The confinement of liposomes and Chinese hamster ovary (CHO) cells by infrared (IR) optical tweezers is shown to result in sample heating and temperature increases by several degrees centigrade, as measured by a noninvasive, spatially resolved fluorescence detection technique. For micron-sized spherical liposome vesicles having bilayer membranes composed of the phospholipid 1,2-diacyl-pentadecanoyl-glycero-phosphocholine (15-OPC), a temperature rise of approximately 1.45 +/- 0.15 degrees C/100 mW is observed when the vesicles are held stationary with a 1.064 microns optical tweezers having a power density of approximately 10(7) W/cm2 and a focused spot size of approximately 0.8 micron. The increase in sample temperature is found to scale linearly with applied optical power in the 40 to 250 mW range. Under the same trapping conditions, CHO cells exhibit an average temperature rise of nearly 1.15 +/- 0.25 degrees C/100 mW. The extent of cell heating induced by infrared tweezers confinement can be described by a heat conduction model that accounts for the absorption of infrared (IR) laser radiation in the aqueous cell core and membrane regions, respectively. The observed results are relevant to the assessment of the noninvasive nature of infrared trapping beams in micromanipulation applications and cell physiological studies.

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constants obtained from that work are in general higher than the previous shock tube data, which are all in the higher temperature range. However, the differences all give rate constants that differ by no more than about 10% in any temperature range. Conclusionsto directly measure the rate constant for the reaction at temperatures up to 1550 K. The results are compared to the previous data, including shock tube data obtained for the reverse reaction and were found to give excellent agreement. The rate constant expression gives rate constants that are within 10% of accurately measured values, which we feel is the accuracy of the expression within the temperature range of the measurements. We suggest that this expression be adopted as the preferred rate constant for reaction 1, as has already been done in some cases.I8We have studied the solvent dependence of the steady-state absorption, emission, and time-resolved emission of 7-azaindole (7AI) and two nonreactive analogues, N-methyl-7-amindole (NMAI) and 7-methyl-7H-pyrrolo[2,3-b]pyridine (7MPP), in an effort to understand the apparently anomalous behavior of 7AI in water. We find that 7AI undergoes the same tautomerization reaction via solventcatalyzed double proton transfer in water as it does in alcohol solvents. Kinetic modeling shows that the unusual features of the 7AI emission in water arise mainly from quantitative changes in two key rate parameters.In water the rate constant for tautomerization (H20 1.2 X lo9 s-I and DzO 0.35 X lo9 s-' at 24 "C) is much slower and simultaneously the nonradiative decay rate of the product is much faster (5 X lo9 s-' in both H20 and DzO) than in most alcohols, making observation of the reaction difficult. The reason the reaction rate in water is unusually slow appears to result from differences in the hydrogen bonding structure and dynamics of water compared to monoalcohols.

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The fluorescent membrane probes 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) and 6-dodecanoyl-2-dimethylamino-naphthalene (laurdan) have been studied for use as optical thermometers in living cells. The thermal sensitivity of NBD is primarily a consequence of rapid, heat-induced electronic changes, which increase the observed fluorescence decay rate. As a result, fluorescence intensity and lifetime variations of membrane-bound NBD-conjugated phospholipids and fatty acids can be directly correlated with cellular temperature. In contrast, laurdan fluorescence undergoes a dramatic temperature-dependent Stokes shift as the membrane undergoes a gel-to-liquid-crystalline phase transition. This facilitates the use of fluorescence spectra to record the indirect effect of microenvironmental changes, which occur during bilayer heating. Microscope and suspension measurements of cells and phospholipid vesicles are compared for both probes using steady-state and fluorescence lifetime (suspension only) data. Our results show that NBD fluorescence lifetime recordings can provide reasonable temperature resolution (approximately 2 degrees C) over a broad temperature range. Laurdan's microenvironmental sensitivity permits better temperature resolution (0.1-1 degree C) at the expense of a more limited dynamic range that is determined solely by bilayer properties. The temperature sensitivity of NBD is based on rapid intramolecular rotations and vibrations, while laurdan relies on a slower, multistep mechanism involving bilayer rearrangement, water penetration and intermolecular processes. Because of these differences in time scale, NBD appears to be more suitable for monitoring ultrafast phenomena, such as the impact of short-pulse microirradiation on single cells.