Perceptual learning and free classification

Two experiments are reported that investigate the effects of stimulus preexposure on discrimination performance in a free classification task, using adult humans as subjects. In free classification subjects are asked to put stimuli into groups in any way that seems reasonable or sensible to them. Experiment 1 shows that the effect of preexposure is contingent on stimulus structure. Experiment 1b is the first demonstration of a retardation in learning as a consequence of simple preexposure in adult human subjects (previous demonstrations have relied on incidental or masked preexposure). Experiment 2 further supports the conclusions of Experiment 1 and extends them with the demonstration that stimulus similarity is a crucial factor. Taken together, these experiments rule out a class of attention-based explanations of the phenomena reported here. The experiments also provide novel information about the effects of preexposure. Preexposure can change the actual classifications subjects form in addition to altering the rate at which they are formed. Implications of these results for current theories of category formation and perceptual learning are considered.     

Non-traditional mothers: single heterosexual/lesbian women and lesbian couples electing motherhood via donor insemination

The conformational properties of alpha,alpha-dialkylated amino acid residues possessing acyclic (diethylglycine, Deg; di-n-propylglycine, Dpg; di-n-butylglycine, Dbg) and cyclic (1-aminocycloalkane-1-carboxylic acid, Acnc) side chains have been compared in solution. The five peptides studied by nmr and CD spectroscopy are Boc-Ala-Xxx-Ala-OMe, where Xxx = Deg (I), Dpg (II), Dbg (III), Ac6c (IV), and Ac7c (V). Delineation of solvent-shielded NH groups have been achieved by solvent and temperature dependence of NH chemical shifts in CDCl3 and (CD3)2SO and by paramagnetic radical induced line broadening in peptide III. In the Dxg peptides the order of solvent exposure of NH groups is Ala(1) > Ala(3) > Dxg(2), whereas in the Acnc peptides the order of solvent exposure of NH groups is Ala(1) > Acnc(2) > Ala(3). The nmr results suggest that Acnc peptides adopt folded beta-turn conformations with Ala(1) and Acnc(2) occupying i + 1 and i + 2 positions. In contrast, the Dxg peptides favor extended C5 conformations. The conformational differences in the two series are clearly borne out in CD studies. The solution conformations of peptides I-III are distinctly different from the beta-turn structure observed in crystals. Low temperature nmr spectra recorded immediately after dissolution of crystals of peptide II provide evidence for a structural transition. Introduction of an additional hydrogen-bonding function in Boc-Ala-Dpg-Ala-NHMe (VI) results in a stabilization of a consecutive beta-turn or incipient 3(10)-helix in solution.     

Lower prevalence and incidence of HIV-1 syncytium-inducing phenotype among injecting drug users compared with homosexual men

OBJECTIVE: To study the prevalence, incidence and predictive value for progression to AIDS of the HIV-1 syncytium-inducing (SI) phenotype in HIV-infected injecting drug users (IDU) compared with HIV-infected homosexual men. DESIGN: Two prospective cohort studies on HIV-1 infection among IDU and homosexual men. METHODS: HIV-infected IDU (n = 225) and homosexual men (n = 366) without AIDS were studied from March 1989 through December 1993. Data on laboratory markers, including the presence of SI variants, demographics, behavioural characteristics and clinical events were collected at every visit. RESULTS: At baseline, SI variants were detected in 4% of IDU and 17% of homosexual men. During the study period 18 IDU and 68 homosexual men switched from non-SI to SI phenotype (4-year cumulative incidence, 14.6 and 28.4%, respectively) before AIDS diagnosis. Among participants with a documented date of HIV infection the cumulative incidence of SI was lower among IDU than homosexual men (4-year cumulative incidence, 6.2 and 20.7%, respectively). At AIDS diagnosis, 21% of all AIDS cases among IDU had the SI phenotype compared with 54% among homosexual men. In both risk groups an accelerated CD4 decline was found after the non-SI-to-SI switch. The SI phenotype appeared to be a predictor of AIDS (multivariate relative hazard, 5.33), independent of CD4 cell count and p24 antigen at baseline. In the multivariate time-dependent analysis, the relative hazard of SI phenotype decreased considerably, which is consistent with the hypothesis that the effect of SI phenotype on progression to AIDS is mediated by CD4 cell count. CONCLUSION: The SI phenotype is associated with accelerated CD4 decline and progression to AIDS in both risk groups. The remarkable lower prevalence and incidence of the SI phenotype among IDU may implicate a difference in pathogenesis and natural history of HIV infection linked to transmission group.     

Conjugation of classic Bowman-Birk soy inhibitor with a copolymer of ethylene oxide and propylene oxide

A classical soybean inhibitor of the Bowman-Birk type (BBI) with a copolymer of ethylene oxide and propylene oxide (PE) has been synthesized. The BBI-PE conjugate contain five covalently bound polymeric chains per one protein molecule and retains its capacity to inhibit trypsin (Ki = 10(-10) M), alpha-chymotrypsin (Ki = 7 x 10(-8) M) and human granulocyte elastase (Ki = 3 x 10(-8) M). The preservation of the antiproteinase activity in the antichymotrypsin center creates a prerequisite for the manifestation of the anticarcinogenic effect of the inhibitor.     

Cholesterol and ergosterol superlattices in three-component liquid crystalline lipid bilayers as revealed by dehydroergosterol fluorescence

We have examined the fractional sterol concentration dependence of dehydroergosterol (DHE) fluorescence in DHE/cholesterol/dimyristoyl-L-alpha-phosphatidylcholine (DMPC), DHE/ergosterol/DMPC and DHE/cholesterol/dipalmitoyl-L-alpha-phosphatidylcholine (DPPC) liquid-crystalline bilayers. Fluorescence intensity and lifetime exhibit local minima (dips) whenever the total sterol mole fraction, irrespective of the DHE content, is near the critical mole fractions predicted for sterols being regularly distributed in hexagonal superlattices. This result provides evidence that all three of these naturally occurring sterols (e.g., cholesterol, ergosterol, and DHE) can be regularly distributed in the membrane and that the bulky tetracyclic ring of the sterols is the cause of regular distribution. Moreover, at the critical sterol mole fractions, the steady-state anisotropy of DHE fluorescence and the calculated rotational relaxation times exhibit distinct peaks, suggesting that membrane free volume reaches a local minimum at critical sterol mole fractions. This, combined with the well-known sterol condensing effect on lipid acyl chains, provides a new understanding of how variations in membrane sterol content change membrane free volume. In addition to the fluorescence dips/peaks corresponding to hexagonal superlattices, we have observed intermediate fluorescence dips/peaks at concentrations predicted by the centered rectangular superlattice model. However, the 22.2 mol% dip for centered rectangular superlattices in DHE/ergosterol/DMPC mixtures becomes diminished after long incubation (4 weeks), whereas on the same time frame the 22.2 mol% dip in DHE/cholesterol/DMPC mixtures remains discernible, suggesting that although all three of these sterols can be regularly distributed, subtle differences in sterol structure cause changes in lateral sterol organization in the membrane.