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Langmuir 2002, 18, 6718-6718
Reply to Comment on “Trehalose Interacts with Phospholipid Polar Heads in Langmuir Monolayers”
In reply to the comment by Disalvo et al. on our paper,1 we put forward the following considerations. The work performed by the group of Disalvo, using complementary techniques to ours, has given important contributions to clarify the mechanisms of interaction between trehalose and phospholipid polar heads at the single bond level. We are sorry for not having cited their papers. However, it must be considered that only a note, mainly devoted to the effect of phloretin on the dipole potential of lipid bilayers,2 appeared before we submitted our work for publication. The two other papers3,4 mentioned by the authors appeared nearly at the same time or even after our paper was accepted for publication. The aim of our work was not only to show that trehalose interacts with phospholipid polar heads but also to provide new experimental support to a general interpretation of surface potential at very expanded molecular areas.5 Indeed, we have shown that at negligible surface pressure, when the monolayer is still in the gaseous phase, the presence of trehalose promotes the structuring of a hydrogen-bonded network at a larger area per molecule than the one in the absence of sugar. This brings about a larger value of the critical area for the first rise of surface potential. In Figure 1 of the above comment, the authors report the change in surface pressure, at constant area, upon subsequent additions of phospholipids (either DMPC (dimyristoylphosphatidylcholine) or DM(ether)PC (ditetradecyl-glycero-phosphocholine)) at the air-water interface in the absence and in the presence of trehalose. We disagree with the interpretation, suggested by the authors, that trehalose has a condensing effect on hydrated lipid headgroups. In fact, it must be considered that the saturation value of surface pressure should correspond to the situation of maximum packing in the monolayer, when * Corresponding author. E-mail:
[email protected]. † Universita ` di Genova. ‡ Universita ` di Palermo. (1) Lambruschini, C.; Relini, A.; Ridi, A.; Cordone, L.; Gliozzi, A. Langmuir 2000, 16, 5476. (2) Diaz, S.; Amalfa, F.; Biondi de Lopez, A. C.; Disalvo, E. A. Langmuir 1999, 15, 5179. (3) Luzardo, M. del C.; Amalfa, F.; Nun˜ez, A. M.; Diaz, S.; Biondi de Lopez, A. C.; Disalvo, E. A. Biophys. J. 2000, 78, 2452. (4) Diaz, S.; Lairio´n, F.; Arroyo, J.; Biondi de Lopez, A. C.; Disalvo, E. A. Langmuir 2001, 17, 852. (5) Leite, V. P.; Cavalli, A.; Oliveira, O. N. Phys. Rev. E 1998, 57, 6835.
further lipid additions give rise to a coexistence of monolayer and bulk structures such as liposomes.6 Therefore, based on the data reported in Figure 1 of the comment, the amount of lipid required for reaching saturation in surface pressure for DMPC, in the presence of trehalose, is twice the amount of lipid required for saturation in the absence of the sugar. Accordingly, the maximum packing area in the presence of trehalose would be half the corresponding area obtained in its absence. Such a value is absolutely not compatible with any reasonable size of a double hydrocarbon chain. We suggest that, most likely, trehalose affects the critical concentration at which molecular aggregates are formed. This interpretation is strongly supported by the peculiar behavior of DMPC in the presence of trehalose, which is nonsigmoidal. In our opinion, this experiment demonstrates not that the area per lipid is much larger in water than in trehalose (a result which is also in contrast with our experimental results, which showed that trehalose has an expanding effect on pressure-area isotherms1) but rather that trehalose interacts strongly with carbonyls, as already proposed by the authors on the basis of Fourier transform infrared measurements.2,3 This strong interaction could perhaps make the two methods of measurement (constant area and pressure-area isotherms) no longer equivalent. The large phospholipid-trehalose interaction is also confirmed by recent experiments at our laboratory. In these experiments, we acquired force-distance curves with the atomic force microscope to measure the hydrogen bond interaction between surfaces exposing carboxyl groups in the absence and in the presence of trehalose (work in progress). Annalisa Relini,*,† Lorenzo Cordone,‡ and Alessandra Gliozzi†
Istituto Nazionale per la Fisica della Materia and Dipartimento di Fisica, Universita` di Genova, Via Dodecaneso 33, I-16146 Genova, Italy, and Istituto Nazionale per la Fisica della Materia and Dipartimento di Scienze Fisiche e Astronomiche, Universita` di Palermo, Via Archirafi 36, I-90123, Palermo, Italy Received December 12, 2001 LA011798I (6) MacDonald, R. C.; Simon, S. A. Proc. Natl. Acad. Sci. U.S.A. 1987, 84, 4089.
10.1021/la011798i CCC: $22.00 © 2002 American Chemical Society Published on Web 07/19/2002
