play a significant role. Inorg Chem

Progress in Nuclear Magnetic Resonance Spectroscopy