Sphingomyelin (SM) is the most prevalent sphingolipid in the majority of mammalian membranes. Proton and 31P nuclear magnetic resonance spectral data were acquired to establish the nature of intra- and intermolecular H-bonds in the monomeric and aggregated forms of SM and to assess possible differences between this lipid and dihydrosphingomyelin (DHSM), which lacks the double bond between carbons 4 and 5 of the sphingoid base. The spectral trends suggest the formation of an intramolecular H-bond between the OH group of the sphingosine moiety and the phosphate ester oxygen of the head group. The narrower linewidth and the downfield shift of the resonance corresponding to OH proton in SM suggest that this H-bond is stronger in SM than in DHSM. The NH group appears to be involved predominantly in intramolecular H-bonding in the monomer. As the concentration of SM increases and the molecules come in closer proximity, these intramolecular bonds are partially disrupted and the NH group becomes involved in lipid-water interactions. The difference between the SM and DHSM appears to be not in the nature of these interactions but rather in the degree to which these intermolecular interactions prevail. As SM molecules cannot come as close together as DHSM molecules can, both the NH and OH moieties remain, on average, more intramolecularly bonded as compared to DHSM.

Streptococcus sanguis demonstrated a high affinity for hydrocarbon solvents. When aqueous suspensions of the organism were mixed with either hexadecane or toluene, the cells tended to bind to the nonaqueous solvent. Increases in temperature resulted in a greater affinity of cells for hexadecane. Interaction between the cells and hexadecane was also enhanced by dilute aqueous sodium chloride and by low pH (pH < 5). The results suggest that the cell surface of S. sanguis has hydrophobic properties. Isolated cell walls also tended to partition into the nonaqueous solvent. Amino acid analyses of the walls revealed the presence of several amino acids which possess hydrophobic side chains. It is likely that the hydrophobic amino acids associated with the cell wall contribute to the hydrophobicity of intact S. sanguis. When the adherence of S. sanguis to saliva-coated hydroxylapatite was measured, it was found that hydrophobic bond-

Ceramide (Cer) has been identified as an active lipid second messenger in the regulation of cell growth, differentiation, and apoptosis. Its analog, dihydroceramide, without the 4 to 5 trans double bond in the sphingoid backbone lacks these biological effects. To establish the conformational features that distinguish ceramide from its analogs, nuclear magnetic resonance spectral data were acquired for diluted samples of ceramides (C2- and C18-Cer), dihydroceramide (C16-DHCer), and deoxydihydroceramide (C18-DODHCer). Our results suggest that in both C2- and C18-Cer, an H-bond network is formed in which the amide proton NH is donated to the OH groups on carbons C1 and C3 of the sphingosine backbone. Two tightly bound water molecules appear to stabilize this network by participating in flip-flop interactions with the hydroxyl groups. In DHCer, the lack of the trans double bond leads to a conformational distortion of this H-bonding motif. Without the critical double bond, the degree with which water molecules stabilize the H bonds between the two OH groups of the sphingolipid is reduced. This structural alteration might preclude the participation of DHCer in signaling-related interactions with cellular targets.

The adherence of Streptococcus sanguis to hydroxylapatite beads has been analyzed by binding isotherms, Langmuir isotherms, and Scatchard plots. For saliva-coated beads, the Scatchard curves contained components with both positive and negative slopes. The results are interpreted as evidence for positive cooperativity in the binding process. Although all Scatchard curves were similar in shape, distinct differences were observed between saliva samples from different individuals. Salivary agglutinins against whole S. sanguis cells did not appear to influence the shapes of the curves or the extent of adherence. In addition, different strains of S. sanguis yielded similar Scatchard plots. When the binding of S. sanguis to buffer-coated hydroxylapatite beads was analyzed by Scatchard plots or binding isotherms, curves were generated which suggested that either direct ligand-ligand or nonspecific interactions were occurring. Hill plots of the adherence data yielded curves with slopes greater than unity for saliva-coated beads, providing additional support for the view that the interactions between S. sanguis and the pellicle involve cooperative phenomena. In contrast, a Hill plot for the binding data of S. sanguis to buffer-coated hydroxylapatite beads gave a curve with a slope of 0.91 ± 0.07, suggesting negative cooperativity or limited specificity. When adherence data were plotted by the Langmuir method, curves were obtained which could not discriminate between the binding of the bacteria to the hydroxylapatite beads coated with either saliva or buffer. It was also observed that several different proteins and whole saliva tended to inhibit adherence. Scatchard plots, however, describing the binding of S. sanguis to the proteincoated beads were unique and revealed possible specific and nonspecific interactions. Scatchard analyses of binding data may be useful in understanding the mechanism(s) of adherence of streptococci to smooth surfaces.Streptococcus sanguis can be observed in early dental plaque and comprises a significant portion of the oral microbiota found on the tooth surface (3,13,15). The mechanism(s) involved in the attachment of S. sanguis to the salivary pellicle which coats the tooth surface remains obscure. Little is known about the structural requirements of pellicle protein receptors for the bacterial ligands. In fact, several groups report that oral streptococci, including Streptococcus mutans as well as S. sanguis, can bind to hydroxylapatite in the absence of any pellicleforming salivary proteins (4,18).In addition to the difficulties in purifying and characterizing specific bacterial surface components and salivary proteins which interact to form stable complexes, there have been only a few efforts to describe the quantitative relationships in adherence phenomena. Clark et al. (4), Gibbons et al. (10), and Appelbaum et al. (1)have employed the Lapgmuir adsorption isotherm to characterize the quantitative aspects of the adherence of oral streptococci to smooth surfaces. Similarly, Wheeler et al. (27...

The conformational features of dihydrosphingomyelin (DHSM), the major phospholipid of human lens membranes, were investigated by 1H and 31P nuclear magnetic resonance spectroscopy. Several postulates emerge from the observed trends: (a) in partially hydrated samples of DHSM in CDCl3 above 13 mM, at which lipid-lipid interactions prevail, the amide proton is mostly involved in intermolecular H-bonds that link neighboring phospholipids through bridging water molecules. In the absence of water, the NH group is involved in an intramolecular H-bond that restricts the mobility of the phosphate group. (b) In the monomeric form of the lipid molecule, the amide proton of the major conformer is bound intramolecularly with one of the anionic and/or ester oxygens of the phosphate group. A minor conformer may also be present in which the NH proton participates in an intramolecular H-bond linking to the OH group of the sphingoid base. (c) Complete hydration leads to an extension of the head group as water molecules bind to the phosphate and NH groups via H-bonds, thus disrupting the intramolecular H-bonds prevalent at low concentrations.