Angiotensin I is known to undergo a reversible conformational transition and a change in potency in rat uterus in vitro with pK -6.5. We have shown by carbon-13 NMR that the conformational transition involves all-trans to partly cis isomerization of the His6-Pro7 peptide bond. Isomerization from all-trans at pH 6.8 to "16% cis at pH 8.0 is therefore correlated with a 10-fold increase in biological activity for [Asp',IJeWJ angiotensin II in rat uterus in vitro. Isomerization from all-trans at pH 6.8 to "16% cis at pH 8.0 in the competitive inhibitor [Phe4,Tyr8Jangiotensin II is correlated with exhibition of virtually no agonist activity at low pH to full agonist activity at high pH. An angiotensin II conformation with Pro7 in the cis form may therefore be the conformation with maximal binding or biological activity at the cellular receptor.Our understanding of the physical and chemical basis for the biological activity of angiotensin II has progressed along two largely independent pathways. The investigation of the biological activity of a large number of analogs (summarized in refs. 1-3) has clarified the role that various residues or groups of residues play in that activity. The investigation of the solution conformation of angiotensin II and of various analogs has made use of most biophysical methods available (summarized in refs. 4 and 5), and several models for the solution conformation have been proposed (5-9). Although most investigators agree that a compact and more or less rigid conformation does exist, no agreement on a definite model has been reached. We suggest that the biological potency of "native" angiotensin II § and of [Phe4,Tyr8]angiotensin II as a function of pH may be related to the amount of cis configuration of Pro7 present in solution. The [Phe4,Tyr8] analog of angiotensin II has been observed to be an antagonist at low pH and an agonist above pH 7 (11).The present study is based on our earlier observations (10) that at low pH in aqueous solution angiotensin II is present in a conformation having proline in the trans configuration and above pH 6.5 two conformations are observed by nuclear magnetic resonance (NMR), an abundant conformation (88%) with proline in the trans configuration and a minor conformation (12%) with proline in the cis configuration. The evidence for this conclusion was that the carbon-13 NMR spectrum at and above neutral pH of the COOH-terminal hexapeptide fragment of [Val5]angiotensin II showed a resonance of the required intensity at the position expected for proline C-'y when proline is in the cis configuration. The pH dependence of the conformation of angiotensin observed by NMR is well documented (4,5,(12)(13)(14) and the possible relationship of this conformational change with pK -6.5 to myotropic activity has been discussed (5, 13). We show here by '3C NMR that both [Asp1,Tyr8]angiotensin II isomerize from alltrans at acidic p2H to partly cis at alkaline p2H and that this isomerization is reported by the His6 C-2 (and C-4) protons. Other conformational...
Abstract. A method combining the techniques of microwave-assisted process (MAP) and gas chromatography (GC) has been developed for the determination of volatile organic compounds in water. For comparison purposes, MAP and conventional gas-phase extractions using static headspace methods were performed on laboratory-spiked water samples. The analytes used as models in this preliminary investigation were benzene, toluene, ethylbenzene, chlorobenzene, 0-, m-, and p-xylenes, and 1,2-, 1,3-, and 1,4-dichlorobenzenes. MAP uses microwaves to very rapidly heat the bulk of the matrix and achieve a selective vaporization of the analytes that are then transferred onto the gas chromatographic apparatus via identical sampling conditions. The method was tested over a 10-5000 ppb range and proved capable of yielding excellent linearity (R2 > 0.99999) and precision (RSD 1.4%). Compared to the conventional static headspace extraction method, the MAP extraction method offered enhanced sensitivity as evidenced by a response in signal greater by 35% for aliquots from the same aqueous solutions. This level of performance is achieved within one minute of overall sample preparation time for the MAP method, whereas the conventional static headspace method used for comparison purposes required 30 minutes of equilibration time. Similar performance was observed with actual samples. Key words: microwave-assisted process, MAP, gas-phase extraction, headspace analysis, VOCs, water SAFETY CONSIDERATIONSThe microwave-assisted process is a simple technology that can be readily understood in terms of the operating steps to be performed. However, the application of microwave energy to various matrices in closed vessels, such as headspace vials, can pose serious hazards in inexperienced hands. The chemical and physical principles underlying the technology are deceptively simple and a wide range of temperature and pressure conditions can be produced according to the nature of the matrix being used. Hence, an extraordinary level of safety and attention to details when planning and performing experiments must be used by all personnel dealing with microwaves. The authors urge all readers to ensure that they seek proper information from knowledgeable sources and that they do not attempt to implement these techniques unless proper guidance is provided and only approved equipment and scientifically sound procedures are used at all times. The feeling of simplicity associated with microwaves can enhance the level of hazard and exposure to accidents if procedures are
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