This is the first time that ToF-SIMS has been applied to trace the distribution of a hapten in human skin. Proof of principle was shown for nickel, and the technique can, in the future, be expanded for investigation of the skin distribution of clinically relevant sensitizers in general.

Ion pairing between the major phospholipids of the Staphylococcus aureus plasma membrane (phosphatidylglycerol -PG and lysyl-phosphatidylglycerol -LPG) confers resistance to antimicrobial peptides and other antibiotics. We developed 3adLPG, a stable synthetic analogue which can substitute for the highy-labile native LPG, in biophysical experiments examining the membrane-protecting role of lipid ion pairing, in S. aureus and other important bacteria. Here we examine the surface charge and lipid packing characteristics of synthetic biomimetic mixtures of DPPG and DP3adLPG in Langmuir monolayers, using a combination of complementary surfaceprobing techniques such as infrared reflection-absorption spectroscopy and grazing-incidence x-ray diffraction. The resultant phase diagram for the ion paired lipids sheds light on the mixing behavior of lipids in monolayer models of resistant phenotype bacterial membranes, and provides a platform for future biophysical studies.The aminoacyl lipids produced by a wide range of bacteria are becoming increasingly recognized as clinically relevant virulence factors, due to the role they play in phenotypic adaptations to the physical and biochemical stressors which confer intrinsic defence against infection. [1,2] The most widely studied of these lipids is lysyl-phosphatidylglycerol (LPG), for which the genomic regulation and biosynthetic pathways in Staphylococcus aureus, have recently been elucidated in some detail. [3,4] Data from microbiological assays, has shown that an increased proportion of LPG in S. aureus membranes correlates with resistance to both host defensive peptides [5] and membrane-active therapeutic antibiotics. [6] The mechanisms facilitating such resistances are assumed to involve the tuning of target membrane physical properties, notably those of interfacial charge and lipid ordering, which are influenced by the LPG content in the bacteria membranes. [7] Biophysical investigations into these phenomena have been hampered by the labile nature of native LPG, which is readily hydrolysed under mild conditions, therefore exposing any such study to the risk of artefact. [8,9] To this end, stable LPG analogues have been synthesized. One such analogue, lysyl-phosphatidylethanolamine (LPE) exhibited an inhibitory effect on antimicrobial peptide activity when incorporated into vesicles containing phosphatidylglycerol (PG). [10] A second analogue, 3-aza-dehydroxy lysyl-phosphatidylglycerol (3adLPG), facilitated enhanced membrane ordering and antimicrobial peptide resistance when mixed with PG under mildly acidic conditions, [11] which are known to promote PG/LPG ion pair formation in model bacterial membranes. [12] In order to gain a better understanding of the influence of lipid ion pairing on membrane structure and interfacial properties, we conducted a high-resolution study of biomimetic mixtures of dipalmitoyl-3adLPG and DPPG in Langmuir monolayers.The natural phospholipid composition ( Figure 1A) observed in different S. aureus strains is dominated by PG ( � 40-70 %) and...