Photon correlation spectroscopy of interacting and dissociating hemoglobin

Nanosecond time scale transient optoplasmonic detection of single proteins

Optical detection of individual proteins with high bandwidth holds great promise for understanding important biological processes on the nanoscale and for high-throughput fingerprinting applications. As fluorescent labels impose restrictions on detection bandwidth and require time-intensive and invasive processes, label-free optical techniques are highly desirable. Here, we read out changes in the resonantly scattered field of individual gold nanorods interferometrically and use photothermal spectroscopy to optimize the experiment’s parameters. This interferometric plasmonic scattering enables the observation of single proteins as they traverse plasmonic near fields of gold nanorods with unprecedented temporal resolution in the nanosecond-to-microsecond range.

Free-solution, label-free protein-protein interactions characterized by dynamic light scattering

We report a free-solution, label-free method for quantitative characterization of macromolecular interactions using dynamic light scattering, a temperature controlled plate reader, and a multiwell concentration gradient. This nondestructive technique enabled determination of stoichiometry of binding, equilibrium dissociation constant, and thermodynamic parameters, as well as the impact of temperature, buffer salinity, and a small-molecule inhibitor. The low volume capability of dynamic light scattering reduced the required sample to 426 pmol/experiment, with detection limits for 150-kDa proteins anticipated to be in the low femtomole range.

Free heme in micromolar amounts enhances the attraction between sickle cell hemoglobin molecules

We probe the role of free heme in the interactions between sickle cell hemoglobin (HbS) molecules in simulated physiological solutions: polymerization of deoxy-HbS is the primary pathogenic event of sickle cell anemia, and HbS releases heme after autoxidation more readily than normal adult hemoglobin. We characterize these interactions in terms of osmotic virial coefficients, which we determine by static light scattering. We analyze the results in the heme-hemoglobin system using the Kirkwood-Goldberg model. We show that in the absence of heme, the HbS molecules weakly attract and the attraction is not due to the lowered-as a result of the sickle cell mutation-molecular charge. We show that the part of the interface between the two alphabeta dimers, exposed in the deoxy-state, plays a crucial role in this attraction. We show that heme at micromolar concentrations induces strong attraction between the hemoglobin molecules. We show that the high efficacy of the heme results from the statistics of electrostatic and hydrophobic interactions between the heme and hemoglobin molecules.

Collective diffusion coefficient of proteins with hydrodynamic, electrostatic, and adhesive interactions

A theory is presented for lambdaC, the coefficient of the first-order correction in the density of the collective diffusion coefficient, for protein spheres interacting by electrostatic and adhesive forces. An extensive numerical analysis of the Stokesian hydrodynamics of two moving spheres is given so as to gauge the precise impact of lubrication forces. An effective stickiness is introduced and a simple formula for lambdaC in terms of this variable is put forward. A precise though more elaborate approximation for lambdaC is also developed. These and numerically exact expressions for lambdaC are compared with experimental data on lysozyme at pH 4.5 and a range of ionic strengths between 0.05M and 2M.

Alprazolam induced conformational change in hemoglobin

Alprazolam (ALP) is a widely prescribed sedative and antidepressant benzodiazepine group of drugs. The wide uses of this drug lead us to investigate its possible interaction with hemoglobin (Hb). Spectrophotometric and spectofluorimetric studies showed strong binding of ALP with Hb. Circular dichroic spectra showed that alpha-helical structure of Hb-subunits has been largely changed. On ALP treatment partial pressure of O(2) is increased in the blood indicating release of O(2) from erythrocytes. Further, the binding of ALP-induced conformational changes in Hb resulting in larger Hb particle size was demonstrated by dynamic light scattering experiment. Thus, the present study unambiguously raises question of danger of random usage of ALP, which binds with and changes the function of Hb.

Metastable mesoscopic clusters in solutions of sickle-cell hemoglobin

Sickle cell hemoglobin (HbS) is a mutant, whose polymerization while in deoxy state in the venous circulation underlies the debilitating sickle cell anemia. It has been suggested that the nucleation of the HbS polymers occurs within clusters of dense liquid, existing in HbS solutions. We use dynamic light scattering with solutions of deoxy-HbS, and, for comparison, of oxy-HbS and oxy-normal adult hemoglobin, HbA. We show that solutions of all three Hb variants contain clusters of dense liquid, several hundred nanometers in size, which are metastable with respect to the Hb solutions. The clusters form within a few seconds after solution preparation and their sizes and numbers remain relatively steady for up to 3 h. The lower bound of the cluster lifetime is 15 ms. The clusters exist in broad temperature and Hb concentration ranges, and occupy 10(-5)-10(-2) of the solution volume. The results on the cluster properties can serve as test data for a potential future microscopic theory of cluster stability and kinetics. More importantly, if the clusters are a part of the nucleation mechanism of HbS polymers, the rate of HbS polymerization can be controlled by varying the cluster properties.

The Studies of Density, Apparent Molar Volume, and Viscosity of Bovine Serum Albumin, Egg Albumin, and Lysozyme in Aqueous and RbI, CsI, and DTAB Aqueous Solutions at 303.15 K

Density (rho), apparent molar volume (V(phi)), and viscosity (eta) of 0.0010 to 0.0018% (w/v) of bovine serum albumin (BSA), egg albumin, and lysozyme in 0.0002, 0.0004, and 0.0008 M aqueous RbI and CsI, and (dodecyl)(trimethyl)ammonium bromide (DTAB) solutions were obtained. The experimental data were regressed against composition, and constants are used to elucidate the conformational changes in protein molecules. With salt concentration, the density of proteins is found to decrease, and the order of the effect of additives on density is observed as CsI > RbI > DTAB. The trend of apparent molar volume of proteins is found as BSA > egg-albumin > lysozyme for three additives. In general, eta values of BSA remain higher for all compositions of RbI than that of egg-albumin for CsI and DTAB. These orders of the data indicate the strength of intermolecular forces between proteins and salts, and are helpful for understanding the denaturation of proteins.

Aggregation properties of a HPMA-camptothecin copolymer in isotonic solutions

Copolymers of camptothecin (CPT) and [N-(2-hydroxypropyl) methacrylamide] (HPMA) are novel anticancer drugs that show improved pharmacological profile in animal models as compared to the free drug CPT. We investigate here the aggregation properties of a HPMA-glycyl-6-aminohexanoyl-glycyl-CPT copolymer ( approximately 20,000 Da). The molecular size of HPMA-copolymer CPT is followed over 5 orders of magnitudes of concentration in isotonic buffer by measuring either the time resolved fluorescence anisotropy (FA) of CPT or the autocorrelation function of the light scattered by the copolymer. A detailed analysis of these data suggests the presence of elongated structures with axial ratio approximately 3 in the range 0.1-0.5 microg/ml and aggregates with association number higher than 2 in more concentrated solutions (up to 10 mg/ml). The binding affinity of HPMA-copolymer CPT for serum albumin is inversely dependent on the degree of aggregation of the copolymer. We also show that the copolymer concentration in plasma from mice treated with an active, non-toxic, dose of HPMA-copolymer CPT, decreases from 3 to 0.01 mg/ml in 72 h. In the same range of concentrations in vitro, we do not detect hydrophobic aggregates of polymers with high (>3) association number. Our study indicates that the circulating HPMA-copolymer CPT in mice should not undergo extensive aggregation and should interact with serum albumin more weakly than free CPT.

Intermolecular interactions, nucleation, and thermodynamics of crystallization of hemoglobin C

The mutated hemoglobin HbC (beta 6 Glu-->Lys), in the oxygenated (R) liganded state, forms crystals inside red blood cells of patients with CC and SC diseases. Static and dynamic light scattering characterization of the interactions between the R-state (CO) HbC, HbA, and HbS molecules in low-ionic-strength solutions showed that electrostatics is unimportant and that the interactions are dominated by the specific binding of solutions' ions to the proteins. Microscopic observations and determinations of the nucleation statistics showed that the crystals of HbC nucleate and grow by the attachment of native molecules from the solution and that concurrent amorphous phases, spherulites, and microfibers are not building blocks for the crystal. Using a novel miniaturized light-scintillation technique, we quantified a strong retrograde solubility dependence on temperature. Thermodynamic analyses of HbC crystallization yielded a high positive enthalpy of 155 kJ mol(-1), i.e., the specific interactions favor HbC molecules in the solute state. Then, HbC crystallization is only possible because of the huge entropy gain of 610 J mol(-1) K(-1), likely stemming from the release of up to 10 water molecules per protein intermolecular contact-hydrophobic interaction. Thus, the higher crystallization propensity of R-state HbC is attributable to increased hydrophobicity resulting from the conformational changes that accompany the HbC beta 6 mutation.

Molecular engineering of a polymer of tetrameric hemoglobins

We have engineered a recombinant mutant human hemoglobin, Hb Prisca beta(S9C+C93A+C112G), which assembles in a polymeric form. The polymerization is obtained through the formation of intermolecular S-S bonds between cysteine residues introduced at position beta9, on the model of Hb Porto Alegre (beta9Ser --> Cys) (Bonaventura and Riggs, Science 1967;155:800-802). Cbeta93 and Cbeta112 were replaced in order to prevent formation of spurious S&bond;S bonds during the expression, assembly, and polymerization events. Dynamic light scattering measurements indicate that the final polymerization product is mainly formed by 6 to 8 tetrameric hemoglobin molecules. The sample polydispersity Q = 0.07 +/- 0.02, is similar to that of purified human hemoglobin (Q = 0.02 +/- 0.02), consistent with a good degree of homogeneity. In the presence of strong reducing agents, the polymer reverts to its tetrameric form. During the depolymerization process, a direct correlation is observed between the hydrodynamic radius and the light scattering of the system, which, in turn, is proportional to the mass of the protein. We interpret this to indicate that the hemoglobin molecules are tightly packed in the polymer with no empty spaces. The tight packing of the hemoglobin molecules suggests that the polymer has a globular shape and, thus, allows estimation of its radius. An illustration of an arrangement of a finite number of tetrameric hemoglobin molecules is presented. The conformational and functional characteristics of this polymer, such as heme pocket conformation, stability to denaturation, autoxidation rate, oxygen affinity, and cooperativity, remain similar to those of tetrameric human hemoglobin.

Photoinduced oxygen dynamics in lyophilized hemoglobin

Reversible laser induced deoxygenation in the lyophilized phase of hemoglobin is demonstrated by means of resonant Raman scattering, luminescence, and optical transmission. Specific Raman modes, which are both sensitive to the spin states of Fe(II) in the hemes and resonant in the visible, are monitored as a function of time to evaluate the effect of the illuminating laser. These modes act as in-situ markers of the oxygen content of the protein. The reversible photoinduced deoxygenation can be observed through both the Raman spin-markers and the optical transmission experiments. In the former, reversible changes in the intensities of specific Raman modes are observed, while in the latter, the oscillator strength of the two main absorptions of oxyhemoglobin in the visible are seen to vary accordingly. The luminescence in lyophilized hemoglobin is found to have at least two different contributions, (i) a resonant component with the Raman modes and; (ii) a nonresonant contribution, which increases at high input laser powers and eventually masks the Raman signals. The nonresonant contribution is the luminescence of the photoproduct achieved by thermal denaturation of the protein and remains standing as a permanent nonreversible damage in the illuminated spot. Semiempirical electronic calculations of the wavefunction and total energy of the iron porphyrin reveal the underlying physical origin of the laser induced deoxygenation process in the hemes and are also presented.

Polyion character of globular proteins detected by translational and rotational diffusion

Photon correlation spectroscopy measurements are performed on a solution of lysozyme, a small, highly charged protein at pH=4, in glycerol-buffer mixtures. From the correlation functions of the polarized and depolarized scattered intensity, we derive an effective protein radius by inverting the Stokes Einstein relation for either the translational or the rotational diffusion, as a function of the protein concentration 0< or =C< or =40 g/L. The translational radius decreases with the protein concentration and extrapolates at C approximately 0 to a value slightly larger than the mean rotational radius, which is independent of the protein concentration. By employing recent calculations on electrolyte friction effects (eletrolyte-protein interactions) on both the translational and rotational radius and by keeping also into account the protein-protein electrostatic interactions, we are able to account for the observed differences, suggesting also that lysozyme is more highly hydrated in glycerol mixtures than in the pure buffer. These results indicate that depolarized photon correlation spectroscopy measurements can be used as a valuable tool to detect small changes in the overall protein size.

Photon cross-correlation spectroscopy to 10-ns resolution

We describe a low-cost modification based on the research of Arecchi et al. [Opt. Commun. 3, 284-288 (1971)] on a standard photon correlation spectroscopy setup for measuring small (approximately equal 1%) and fast (resolution, approximately equal 10 ns) light fluctuations. Details of the electronics and optics permit the apparatus to be easily reproduced. Careful tests show that spurious correlations of <or=1% can be achieved with standard detectors only when taking precautions with the electronics. Moreover, measurements on latex spheres, ellipsoids, and protein solutions show that fast and very small light fluctuations ( approximately equal 1-5%) can be studied versus scattering vector and light polarization.