Biomedical subjects

W D Tian

Publications and source records attributed to W D Tian.

6 recordsLinked to original sources

PubMed2001

Aging and effects of ultraviolet A exposure may be quantified by fluorescence excitation spectroscopy in vivo.

The fluorescence properties of skin chromophores such as tryptophan and collagen cross-links might be useful markers of aging and photoaging. As the fluorescence of pepsin-digestible collagen cross-links was found to increase with aging and decrease with photoaging we investigated the characteristics of this dependence. In vivo fluorescence excitation spectra (emission at 380 nm) of SKH hairless mouse model skin are characterized by two bands centered near 295 nm and 335 nm due, respectively, to epidermal tryptophan moieties and pepsin-digestible collagen cross-links. Several groups of hairless mice were followed over a period of 18 mo to document changes in skin fluorescence with aging. Other groups of animals were exposed to either broad band or narrowband ultraviolet A radiation to determine the effects of ultraviolet A exposure on the fluorescence of the dermal collagen cross-links and to determine an action spectrum for the induced changes. We also found that the intensity of pepsin-digestible collagen cross-links in vivo increases linearly with age and that the fluorescence of epidermal tryptophan decreases linearly with age. We found that the fluorescence of pepsin-digestible collagen cross-links decreases immediately following exposure to ultraviolet A whereas epidermal tryptophan fluorescence increases. Both changes were dose dependent but the increase in tryptophan fluorescence occurred exclusively in young animals (2--6 mo old). We found that the ultraviolet-induced fluorescence decrease of pepsin-digestible collagen cross-links is wavelength specific. The action spectrum for the ultraviolet A effect on the in vivo fluorescence of pepsin-digestible collagen cross-links shows a distinct maximum at 335 nm that corresponds to the maximum in the fluorescence excitation spectrum due to pepsin-digestible collagen cross-links. Our results seem to indicate that in vivo fluorescence of epidermal tryptophan moieties and collagen cross-links in the dermal matrix may serve as markers for skin aging, for photoaging, and for immediate assessment of exposure to ultraviolet A radiation.

Aging↗

PubMed2001

Analysis of ingredient and heating value of municipal solid waste.

Great differences between municipal solid wastes (MSW) produced at different places and different times in terms of such parameters as physical ingredient and heating value lead to difficulty in effective handling of MSW. In this paper, ingredient, heating value and their temporal varying trends of typical MSW in Beijing were continuously measured and analyzed. With consideration of the process in pyrolysis and incineration, correlation between physical ingredients and heating values was induced, favorable for evaluation of heating value needed in handling of MSW from simple analysis of physical ingredients of it.

China↗

PubMed2001

Internal circulating fluidized bed incineration system and design algorithm.

The internal circulating fluidized bed (ICFB) system is characterized with fast combustion, low emission, uniformity of bed temperature and controllability of combustion process. It is a kind of novel clean combustion system, especially for the low-grade fuels, such as municipal solid waste (MSW). The experimental systems of ICFB with and without combustion were designed and set up in this paper. A series of experiments were carried out for further understanding combustion process and characteristics of several design parameters for MSW. Based on the results, a design routine for the ICFB system was suggested for the calculation of energy balance, airflow rate, heat transfer rate, and geometry arrangement. A test system with ICFB combustor has been set up and the test results show that the design of the ICFB system is successful.

Air Pollution↗

PubMed1996

Probing heme protein conformational equilibration rates with kinetic selection.

Double-pulse flash photolysis experiments on solutions of carbonmonoxymyoglobin (MbCO) are used to determine the time scale for protein conformational averaging. The interconversion times for transitions between the "open" and "closed" subpopulations of MbCO are found to be 10(-6)-10(-4)s, depending on solvent composition and temperature. In aqueous solution at 273 K, the interconversion rate is found to be 1.4 x 10(6)s. Since the interconversion rate is comparable to or slower than the geminate rebinding rate, we describe the geminate phase of the kinetics as a superposition of contributions from the open and closed states. Although geminate kinetics remain intrinsically nonexponential for both open and closed states near room temperature, we find that substates within these two subpopulations interconvert more rapidly than the geminate rebinding. These observations cannot be explained by a superposition of contributions from a quasicontinuous conformational distribution (Steinbach et al., 1991) and are probably due to the long-time tail of the relaxation of the protein (Tian et al., 1992). Bimolecular rebinding takes place at a statistically averaged rate, since the interconversion and relaxation rates are faster than the bimolecular kinetics. The geminate and bimolecular kinetics are analyzed quantitatively as a function of pH using this approach and the spectroscopically determined populations of the open and closed states. The analysis accounts for the observed kinetics and also successfully predicts the kinetic response observed in the double-pulse experiments. In aqueous solution at 273 K, the geminate amplitudes and rates are found to be I(0)g = 32% and k(0)g = 1.3 x 10(7)s(-1) for the open state and I(1)g = 9.3% and k(1)g = 1.4 x 10(6)s(-1) for the closed state. In 75% glycerol solution at 264 K, the dominant component of the geminate rebinding is characterized by I(0)g1 = 89% and k(0)g1 = 3.1 x 10(6)s(-1) for the open state and I(1)g1 = 26% and k(1)g1 = 3.1 x 10(6)s(-1) for the closed state. The fact that the interconversion rate is comparable to the geminate rate of the closed state in aqueous solution is consistent with the idea that the open state provides an important pathway for ligand escape from (or entry to) the heme pocket (Tian et al., 1993). The increased viscosity of 75% glycerol solution delays the closed--> open interconversion until the end of the geminate phase, which forces the ligand to find alternative pathways to the solution. This observation, in conjunction with the near equivalence of the geminate rates for the open and closed states in 75% glycerol solution, suggests that the solvent composition fundamentally alters the protein-ligand dynamics.

Animals↗

PubMed1995

Measurements of CO geminate recombination in cytochromes P450 and P420.

The kinetics of CO geminate recombination in cytochrome P450cam are studied at room temperature subsequent to laser photolysis. The geminate rebinding kinetics of P450 are strongly affected by the presence of the camphor substrate. We observe a approximately 2% geminate yield for substrate-bound P450 and a 90% geminate yield when the substrate is absent. The drastic difference in the geminate kinetics suggests that the presence of camphor significantly alters the CO rebinding and escape rates by modifying the heme pocket environment. Two geminate phases and two bimolecular rebinding phases in the substrate free protein were observed, which could arise from slowly interconverting protein conformations. When the temperature or the viscosity of the solution is changed, the fast geminate rate remains the same, whereas the slow geminate rate and the two bimolecular rates change significantly. The geminate rebinding yield of substrate-free P420 is smaller than that of substrate free P450, but its geminate rebinding rate is faster. This demonstrates that in the absence of substrate, CO escapes from the pocket of P420 much more rapidly than from P450 and suggests that the distal pocket environment is altered in the P420 form.

Carbon Monoxide↗

PubMed1993

Investigations of ligand association and dissociation rates in the "open" and "closed" states of myoglobin.

Kinetic and Raman spectroscopic studies are combined to analyze ligand association and dissociation rates as a function of pH in aqueous solutions of myoglobin. A double-pulse flash photolysis protocol is used to kinetically select a rapidly rebinding (open pocket) fraction of the myoglobin ensemble and determine the timescale for averaging (approximately 1 to 10 microseconds) between the "open" and "closed" distal pocket protein conformations. Since this timescale is fast compared to the rate of ligand migration from the solution to the heme pocket (approximately 10(-4)s), a time-averaged population analysis, rather than a superposition of states, can be used to describe the ligand association and dissociation kinetics. Raman spectroscopy provides the relative populations of the open and closed distal pocket states as a function of pH which, in parallel with kinetics measurements, are used to determine the rates for ligand association and dissociation specific to these states. In aqueous solution at 293 K (1 mM CO) we find kon0 = 5.6 x 10(3) s-1, koff0 = 8.5 x 10(-2) s-1 for the open state and kon1 = 5.0 x 10(2) s-1, koff1 = 1.3 x 10(-2) s-1 for the closed state. The order of magnitude increase in the dissociation and association rates of the open form suggests that it may play a significant role in the ligand binding process, even though it comprises only approximately 5% of the time-averaged population at pH 7. For oxygen binding at 293 K (1.36 mM O2) we find kon0 = 4.6 x 10(4) s-1, koff0 approximately 10(4 +/- 2) s-1 for the open state and kon1 = 2.0 x 10(4) s-1, koff1 = 13 s-1 for the closed state. The dramatic increase in the dissociation rate of the open form is probably due to the loss of the hydrogen bond with the distal histidine, which stabilizes the bound O2 in the closed state. Overall, these results demonstrate that the open conformation plays a significant role in determining the ligand association and dissociation rates and suggest that environmentally induced modulations of the open population could be used as a biomolecular control mechanism for the uptake and delivery of oxygen in muscle cells.

Carbon Monoxide↗