The Saffman-Delbrück approximation is commonly used in biophysics to relate the membrane inclusion size to its translational diffusion coefficient and membrane viscosity. However, this approximation has a restricted validity range, and its application to determination of inclusion sizes from diffusion data may in certain cases lead to unreliable results. At the same time, the model by Hughes et al. (Hughes, B. D., B. A. Pailthorpe, and C. R. White. 1981. J. Fluid Mech. 110:349-372.), providing diffusion coefficients of membrane inclusions for arbitrary inclusion sizes and viscosities of the membrane and surrounding fluids, involves substantial computational efforts, which prevents its use in practical data analysis. We develop a simple and accurate analytical approximation to the Hughes et al. model and demonstrate its performance and utility by applying it to the recently published experimental data on translational diffusion of micrometer-sized membrane domains.

The formation of nanoassemblies of CdSe/ZnS quantum dots (QD) and pyridyl-substituted free-base porphyrin (H(2)P) molecules has been spectroscopically identified by static and time-resolved techniques. The formation of nanoassemblies has been engineered by controlling the type and geometry of the H(2)P molecules. Pyridyl functionalization gives rise to a strong complex formation accompanied by QD photoluminescence (PL) quenching. For some of the systems, this quenching is partly related to fluorescence resonance energy transfer (FRET) from the QD to H(2)P and can be explained according to the Förster model. The quantitative interpretation of PL quenching due to complexation reveals that (i) on average only about (1)/(5) of the H(2)P molecules at a given H(2)P/QD molar ratio are assembled on the QD and (ii) only a limited number of "vacancies" accessible for H(2)P attachment exist on the QD surface.

We report on modification of the spontaneous emission of dye molecules embedded in a threedimensional solid-state photonic crystal exhibiting a stop band in the visible range. Molecules embedded in artificial opal filled with a polymer show a dip in the fluorescence spectrum and nonexponential spontaneous decay kinetics containing both accelerated and inhibited components compared to the dye fluorescence in a reference polymer matrix. Results are interpreted in terms of redistribution of the photon density of states in the photonic crystal. [S0031-9007(98)06494-1] PACS numbers: 42.50. -p

Diffusion and segmental dynamics of the double-stranded -phage DNA polymer are quantitatively studied over the transition range from stiff to semiflexible chains. Spectroscopy of fluorescence fluctuations of single-end fluorescently labeled monodisperse DNA fragments unambiguously shows that doublestranded DNA in the length range of 10 2 -2 10 4 base pairs behaves as a semiflexible polymer with segmental dynamics controlled by hydrodynamic interactions. DOI: 10.1103/PhysRevLett.97.258101 PACS numbers: 87.14.Gg, 82.35.Lr, 87.15.He, 87.15.Vv The dynamic behavior of individual macromolecules in solution is governed by chain connectivity and hydrodynamic interactions [1]. Understanding polymer dynamics and quantitative verification of polymer theories requires detailed information on segmental motion of individual polymer molecules. However, the classical experimental techniques, such as dynamic light scattering (DLS) or transient electric birefringence (TEB), predominantly deliver information on large-scale shape fluctuations of macromolecules, and development of new experimental approaches providing detailed information on internal dynamics of individual molecules is of high importance.Precise experiments in polymer physics are impossible without well-defined monodisperse polymer samples covering a wide range of molecular weights. The recent progress in molecular biotechnology resulted in a variety of techniques to produce monodisperse DNA fragments, which stimulated the use of DNA as a model compound in studies of polymer dynamics in solution [2]. Doublestranded DNA (dsDNA) is a biopolymer characterized by a large persistence length l p 50 nm [3]. As a result, dsDNA fragments exhibit rodlike, semiflexible, or even flexible polymer behavior, depending on their length. Thus, simple generic models of polymer dynamics [1] are not expected to provide a quantitative description of dsDNA behavior, and more advanced models accounting for the persistence of the polymer chain [4] are required.The dynamics of dsDNA in solution has been a subject of a number of experimental investigations carried out by various techniques, including DLS [5], single-molecule fluorescence microscopy [6], electrophoresis [7], and TEB [8]. Fluorescence correlation spectroscopy (FCS) [9], a single-molecule technique that can provide more detailed information on the macromolecular dynamics than the classical ensemble-based methods, has been recently applied to investigation of DNA in solution [10 -12], which has lead to a controversy whether dsDNA dynamics in dilute solution is controlled by hydrodynamic interactions [10,12] or not [11]. Apart from this controversy, for different reasons, none of these FCS-based experiments produced parameters of DNA dynamics. Generally, to the best of our knowledge, no experimental studies were published so far where diffusion and intramolecular dynamics of dsDNA have been simultaneously investigated over the transition range from stiff to semiflexible chains. In the present Letter, we fill this gap and car...