Conformational investigation of alpha,beta-dehydropeptides. XV: N-acetyl-alpha,beta-dehydroamino acid N 'N '-dimethylamides: conformational properties from infrared and theoretical studies.

The FTIR spectra were analysed in the region of the nu(s)(N-H), AI(C=O) and nu(s)(Calpha=Cbeta) bands for a series of Ac-DeltaXaa-NMe2, where DeltaXaa = DeltaAla, (Z)-DeltaAbu, (Z)-DeltaLeu, (Z)-DeltaPhe and DeltaVal, to determine a predominant solution conformation of these alpha,beta-dehydropeptide-related molecules. Measurements were taken in CCl4, DCM and MeCN solutions. In the same way, spectra of saturated analogues Ac-Xaa-NMe2, where Xaa = Ala, Abu, Leu, Phe and Val, were investigated. To help interpret the spectroscopic results, conformational maps were calculated by the B3LYP/6-31+G** method. Also, the relative energies of all conformers of the dehydro compounds in vacuo as well as in the studied solvents in addition to the theoretical IR frequencies of these conformers were calculated. For comparison, molecules of two saturated analogues, Ac-L-Ala-NMe2 and Ac-L-Phe-NMe2, were calculated in a similar way. Both unsaturated and saturated compounds, which have an aliphatic side chain, occur in CCl4 and DCM mainly as a mixture of extended conformers with the C5 H-bond and open conformers. As solvent polarity increases, participation of the open conformers also increases, and in MeCN, the model amides are almost exclusively in the open form, except Ac-DeltaAla-NMe2, which shows a small amount of the H-bonded conformer. Ac-DeltaAla-NMe2 and Ac-DeltaAbu-NMe2 have stronger C5 hydrogen bonds than those of their saturated counterparts. As the calculations indicate, the open conformation of the unsaturated amides is conformer H/F with phi, psi -44 +/- 5 degrees, 127 +/- 4 degrees. This is the second lowest in energy conformer in vacuo and in CCl4 and the lowest one in more polar solvents. The open conformation of Ac-L-Ala-NMe2 constitutes conformer C with phi, psi -101.5 degrees, 112.7 degrees. For Ac-DeltaAla-NMe2 and Ac-DeltaAbu-NMe2, FTIR also reveals the presence of a third conformer. Calculations indicate that is the semiextended conformer D with the N1-H1...N2 hydrogen bond/contact. In all solvents, Ac-L-Phe-NMe2 and Ac-(Z)-DeltaPhe-NMe2 show only the extended E and the open H/F, respectively. In both there is an amide/pi(Ph) interaction.     

Comparison of the prolonged swimming performances of closely related, morphologically distinct three‐spined sticklebacks Gasterosteus spp.

Prolonged swimming performances of two as yet unnamed species of three‐spined stickleback, Gasterosteus spp., were compared. The two fishes (not yet formally described, referred to here as benthic and limnetic) inhabit different niches within Paxton Lake, Texada Island, British Columbia, Canada, and are recent, morphologically distinct species. Limnetics had longer endurance during prolonged swimming than did benthics. The mean regression of the log₁₀ of fatigue time (F_t, s) on swimming speed (U, standard length, L_S s^?1) for limnetics (log₁₀F_t = 7·03 ? 0·46U) had a similar slope, but a significantly higher intercept than that for benthics (log₁₀F_t = 5·55 ? 0·43U). Adult benthics were larger, heavier and deeper‐bodied fish than limnetics. Limnetics, however, had a significantly greater pectoral fin edge:base ratio (mean ± s .e .: limnetics, 4·58 ± 0·43; benthics, 3·63 ± 0·27). In addition, limnetics had significantly lower drag coefficients (C_D) than benthics (limnetics, log₁₀C_D = ?0·49log₁₀R_e + 0·66; benthics, log₁₀C_D = ?0·26log₁₀R_e ? 0·30) where R_e is the Reynolds number [(L_SU (ν^?1), where U and ν are swimming velocity and the kinematic viscosity of the water, respectively]. Compared to their ancestral form, the anadromous three‐spined stickleback Gasterosteus aculeatus L., limnetics and benthics had significantly longer and shorter endurance times, respectively. In addition, both these fishes had significantly higher fast‐start velocities than their ancestral form. Selection due to differential resource use may have lead to divergence of body form, and, therefore, of steady swimming performance. Therefore predation may be the selective force for the similar high escape performance in these two fishes.     

THE MORPHOLOGY OF HYOLITHIDS AND ITS FUNCTIONAL IMPLICATIONS

Abstract:  The exceptionally preserved hyolithids Gompholites striatulus , Maxilites robustus , Maxilites snajdri and Maxilites sp. are described with particular emphasis on helen and muscle scar morphology. These two aspects of hyolithid morphology have remained controversial. In life position, each helen curved ventrally. When the operculum closed the aperture of the conch, each helen was locked at the commissure slit with its dorsal edge tilted forward. Inside the conch, it was held in the dorsal apertural plane and clear of the inner surface of the operculum. Previously unidentified muscle scars are described from both the operculum and the conch. Dorsal scars on the conch aperture held muscles directed to the operculum. Comparative study of the muscle insertion pattern indicates that hyolithids did not have serially arranged muscles and that all hyolithids may have had a common skeleto-muscular system. The arrangement of the muscle scars with respect to the helens suggests that the latter were capable of relatively complex movements and could have been used to propel the organism over the substrate. The general morphology and orientation of the helens suggests that in addition they functioned to stabilize the organism on the sea-floor.     

Food webs in tropical Australian streams: shredders are not scarce

1. Macroinvertebrates were collected in dry and wet seasons from riffles and pools in two streams in tropical north Queensland. Total biomass, abundance and species richness were higher in riffles than in pools but did not differ between streams or seasons. 2. Gut contents of all species were identified. Cluster analysis based on gut contents identified five dietary groups: I, generalist collectors; II, generalist shredders and generalist predators; III, generalist scrapers; IV, specialist shredders; and V, specialist predators. Species were allocated to functional feeding groups (FFGs) based on these dietary groups. 3. Many species were generalist in their diets, but specialist predators and shredders were particularly prominent components of the invertebrate assemblages in terms of biomass and species richness. 4. Community composition (proportions of biomass, abundance and species richness of the different FFGs) varied between habitat types, but not between streams or seasons, although differences between riffles and pools varied with season. 5. Comparison of the fauna of 20 streams showed that our study sites were similar to, or not atypical of, low‐order streams in the Queensland wet tropics.     

The response of macroinvertebrate production to a pollution gradient in a headwater stream

1. This study quantified patterns of macroinvertebrate secondary production and stored benthic organic matter along a gradient of pollution and habitat channelisation over a 3‐km reach of Goosefare Brook, a first‐order stream in southern Maine (U.S.A.). 2. Whole‐community invertebrate production decreased from 26.4 g ash‐free dry mass (AFDM) m⁻² year⁻¹ at the reference station to 1.1 g AFDM m⁻² year⁻¹ at stations with the greatest levels of pollution. Production decreased along the pollution gradient for most taxa, although decreases were partly offset by production increases in tolerant taxa. Biomass turnover rates (P/B) were less affected by the stresses than was production. 3. Differences in functional characteristics of the community were evident at stations with channelised habitat, but overall production declined in a linear pattern that mirrored the pollution gradient. Stored organic matter showed a decline along the gradient, but was also lower at channelised stations. Populations of taxa with documented pollution tolerance were more likely to maintain or increase production and P/B. 4. Decreasing biomass because of decreasing stored organic matter and lethal effects of pollutants resulted in shifts in the pathways of energy flow observed at stations exposed to moderate physical or chemical stress, to the loss of most taxa and an extreme (96%) decrease in production at the stations receiving the highest levels of metal pollution. 5. The shifting prominence of different taxa along a continuum of stress in Goosefare Brook shows that describing the nature of an impairment in a functional context requires consideration of chemical stressors, habitat alterations and food resources.     

Driving forces shaping phytoplankton assemblages in two subtropical plateau lakes with contrasting trophic status

1. We studied driving forces shaping phytoplankton assemblages in two subtropical plateau lakes with contrasting trophic status, the oligotrophic deep Lake Fuxian and the eutrophic shallow Lake Xingyun. 2. Phytoplankton samples were taken monthly for a year and phytoplankton species were sorted into the main taxonomic groups and associations proposed by Reynolds. A canonical correspondence analysis (CCA) was used to test the occurrence of these classification schemes and to determine their discriminatory power. 3. The results suggest that the major driving forces in Lake Fuxian were physical variables, and particularly the underwater light climate, whereas, nutrients were the important driving force in Lake Xingyun. 4. Top–down control through zooplankton grazing in Lake Fuxian was hardly ever a significant determinant itself, because of the scarcity of zooplankton and their low grazing efficiency of predation while a dominance of inedible cyanobacteria throughout the year rendered top–down controls ineffective failing in Lake Xingyun. Hence phytoplankton communities in both lakes appear to be regulated primarily by bottom–up controls.     

A flow-injection chemiluminescence method for the determination of human serum albumin, using the reaction of fluorescein-human serum albumin-sodium hypochlorite by the enhancement effect of the cationic surfactant cetyltrimethylammonium bromide.

The interaction between surfactant and fluorescein was studied, using a fluorescence spectroscopy and flow-injection (FI) chemiluminescence (CL) method. It was found that the cationic surfactant cetyltrimethylammonium bromide (CTAB) could cause the structural transformation of fluorescein from the quinone to the spirolactone form, and greatly enhance the CL intensity of the fluorescein-human serum albumin (HSA) complex. Based on this finding, a rapid and sensitive FI-CL method was developed for the determination of HSA. Under the optimum conditions, the proposed method has a linear range of 0.05-24.0 microg/mL, with a detection limit of 0.03 microg/mL for HSA (3sigma). The relative standard deviation (RSD) of 1.2 microg/mL HSA (n = 8) is 0.8%. The method was applied to the determination of protein content in urine samples, with satisfactory results. Density functional theory was used to study the mechanism of surfactant-enhanced CL intensity of the fluorescein-HSA complex.     

SPATIOTEMPORAL VARIATION OF THE ABUNDANCE OF CALCAREOUS GREEN MACROALGAE IN THE FLORIDA KEYS: A STUDY OF SYNCHRONY WITHIN A MACROALGAL FUNCTIONAL‐FORM GROUP1

The abundance of calcareous green algae was recorded quarterly at 28 sites within the Florida Keys National Marine Sanctuary (FKNMS) for a period of 7 years as part of a sea grass monitoring program. To evaluate the validity of using the functional‐form group approach, we designed a sampling method that included the functional‐form group and the component genera. This strategy enabled us to analyze the spatiotemporal patterns in the abundance of calcareous green algae as a group and to describe synchronous behavior among its genera through the application of a nonlinear regression model to both categories of data. Spatial analyses revealed that, in general, all genera displayed long‐term trends of increasing abundance at most sites; however, at some sites the long‐term trends for genera opposed one another. Strong synchrony in the timing of seasonal changes was found among all genera, possibly reflecting similar reproductive and seasonal growth pattern, but the variability in the magnitude of seasonal changes was very high among genera and sites. No spatial patterns were found in long‐term or seasonal changes; the only significant relation detected was for slope, with sites closer to land showing higher values, suggesting that some factors associated with land proximity are affecting this increase. We conclude that the abundances of genera behaved differently from the functional‐form group, indicating that the use of the functional‐form group approach may be unsuitable to detect changes in sea grass community structure in the FKNMS at the existing temporal and spatial scale of the monitoring program.     

Differential water resource use by herbaceous and woody plant life-forms in a shortgrass steppe community

We conducted a study to test the predictions of Walter's two-layer model in the shortgrass steppe of northeastern Colorado. The model suggests that grasses and woody plants use water resources from different layers of the soil profile. Four plant removal treatments were applied in the spring of 1996 within a plant community codominated by Atriplex canescens (a C₄ shrub) and Bouteloua gracilis (a C₄ grass). During the subsequent growing season, soil water content was monitored to a depth of 180 cm. In addition, stem and leaf tissue of Atriplex, Bouteloua and the streamside tree Populus sargentii were collected monthly during the growing seasons of 1995 and 1996 for analysis of the δ¹⁸O value of plant stem water (for comparison with potential water sources) and the δ¹³C value of leaves (as an indicator of plant water status). Selective removal of shrubs did not significantly increase water storage at any depth in the measured soil profile. Selective removal of the herbaceous understory (mainly grasses) increased water storage in the top 60 cm of the soil. Some of this water gradually percolated to lower layers, where it was utilized by the shrubs. Based on stem water δ¹⁸O values, grasses were exclusively using spring and summer rain extracted from the uppermost soil layers. In contrast, trees were exclusively using groundwater, and the consistent δ¹³C values of tree leaves over the course of the summer indicated no seasonal changes in gas exchange and therefore minimal water stress in this life-form. Based on anecdotal rooting-depth information and initial measurements of stem water δ¹⁸O, shrubs may have also had access to groundwater. However, their overall δ¹⁸O values indicated that they mainly used water from spring and summer precipitation events, extracted from subsurface soil layers. These findings indicate that the diversity of life-forms found in this shortgrass steppe community may be a function of the spatial partitioning of soil water resources, and their differential use by grasses, shrubs, and trees. Consequently, our findings support the two-layer model in a broad sense, but indicate a relatively flexible strategy of water acquisition by shrubs. Received: 23 December 1997 / Accepted: 16 September 1998