Introduction The elastic properties of biological membranes are connected to the problem of cell stability and resistance to external in¯uences and to many physiological pro- cesses such as shape changes, vesicle endocytosis and speci®c interactions with transmembrane proteins. Be- cause of their essential role, the elastic properties have been investigated by dierent techniques, which include the mechanical deformation of the membrane by suction [1, 2] or local compression [3] or the excitation of modes of vibration induced arti®cially by electrical stress [4, 5] or naturally by thermal eects [6±10]. The experimental observation of the response to perturbations is usually performed by optical video microscopy, which implies that the membranes are arranged in the form of giant vesicles (few microns). On the other hand, a dynamic scattering technique, namely neutron spin echo, has been pro®tably used to investigate the elastic properties of the hydrophilic± hydrophobic interface of microemulsion small droplets (size 5±10 nm) [11±13]. This technique is based on some general features of scattering, namely that each dierent mode of the ¯uctuation of an interface, characterized by a particular frequency, contributes to the scattered intensity according to dierent q-dependent functions. Moreover, the zero-order mode, corresponding to the form factor of the average shape of the interface, which is usually the most important contribution to the scattered intensity, has a pronounced minimum in the same q position where the second-order mode, usually the most excitable mode, has a maximum. Then, it is possible to ®nd experimental conditions which allow the observation of both the corresponding characteristic times. The application of analogous dynamic scattering techniques is promising also in the case of small vesicles (about 100 nm) to assess the characteristic ¯uctuation times of the closed bilayer, provided that the suitable q range can be attained experimentally. In fact, the extension of the neutron spin echo technique to the case of vesicles has been desired for a long time [14], and Progr Colloid Polym Sci (2000) 115 : 181±185 Ó Springer-Verlag 2000 BIOSYSTEMS P. Brocca L. CantuÁ M. Corti E. Del Favero Thermal ¯uctuations of small vesicles: observation by dynamic light scattering P. Brocca á L. CantuÁ á M. Corti (&) E. Del Favero INFM, Dipartimento di Chimica e Biochimica Medica UniversitaÁ di Milano L.I.T.A., via F. lli Cervi 93 20090 Segrate, Italy e-mail: mario.corti@unimi.it Abstract The laser light scattering technique can be used in a noncon- ventional fashion to study dynamic properties of vesicles which are too small to be observed by microscopy. In fact, in suitable experimental conditions, the correlation function of the scattered light contains a contribution from bilayer ¯uctua- tions, besides the usual diusion one. Characteristic ¯uctuation times have been determined for single- component phospholipid vesicles of 60 nm radius, prepared by extru- sion. The addition of small amounts of a glycolipid (to 2% mole frac- tion), induces a signi®cative increase in the ¯uctuation times (of the order of 20%) but still does not aect the diusive motion, indicating a soft- ening of the membrane. Being so sensitive, this technique is quite promising both for the study of membrane properties in the presence of defects and for applications to biology and pharmacology. Key words Thermal ¯uctuations á Small vesicles á Dynamic light scattering