Hemoglobin site-mutants reveal dynamical role of interhelical H-bonds in the allosteric pathway: time-resolved UV resonance Raman evidence for intra-dimer coupling

Probing Structure and Reaction Dynamics of Proteins Using Time-Resolved Resonance Raman Spectroscopy

The mechanistic understanding of protein functions requires insight into the structural and reaction dynamics. To elucidate these processes, a variety of experimental approaches are employed. Among them, time-resolved (TR) resonance Raman (RR) is a particularly versatile tool to probe processes of proteins harboring cofactors with electronic transitions in the visible range, such as retinal or heme proteins. TR RR spectroscopy offers the advantage of simultaneously providing molecular structure and kinetic information. The various TR RR spectroscopic methods can cover a wide dynamic range down to the femtosecond time regime and have been employed in monitoring photoinduced reaction cascades, ligand binding and dissociation, electron transfer, enzymatic reactions, and protein un- and refolding. In this account, we review the achievements of TR RR spectroscopy of nearly 50 years of research in this field, which also illustrates how the role of TR RR spectroscopy in molecular life science has changed from the beginning until now. We outline the various methodological approaches and developments and point out current limitations and potential perspectives.

Tertiary dynamics of human adult hemoglobin fixed in R and T quaternary structures

Protein dynamics of human adult hemoglobin and its mutants restricted in R and T quaternary states following ligand photolysis were studied by time-resolved resonance Raman spectroscopy. In the time-resolved spectra, we observed spectral changes of in-plane stretching modes of heme and the iron-histidine stretching mode of the Fe-His bond for all the hemoglobin samples. The βD99N mutant, which adopts the R state in both the ligand-bound and the deoxy forms, showed similar temporal behaviors in time-resolved resonance Raman spectra as wild-type recombinant hemoglobin until 10 μs, consistent with the fact that the mutant undergoes only the tertiary structural changes in the R state. The βN102T mutant, which adopts the T state in both the ligand-bound and the deoxy forms, showed much slower tertiary structural changes, suggesting that the EF helical motion is decelerated by the change of the intersubunit interactions. The present data indicate that the allosteric kinetic response between the interhelical hydrogen bonds of the EF helices and the intersubunit hydrogen bonds is bidirectional. The implications of these results for understanding the allosteric pathway of Hb are discussed in detail.

A role of heme side-chains of human hemoglobin in its function revealed by circular dichroism and resonance Raman spectroscopy

Structural changes of heme side-chains of human adult hemoglobin (Hb A) upon ligand (O₂ or CO) dissociation have been studied by circular dichroism (CD) and resonance Raman (RR) spectroscopies. We point out the occurrence of appreciable deformation of heme side-chains like vinyl and propionate groups prior to the out-of-plane displacement of heme iron. Referring to the recent fine resolved crystal structure of Hb A, the deformations of heme side-chains take place only in the β subunits. However, these changes are not observed in the isolated β chain (β₄ homotetramer) and, therefore, are associated with the α-β inter-subunit interactions. For the communications between α and β subunits in Hb A regarding signals of ligand dissociation, possible routes are proposed on the basis of the time-resolved absorption, CD, MCD (magnetic CD), and RR spectroscopies. Our finding of the movements of heme side-chains would serve as one of the clues to solve the cooperative O₂ binding mechanism of Hb A.

Alteration of the α1β2/α2β1 subunit interface contributes to the increased hemoglobin-oxygen affinity of high-altitude deer mice

BACKGROUND

Deer mice (Peromyscus maniculatus) that are native to high altitudes in the Rocky Mountains have evolved hemoglobins with an increased oxygen-binding affinity relative to those of lowland conspecifics. To elucidate the molecular mechanisms responsible for the evolved increase in hemoglobin-oxygen affinity, the crystal structure of the highland hemoglobin variant was solved and compared with the previously reported structure for the lowland variant.

RESULTS

Highland hemoglobin yielded at least two crystal types, in which the longest axes were 507 and 230 Å. Using the smaller unit cell crystal, the structure was solved at 2.2 Å resolution. The asymmetric unit contained two tetrameric hemoglobin molecules.

CONCLUSIONS

The analyses revealed that αPro50 in the highland hemoglobin variant promoted a stable interaction between αHis45 and heme that was not seen in the αHis50 lowland variant. The αPro50 mutation also altered the nature of atomic contacts at the α1β2/α2β1 intersubunit interfaces. These results demonstrate how affinity-altering changes in intersubunit interactions can be produced by mutations at structurally remote sites.

Solution NMR Spectroscopy for the Study of Enzyme Allostery

Allostery is a ubiquitous biological regulatory process in which distant binding sites within a protein or enzyme are functionally and thermodynamically coupled. Allosteric interactions play essential roles in many enzymological mechanisms, often facilitating formation of enzyme-substrate complexes and/or product release. Thus, elucidating the forces that drive allostery is critical to understanding the complex transformations of biomolecules. Currently, a number of models exist to describe allosteric behavior, taking into account energetics as well as conformational rearrangements and fluctuations. In the following Review, we discuss the use of solution NMR techniques designed to probe allosteric mechanisms in enzymes. NMR spectroscopy is unequaled in its ability to detect structural and dynamical changes in biomolecules, and the case studies presented herein demonstrate the range of insights to be gained from this valuable method. We also provide a detailed technical discussion of several specialized NMR experiments that are ideally suited for the study of enzymatic allostery.

The time scale of the quaternary structural changes in hemoglobin revealed using the transient grating technique

The quaternary structural transition between the R and T states of human hemoglobin was investigated using the transient grating technique. The results presented herein reveal that the quaternary structural change accompanied by the R-T transition occurs within a few microseconds.

Differential control of heme reactivity in alpha and beta subunits of hemoglobin: a combined Raman spectroscopic and computational study

The use of hybrid hemoglobin (Hb), with mesoheme substituted for protoheme, allows separate monitoring of the α or β hemes along the allosteric pathway. Using resonance Raman (rR) spectroscopy in silica gel, which greatly slows protein motions, we have observed that the Fe-histidine stretching frequency, νFeHis, which is a monitor of heme reactivity, evolves between frequencies characteristic of the R and T states, for both α or β chains, prior to the quaternary R-T and T-R shifts. Computation of νFeHis, using QM/MM and the conformational search program PELE, produced remarkable agreement with experiment. Analysis of the PELE structures showed that the νFeHis shifts resulted from heme distortion and, in the α chain, Fe-His bond tilting. These results support the tertiary two-state model of ligand binding (Henry et al., Biophys. Chem. 2002, 98, 149). Experimentally, the νFeHis evolution is faster for β than for α chains, and pump-probe rR spectroscopy in solution reveals an inflection in the νFeHis time course at 3 μs for β but not for α hemes, an interval previously shown to be the first step in the R-T transition. In the α chain νFeHis dropped sharply at 20 μs, the final step in the R-T transition. The time courses are fully consistent with recent computational mapping of the R-T transition via conjugate peak refinement by Karplus and co-workers (Fischer et al., Proc. Natl. Acad. Sci. U. S. A. 2011, 108, 5608). The effector molecule IHP was found to lower νFeHis selectively for α chains within the R state, and a binding site in the α1α2 cleft is suggested.

Subunit disassembly pathway of human hemoglobin revealing the site-specific role of its cysteine residues

Cysteine residues play a unique role in human hemoglobin (Hb) by affecting its cooperative oxygen binding behavior and the stability of its tetrameric structure. However, how these cysteine residues fulfill their biophysical functions from the molecular level is yet unclear. Here we study the subunit disassembly pathway of human hemoglobin using the sulfhydryl reagent, p-hydroxymercuribenzoate (PMB) and investigate the functional roles of cysteine residues in human hemoglobin. We show evidence from the matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry that all three types of cysteine residues, including the surface-exposed βCys93 and the shielded αCys104 and βCys112 are reactive to PMB, resolving an issue long under debate. It is demonstrated that all three types of cysteine residues must be blocked by PMB to accomplish the subunit disassembly, and the PMB-cysteine reactions proceed in a stepwise manner with an order of βCys93, αCys104, and βCys112. The PMB reactions with the three different cysteine residues demonstrate strong site-specificity. The possible influence of PMB-cysteine reactions to the stability of various intersubunit salt bridges has been discussed based on the crystallographic structure of hemoglobin, providing insights in understanding the hemoglobin subunit disassembly pathway and the site-specific functional role of each cysteine residue in hemoglobin.

Deer mouse hemoglobin exhibits a lowered oxygen affinity owing to mobility of the E helix

The deer mouse, Peromyscus maniculatus, exhibits altitude-associated variation in hemoglobin oxygen affinity. To examine the structural basis of this functional variation, the structure of the hemoglobin was solved. Recombinant hemoglobin was expressed in Escherichia coli and was purified by ion-exchange chromatography. Recombinant hemoglobin was crystallized by the hanging-drop vapor-diffusion method using polyethylene glycol as a precipitant. The obtained orthorhombic crystal contained two subunits in the asymmetric unit. The refined structure was interpreted as the aquo-met form. Structural comparisons were performed among hemoglobins from deer mouse, house mouse and human. In contrast to human hemoglobin, deer mouse hemoglobin lacks the hydrogen bond between α1Trp14 in the A helix and α1Thr67 in the E helix owing to the Thr67Ala substitution. In addition, deer mouse hemoglobin has a unique hydrogen bond at the α1β1 interface between residues α1Cys34 and β1Ser128.

Heme reactivity is uncoupled from quaternary structure in gel-encapsulated hemoglobin: a resonance Raman spectroscopic study

Encapsulation of hemoglobin (Hb) in silica gel preserves structure and function but greatly slows protein motion, thereby providing access to intermediates along the allosteric pathway that are inaccessible in solution. Resonance Raman (RR) spectroscopy with visible and ultraviolet laser excitation provides probes of heme reactivity and of key tertiary and quaternary contacts. These probes were monitored in gels after deoxygenation of oxyHb and after CO binding to deoxyHb, which initiate conformational change in the R-T and T-R directions, respectively. The spectra establish that quaternary structure change in the gel takes a week or more but that the evolution of heme reactivity, as monitored by the Fe-histidine stretching vibration, ν(FeHis), is completed within two days, and is therefore uncoupled from the quaternary structure. Within each quaternary structure, the evolving ν(FeHis) frequencies span the full range of values between those previously associated with the high- and low-affinity end states, R and T. This result supports the tertiary two-state (TTS) model, in which the Hb subunits can adopt high- and low-affinity tertiary structures, r and t, within each quaternary state. The spectra also reveal different tertiary pathways, involving the breaking and reformation of E and F interhelical contacts in the R-T direction but not the T-R direction. In the latter, tertiary motions are restricted by the T quaternary contacts.

Differences between protein dynamics of hemoglobin upon dissociation of oxygen and carbon monoxide

Protein dynamics of human adult hemoglobin (HbA) upon ligand photolysis of oxygen (O(2)) and carbon monoxide (CO) was investigated using time-resolved resonance Raman (TR(3)) spectroscopy. The TR(3) spectra of the both photoproducts at 1-ns delay differed from that of the equilibrium deligated form (deoxy form) in the frequencies of the iron-histidine stretching [ν(Fe-His)] and methine wagging (γ(7)) modes, and the band intensity of pyrrole stretching and substituent bending (ν(8)) modes. Spectral changes of the O(2) photoproduct in the submicrosecond region were faster than those of the CO photoproduct, indicating that the structural dynamics following the photodissociation is ligand dependent for HbA. In contrast, no ligand dependence of the dynamics was observed for myoglobin, which has a structure similar to that of the subunit of HbA. The structural dynamics and relevance to the functionality of HbA also are discussed.

Backbone dynamics of deoxy and carbonmonoxy hemoglobin by NMR/SRLS

The slowly relaxing local structure (SRLS) approach, developed for NMR spin relaxation analysis in proteins, is applied herein to amide ¹⁵N relaxation in deoxy and carbonmonoxy hemoglobin. Experimental data including ¹⁵N T₁, T₂ and ¹⁵N-{¹H} NOE, acquired at 11.7 and 14.1 T, and 29 and 34 °C, are analyzed. The restricted local motion of the N-H bond is described in terms of the principal value (S(0)(2)) and orientation (β(D)) of an axial local ordering tensor, S, and the principal values (R(||)(L) and R(⊥)(L)) and orientation (β(O)) of an axial local diffusion tensor, R(L). The parameters c₀² (the potential coefficient in terms of which S(0)(2) is defined), R(||)(L), β(D), and β(O) are determined by data fitting; R(⊥)(L) is set equal to the global motional rate, R(C), found previously to be (5.2-5.8) × 10⁶ 1/s in the temperature range investigated. The principal axis of S is (nearly) parallel to the C(i-1)(α)-C(i)(α) axis; when the two axes are parallel, β(D) = -101.3° (in the frame used). The principal axis of R(L) is (nearly) parallel to the N-H bond; when the two axes are parallel, β(O) = -101.3°. For "rigid" N-H bonds located in secondary structure elements the best-fit parameters are S(0)(2) = 0.88-0.95 (corresponding to local potentials of 8.6-19.9 k(B)T), R(||)(L) = 10⁹-10¹⁰ 1/s, β(D) = -101.3° ± 2.0°, and β(O) = -101.3° ± 4°. For flexible N-H bonds located in loops the best-fit values are S(0)(2) = 0.75-0.80 (corresponding to local potentials of 4.5-5.5 k(B)T), R(||)(L) = (1.0-6.3) × 10⁸ 1/s, β(D) = -101.3° ± 4.0°, and β(O) = -101.3° ± 10°. These results are important in view of their physical clarity, inherent potential for further interpretation, consistency, and new qualitative insights provided (vide infra).

Unveiling the timescale of the R-T transition in human hemoglobin

Time-resolved wide-angle X-ray scattering, a recently developed technique allowing to probe global structural changes of proteins in solution, was used to investigate the kinetics of R-T quaternary transition in human hemoglobin and to systematically compare it to that obtained with time-resolved optical spectroscopy under nearly identical experimental conditions. Our data reveal that the main structural rearrangement associated with the R-T transition takes place approximately 2 mus after the photolysis of hemoglobin at room temperature and neutral pH. This finding suggests that the 20-mus step observed with time-resolved optical spectroscopy corresponds to a small and localized structural change.

Linking conformation change to hemoglobin activation via chain-selective time-resolved resonance Raman spectroscopy of protoheme/mesoheme hybrids

Time-resolved resonance Raman (RR) spectra are reported for hemoglobin (Hb) tetramers, in which the alpha and beta chains are selectively substituted with mesoheme. The Soret absorption band shift in mesoheme relative to protoheme permits chain-selective recording of heme RR spectra. The evolution of these spectra following HbCO photolysis shows that the geminate recombination rates and the yields are the same for the two chains, consistent with recent results on (15)N-heme isotopomer hybrids. The spectra also reveal systematic shifts in the deoxyheme nu (4) and nu (Fe-His) RR bands, which are anticorrelated. These shifts are resolved for the successive intermediates in the protein structure, which have previously been determined from time-resolved UV RR spectra. Both chains show Fe-His bond compression in the immediate photoproduct, which relaxes during the formation of the first intermediate, R(deoxy) (0.07 micros), in which the proximal F-helix is proposed to move away from the heme. Subsequently, the Fe-His bond weakens, more so for the alpha chains than for the beta chains. The weakening is gradual for the beta chains, but is abrupt for the alpha chains, coinciding with completion of the R-T quaternary transition, at 20 micros. Since the transition from fast- to slow-rebinding Hb also occurs at 20 micros, the drop in the alpha chain nu (Fe-His) supports the localization of ligation restraint to tension in the Fe-His bond, at least in the alpha chains. The mechanism is more complex in the beta chains.

Subunit-selective interrogation of CO recombination in carbonmonoxy hemoglobin by isotope-edited time-resolved resonance Raman spectroscopy

Hemoglobin (Hb) is an allosteric tetrameric protein made up of alphabeta heterodimers. The alpha and beta chains are similar, but are chemically and structurally distinct. To investigate dynamical differences between the chains, we have prepared tetramers in which the chains are isotopically distinguishable, via reconstitution with (15)N-heme. Ligand recombination and heme structural evolution, following HbCO dissociation, was monitored with chain selectivity by resonance Raman (RR) spectroscopy. For alpha but not for beta chains, the frequency of the nu(4) porphyrin breathing mode increased on the microsecond time scale. This increase is a manifestation of proximal tension in the Hb T-state, and its time course is parallel to the formation of T contacts, as determined previously by UVRR spectroscopy. Despite the localization of proximal constraint in the alpha chains, geminate recombination was found to be equally probable in the two chains, with yields of 39 +/- 2%. We discuss the possibility that this equivalence is coincidental, in the sense that it arises from the evolutionary pressure for cooperativity, or that it reflects mechanical coupling across the alphabeta interface, evidence for which has emerged from UVRR studies of site mutants.

Protein dynamics from time resolved UV Raman spectroscopy

Raman spectroscopy can provide unique information on the evolution of structure in proteins over a wide range of time scales; the picosecond to millisecond range can be accessed with pump-probe techniques. Specific parts of the molecule are interrogated by tuning the probe laser to a resonant electronic transition, including the UV transitions of aromatic residues and of the peptide bond. Advances in laser technology have enabled the characterization of transient species at an unprecedented level of structural detail. Applications to protein unfolding and allostery are reviewed.

Effector-induced structural fluctuation regulates the ligand affinity of an allosteric protein: binding of inositol hexaphosphate has distinct dynamic consequences for the T and R states of hemoglobin

The present study reports distinct dynamic consequences for the T- and R-states of human normal adult hemoglobin (Hb A) due to the binding of a heterotropic allosteric effector, inositol hexaphosphate (IHP). A nuclear magnetic resonance (NMR) technique based on modified transverse relaxation optimized spectroscopy (TROSY) has been used to investigate the effect of conformational exchange of Hb A in both deoxy and CO forms, in the absence and presence of IHP, at 14.1 and 21.1 T, and at 37 degrees C. Our results show that the majority of the polypeptide backbone amino acid residues of deoxy- and carbonmonoxy-forms of Hb A in the absence of IHP is not mobile on the micros-ms time scale, with the exception of several amino acid residues, that is, beta109Val and beta132Lys in deoxy-Hb A, and alpha40Lys in HbCO A. The mobility of alpha40Lys in HbCO A can be explained by the crystallographic data showing that the H-bond between alpha40Lys and beta146His in deoxy-Hb A is absent in HbCO A. However, the conformational exchange of beta109Val, which is located in the intradimer (alpha 1beta 1 or alpha 2beta 2) interface, is not consistent with the crystallographic observations that show rigid packing at this site. IHP binding appears to rigidify alpha40Lys in HbCO A, but does not significantly affect the flexibility of beta109Val in deoxy-Hb A. In the presence of IHP, several amino acid residues, especially those at the interdimer (alpha 1beta 2 or alpha 2beta 1) interface of HbCO A, exhibit significant conformational exchange. The affected residues include the proximal beta92His in the beta-heme pocket, as well as some other residues located in the flexible joint (betaC helix-alphaFG corner) and switch (alphaC helix-betaFG corner) regions that play an important role in the dimer-dimer rotation of Hb during the oxygenation process. These findings suggest that, upon IHP binding, HbCO A undergoes a conformational fluctuation near the R-state but biased toward the T-state, apparently along the trajectory of its allosteric transition, accompanied by structural fluctuations in the heme pocket of the beta-chain. In contrast, no significant perturbation of the dynamic features on the ms-micros time scale has been observed upon IHP binding to deoxy-Hb A. We propose that the allosteric effector-induced quaternary structural fluctuation may contribute to the reduced ligand affinity of ligated hemoglobin. Conformational exchange mapping of the beta-chain of HbCO A observed at 21.1 T shows significantly increased scatter in the chemical exchange contribution to the transverse relaxation rate ( R ex) values, relative to those at lower fields, due to the enhanced effect of the local chemical shift anisotropy (CSA) fluctuation. A spring-on-scissors model is proposed to interpret the dynamic phenomena induced by the heterotropic effector, IHP.

A quantum-chemical picture of hemoglobin affinity

Understanding the molecular mechanism of hemoglobin cooperativity remains an enduring challenge. Protein forces that control ligand affinity are not directly accessible by experiment. We demonstrate that computational quantum mechanics/molecular mechanics methods can provide reasonable values of ligand binding energies in Hb, and of their dependence on allostery. About 40% of the binding energy differences between the relaxed state and tense state quaternary structures result from strain induced in the heme and its ligands, especially in one of the pyrrole rings. The proximal histidine also contributes significantly, in particular, in the alpha-chains. The remaining energy difference resides in protein contacts, involving residues responsible for locking the quaternary changes. In the alpha-chains, the most important contacts involve the FG corner, at the "hinge" region of the alpha(1)beta(2) quaternary interface. The energy differences are spread more evenly among the beta-chain residues, suggesting greater flexibility for the beta- than for the alpha-chains along the quaternary transition. Despite this chain differentiation, the chains contribute equally to the relaxed substitute state energy difference. Thus, nature has evolved a symmetric response to the quaternary structure change, which is a requirement for maximum cooperativity, via different mechanisms for the two kinds of chains.

A comparative NMR study of the polypeptide backbone dynamics of hemoglobin in the deoxy and carbonmonoxy forms

Model-free-based NMR dynamics studies have been undertaken for polypeptide backbone amide N-H bond vectors for both the deoxy and carbonmonoxy forms of chain-specific, isotopically (15N and 2H) labeled tetrameric hemoglobin (Hb) using 15N-relaxation parameters [longitudinal relaxation rate (R1), transverse relaxation rate (R2), and heteronuclear nuclear Overhauser effect (NOE)] measured at two temperatures (29 and 34 degrees C) and two magnetic field strengths (11.7 and 14.1 T). In both deoxy and carbonmonoxy forms of human normal adult hemoglobin (Hb A), the amide N-H bonds of most amino acid residues are rigid on the fast time scale (nanosecond to picosecond), except for the loop regions and certain helix-helix connections. Although rigid in deoxy-Hb A, beta146His has been found to be free from restriction of its backbone motions in the CO form, presumably due to the rupture of its hydrogen bond/salt bridge network. We now have direct dynamics evidence for this structural transition of Hb in solution. While remarkably flexible in the deoxy state, alpha31Arg and beta123Thr, neighbors in the intradimer (alpha1beta1) interface, exhibit stiffening upon CO binding. These findings imply a role for alpha31Arg and beta123Thr in the intradimer communication but contradict the results from X-ray crystallography. We have also found that there is considerable flexibility in the intradimer (alpha1beta1) interface (i.e., B, G, and H helices and the GH corner) and possible involvement of several amino acid residues (e.g., alpha31Arg, beta3Leu, beta41Phe, beta123Thr, and beta146His) in the allosteric pathway. Several amino acid residues at the intradimer interfaces, such as beta109Val, appear to be involved in possible conformational exchange processes. The dynamic picture derived from the present study provides new insights into the traditional description of the stereochemical mechanism for the cooperative oxygenation of Hb A based on X-ray crystallographic results.

Dynamics of Allostery in Hemoglobin: Roles of the Penultimate Tyrosine H bonds

The tyrosine residues adjacent to the C termini of the hemoglobin (Hb) subunits, alphaY140 and betaY145, are expected to play important structural roles, because the C termini are the loci of T-state quaternary salt-bridges, and because the tyrosine side-chains bridge the H and F helices via H bonds to the alphaV93 and betaV98 carbonyl groups. These roles have been investigated via measurements of oxygen binding, (1)H NMR spectra, resonance Raman (RR) spectra, and time-resolved resonance Raman (TR(3)) spectra on site mutants in which the Hcdots, three dots, centeredF H bonds are eliminated by replacing the tyrosine residues with phenylalanine. The TR(3) spectra confirm the hypothesis, based on TR(3) studies of wild-type Hb, that the Hcdots, three dots, centeredF H bonds break and then re-form during the sub-microsecond phase of the R-T quaternary transition. The TR(3) spectra support the inference from other mutational studies that the alphabeta dimers act as single dynamic units in this early phase, motions of the E and F helices being coupled tightly across the dimer interface. Formation of T quaternary contacts occurs at about the same rate in the mutants as in HbA. However, these contacts are weakened substantially by the Y/F substitutions. Equilibrium perturbations are apparent also, especially for the alpha-subunits, in which relaxation of the Fe-His bond, strengthening of the Acdots, three dots, centeredE interhelical H bond, and weakening of the "switch" quaternary contact in deoxyHb are all apparent. Structural effects are less marked for the beta-chain Y/F replacement, but the Bohr effect is reduced by 25%, indicating that the salt-bridge and H bond interactions of the adjacent C terminus are loosened. The alpha-chain replacement reduces the Bohr effect much more, consistent with the global perturbations detected by the structure probes.

Tunable kHz deep ultraviolet (193-210 nm) laser for Raman application

The performance characteristics of a kilohertz solid-state laser source for ultraviolet Raman spectroscopy are described. Deep ultraviolet (UV) excitation in the 193-210 nm region is provided by mixing of the fundamental and third harmonics of a Ti-sapphire laser, which is pumped by the second harmonic of a Q-Switched Nd-YLF laser. The combination of tunability, narrow linewidth, high average power, good stability, and kilohertz repetition rate makes this laser suitable for deep UV resonance Raman applications. The short pulse duration (approximately 20 ns) permits nanosecond time resolution in pump-probe applications. The low peak power and high data rate provide artifact-free spectra with a high signal-to-noise ratio. UV Raman cross-section and Raman excitation profiles are reported for gaseous O2 (relative to N), aqueous ClO4-, tyrosine, phenylalanine, tryptophan, histidine, and hemoglobin excited between 193 nm and 210 nm to illustrate laser performance.

Hb Montfermeil [β 130(H8) Tyr→Cys]: suggests a key role for the interaction between helix A and H in oxygen affinity of the hemoglobin molecule

Hb Montfermeil [beta130(H8) Tyr-->Cys] is a high oxygen affinity variant causing erythrocytosis. The cysteine replacement is buried in the inside of the beta chain where it alters the interactions between helix A and H, with a further effect on helix E. This position has already been proposed to contribute to the difference in oxygen affinity between human and bovine hemoglobins. Three dimensional structural considerations and comparison of the functional behavior of other variants suggest that this region is an important determinant of the intrinsic oxygen affinity of the hemoglobin molecule.