PubMed · 5934495
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D J Frager. 1966. A constant-temperature microbath.. https://doi.org/10.1152/jappl.1966.21.2.747
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The UV-induced dissociation of lipid hydroperoxides, naturally present in methyl linoleate and methyl linoelaidate, was monitored by absorption spectroscopy. The decay of hydroperoxides in homogeneous solution, for a fluence rate of 0.82 mW.cm-2 at 240 nm, followed an exponential course with an average rate constant of k = (2.0 +/- 0.1) . 10(-3) s-1. Addition of alpha-tocopherol led to an increase in the magnitude of k until it reached a limiting value of (3.4 +/- 0.2) . 10(-3)s-1. When methyl linoleate (containing adventitous traces of hydroperoxides) was incorporated in SDS micelles, sustained UV irradiation caused initially an increase in the amounts of lipid hydroperoxides, due to linoleate autoxidation, followed by an exponential decrease due to photodissociation. From the initial rate of hydroperoxide formation (due to linoleate autoxidation) and the rate constant of the subsequent hydroperoxide decay, an oxidizibility of 0.61 . 10(-2) (Ms)-1/2 of methyl linoleate in micelles was deduced. It is suggested that UV irradiation and optical absorption afford an easy and reliable means for studying autoxidation of polyunsaturated fatty acids.
BACKGROUND AND OBJECTIVE: Flashlamp pumped dye (FPDL), Q-switched Nd:YAG, and alexandrite lasers are the most clinically used laser lithotriptors. Although calculi are fragmented by laser induced mechanical stresses for all lithotriptors, different fragment sizes and fragmentation efficiencies have been reported. In this work the effect of the pulse duration and pulse shape on the fragmentation processes is studied. MATERIAL AND METHODS: Fragmentation processes are characterized on model stones and on sensing target fibers. Stone fragmentation and cavitation bubble generation are observed by video flash photography. Shock wave occurrence and strength are monitored with an hydrophone. RESULTS: For the FPDL, stone fragmentation is induced by the collapse of the large cavitation bubble formed. For the Q-switched Nd:YAG, fragmentation is already observed during the laser pulse, at the plasma onset, although further fragmentation can occur at the bubble collapse. For pulse durations corresponding to the alexandrite, an intermediate fragmentation regime is observed. CONCLUSION: For the first time the physical basis of the observed differences in the fragmentation efficiencies of current laser lithotriptors is described. For nanosecond durations the fragmentation processes are governed by plasma induced shock waves. On the contrary, for microsecond durations fragmentation is governed by cavitation. The high fragmentation efficiency of microsecond lasers is due to a high laser energy transfer into cavitation.
Brönsted-Bjerrum theory [Brönsted (1922) Z. Phys. Chem. 102, 169-207; (1925) Z. Phys. Chem. 115, 337-364; Bjerrum (1924) Z. Phys. Chem. 108, 82-100] as applied to reactions at interfaces is used to interpret published data on the lipolysis of dinonanoyl phosphatidylcholine monolayers by pancreatic phospholipase A2. Reasonable quantitative agreement between theoretical and experimental results occurs when the reported effects of surface pressure on the amount of adsorbed enzyme are used together with the assumption that the Langmuir-Syskowski [Lange (1967) Nonionic Surfactants (Schick, M., ed.), chap. 4, Marcel Dekker, London and New York] isotherm describes the adsorption of lipid. The equations of Mass Action kinetics are less successful. Equivalent data on the lipolysis of didodecanoyl phosphatidylglycerol by pancreatic lipase can also be interpreted by arguing that the adsorbed enzyme forms significant amounts of enzyme substrate complex which reacts to give products in accordance with the well-known Briggs-Haldane [Lehninger (1975) Biochemistry, 2nd edn., chap. 8, Worth, New York] mechanism. The positions of the observed maxima in rate versus surface pressure plots suggests that the adsorption of an enzyme molecule displaces less lipid than one expects from the pronounced differences in molecular sizes of the two adsorbed species.