The status of our investigation of low-energy K + Xe collisions in the Xenon bubble chamber DIANA is reported. In the charge-exchange reaction K + Xe →

We have continued our investigation of the charge-exchange reaction K + Xe → K 0 pXe ′ in the bubble chamber DIANA. In agreement with our previous results based on part of the present statistics, formation of a narrow pK 0 resonance with mass of 1537 ± 2 MeV/c 2 is observed in the elementary transition K + n → K 0 p on a neutron bound in the Xenon nucleus. Visible width of the peak is consistent with being entirely due to instrumental resolution and allows to place an upper limit on its intrinsic width: Γ < 9 MeV/c 2 . A more precise estimate of the resonance intrinsic width, Γ = 0.36 ± 0.11 MeV/c 2 , is obtained from the ratio between the numbers of resonant and non-resonant charge-exchange events. The signal is observed in a restricted interval of incident K + momentum, that is consistent with smearing of a

The structure of sickle cell hemoglobin (Hb S) (β6 Glu → Val) fibers was probed using UV resonance Raman (UVRR) spectroscopy. For these studies a functional analogue of Hb S, fluoromet Hb S, was used to study structural changes that accompany fiber formation. The amide backbone and aromatic residues of Hb S were selectively investigated using excitation wavelengths of 210, 215, and 230 nm. In the 210 and 215 nm excited fiber spectra, the intensity of all Phe bands increases dramatically. At the excitation wavelengths used, the Phe signal intensity reflects the local environment and increases linearly with increasing ethylene glycol concentration. Thus, UVRR fiber spectra are suggestive of an increase in hydrophobicity of the Phe local environment, which results from the formation of lateral and axial fiber contacts that are primarily nonpolar and hydrophobic in nature. The observed UVRR signal is assigned to the β185 Phe residue, which, together with the β188 Leu residue, forms a hydrophobic lateral contact with the mutated β26 residue. In addition, 230 nm difference spectra are suggestive that H-bonds stabilizing the α1β2 interface are stronger in fibers than in unassociated T-state tetramers. The W3 mode in fiber difference spectra occurs at 1550 and 1565 cm-1, indicative of an increase in Trp spectral heterogeneity. The +6 cm-1 upshift of the W3 mode is attributed to increased hydrophobicity of Trp local environment and is assigned to the β215 Trp residue. Other structural changes include an increase in disorder upon fiber formation, as shown by the frequencies of protein backbone amide vibrational modes. UVRR spectroscopic results are consistent with the structural details of the Hb S double strand observed crystallographically and provide new information regarding local environment and strength of H-bond interactions.

The tertiary and quaternary structure of HbS has been investigated using UV resonance Raman (UVRR) and fluorescence spectroscopic methods. This well-characterized Hb mutant (β6 Glu → Val), which forms polymers under deoxygenating conditions, was studied in the tetrameric form as the fluoromet derivative and compared with HbA under the same conditions. The excitation wavelengths employed in this study preferentially probe the Tyr and Trp residues in the protein. Comparison of UVRR and fluorescence difference spectra generated between the T and R states of FmetHbA and FmetHbS are indicative that the α1β2 intersubunit contacts, monitored through the β37 Trp and the α42 Tyr residues, are essentially the same for the two hemoglobins. (The abbreviations used are the following:  Hb, hemoglobin; FmetHbA, FmetHbS, the fluoromet derivatives of hemoglobin A and hemoglobin S; UVRR, ultraviolet resonance Raman, IHP, inositol hexaphosphate.) The tertiary conformation of FmetHbS, however, is perturbed relative to that of FmetHbA in both the T and R quaternary states, as shown by an overall increase in the intensity of the Trp modes in the UVRR spectra and a decrease in the fluorescence intensity. These spectral differences are attributed to the β15 Trp residue, because of the mutation at the β6 position. The combined spectroscopic studies provide evidence for an altered tertiary structure in HbS where the A-helix is displaced toward the E-helix as monitored by the strength of the β15 Trp···β72 Ser H-bond.

The data on the charge-exchange reaction K + Xe → K 0 pXe ′ , obtained with the bubble chamber DIANA, are reanalyzed using increased statistics and updated