As geologically relevant models of prebiotic environments, systems consisting of clay, water, and amino acids were subjected to cyclic variations in temperature and water content. Fluctuations of both variables produced longer oligopeptides in higher yields than were produced by temperature fluctuations alone. The results suggest that fluctuating environments provided a favorable geological setting in which the rate and extent of chemical evolution would have been determined by the number and frequency of cycles.

Replacement of a carboxyl function by fluorine, fluorodecarboxylation, is a new process that can be accomplished by the reaction of alkanoic acids with xenon difluoride. Primary, tertiary, and benzylic acids perform best in the reaction, which is conducted at room temperature in methylene chloride or chloroform solution. A reaction mechanism is proposed in which the acid is initially converted to a fluoroxenon ester, RCO2XeF. The esters of the primary and secondary acids react by nucleophilic displacement by fluoride, as evidenced by incorporation of 18F− and no reactions common to free radicals or carbocations. The esters of the tertiary and benzylic acids react by converting to free radicals that can be further oxidized to carbocations. Thus incorporation of 18F− and racemization are observed with α-methoxy-α-trifluoromethylphenylacetic acid. Hydroxyl and amino functions inhibit the reaction. Aromatic and vinylic acids do not react.

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A model is proposed for the intermediate stages of prebiotic evolution, based on the characteristics of the adsorption and condensation of amino acids and nucleotides on the surface area of clay minerals in a fluctuating environment. Template replication and translation of adsorbed oligonucleotides and catalytic effects by peptide products on further condensation are proposed, due to specific properties of hypohydrous clay surfaces as well as the biomolecules themselves. Experimental evidence supports some of the proposed interactions, and all of them can be tested experimentally.

SUMMARY We used M-mode echocardiography to measure left ventricular dimensions in diastole (Dd) and systole (Ds) and to assess ventricular performance by computing the percent dimensional shortening (%AD) and the normalized rate of dimensional shortening (Vd) during isometric and isotonic exercise in normal subjects. In 27 subjects, isometric handgrip exercise at 50% of maximum grip until fatigue produced a significant increase in Ds (33 ± 3.4 (SD) vs 30.6 i 3.7 mm, p < 0.001), and a reduction in %AD (34 ± 4 vs 39 ± 5%,p < 0.001) and Vd (1.15 ± 0.15 vs 1.28 0.19 sec-',p < 0.001). Handgrip exercise at 15% of maximum grip produced similar but less marked changes in the 27 subjects, and acute pressure loading with phenylephrine caused similar but more marked changes in 10 of the subjects. In the 20 subjects who performed at least 12 minutes of supine bicycle exercise, Ds decreased significantly (25.6 ± 4.0 vs 31.7 ± 2.8 mm, p < 0.001) and %AD increased (49 ± 6 vs 36 ± 5%,p < 0.001). We observed similar results in the 12 subjects also studied during upright bicycle exercise. Dd was smaller in the upright position but unchanged during either isometric or isotonic exercise. We conclude that: 1) end-diastolic left ventricular size is maintained during isometric exercise and moderate dynamic exercise, even in the upright position; 2) isometric exercise leads to a mild decrease in left ventricular shortening, whereas dynamic exercise results in marked increases in shortening; this difference may be related to the relatively greater increase in blood pressure than in heart rate during isometric exercise; and 3) M-mode echocardiography can be successfully accomplished in selected subjects during various forms of exercise. M-MODE ECHOCARDIOGRAPHY is an accurateand reproducible technique for assessing resting left ventricular size and performance in normal-sized hearts without segmental myocardial disease.'14 The sensitivity of this technique for detecting changes in left ventricular dynamics has been shown in studies evaluating the response to upright tilting,', the Valsalva maneuver,7 8 ventricular premature depolarizations,9 normal phasic respiration'0 and acute pharmacologic interventions with agents such as amyl nitrite, nitroglycerin, atropine and phenylephrine. 1" 12 One advantage of echocardiography is the ability to measure left ventricular diameters intermittently or continuously before, during and after an intervention without risk or discomfort to the subject and without affecting the normal response to the test conditions. Accordingly, we used echocardiography to measure left ventricular size and performance in normal subjects during isometric handgrip exercise and dynamic bicycle exercise in the supine and upright positions. MethodsThe study population consisted of 27 normal subjects who were selected from a group of twice as many because of excellent echocardiograms in the supine position at rest. There were 23 men and four women, ages 19-36 years. Physical fitness varied from subject