Immunoglobulins are somewhat unusual in that they are biologically active proteins capable of carrying out several different functions. Thus, they can bind specifically with antigen, activate the complement system, mediate many cytotropic reactions, and act as antigen receptors on lymphocyte membranes.

In general the characterization of a virus particle depends upon a number of known and independently acquired parameters, all of which require a great deal of time. As a direct method of measurement, the mass of viruses has been largely ignored in virus characterization studies. Where the molecular weight of viruses has been determined by sedimentation and diffusion, sedimentation equilibrium, light scattering, or the electron microscopy-counting procedure, the relationship between this property and mass has been arrived at by employing Avogadro's number. In the present report it will be demonstrated that the disadvantages found in these usual methods of determining molecular weight, such as experimentation, calculation of virus size, and purification of virus preparation, are not present in the quantitative electron microscopy method for determination of mass and molecular weight.

An understanding of sensory information processing in the nervous system will probably require investigations with a variety of ‘model’ systems at different levels of complexity.

Our choice of a suitable model system was constrained by two conflicting requirements: on one hand the information processing properties of the system should be rather complex, on the other hand the system should be amenable to a quantitative analysis. In this sense the fly represents a compromise.

In these two papers we explore how optical information is processed by the fly's visual system. Our objective is to unravel the logical organization of the fly's visual system and its underlying functional and computational principles. Our approach is at a highly integrative level. There are different levels of analysing and ‘understanding’ complex systems, like a brain or a sophisticated computer.

Visual information processing in the nervous system of flies begins with a large array of photoreceptors, which transduce a light intensity pattern, and culminates in a behavioural response that depends on that pattern.

In the previous paper we have given a quantitative description of visual control of flight orientation in the fly. This description can account for fixation, tracking and some instances of spontaneous pattern preference behaviour. The phenomenological theory outlines the basic logical organization of the visual control system of the fly. It requires the neural network between the receptors and the flight muscles to perform two main computations on the visual input. One computation extracts movement information (the term r(ψ)ψ b of the phenomenological equation). The other provides position information (the term D(ψ)).

The magnetic state of iron in haem proteins has long been recognized as a convenient indicator of chemical coordination as well as more subtle biochemical properties. In the trivalent case in particular, there are three magnetically distinct configurations of the five 3d electrons, yielding spin states of S = 1/2, 3/2 and 5/2. The first and third are extremely familiar, and often lie close enough in energy so that a thermal mixture of low-spin S = 1/2 and high-spin S = 5/2 states exists in an ensemble of molecules. Selection rules for common perturbations (ΔS = ∘, ± 1 for the spin-orbit interaction and ΔS = ∘ for the electronic Zeeman interaction) ensure that quantum mixtures, in which the wave function is a true combination of S = 1/2 and S = 5/2 components, are not observed. The mid-spin, S = 3/2, state, and allowed 5/2–3/2 and 1/2–3/2 quantum mixtures including it, are much less well known. These have only rarely been invoked in recent years as an explanation for experimental haem protein magnetic data.

To a certain degree, the events underlying the action potential are understood. For each ion to which the membrane is permeable, there exists an equilibrium potential, whose physical origin is in the activity gradient for that ion. The membrane potential is then the summation of these individual ionic batteries each weighted according to the membrane permeability to that ion. In nerve fibres at rest the potassium permeability is relatively high and the membrane potential is near to the equilibrium potential for potassium. During excitation there is a transitory increase in the permeability to sodium and a slower transitory increase in the potassium permeability also. The membrane potential, therefore, temporarily moves to a value near to the sodium equilibrium potential and then returns to its resting value. Less well understood are the molecular mechanisms responsible for these selective changes in membrane permeability, i.e. how it is that a channel (pathway bywhich an ion traverses the membrane) changes its availability for ion passage as a response to a change in the membrane potential. This is the gating process.

Fluorescence spectroscopy offers particularly sensitive tools for the investigation of physical properties of macromolecules in solution. Differences in molecular structure under varying experimental conditions are often reflected in quantum yield and lifetime of natural or synthetically inserted fluorescent probes. inserted fluorescent probes. Energy transfer between fluorescent groups at known places of a primary sequence can be used to measure distances between the label sites (Eisinger, 1976) as well as to investigate their relative motion, e.g. measurements of the kinetics of variations in the end to end distance of a polymer (E. Haas & I. Steinberg, personal communication).

Theoretical methods of conformational analysis have been applied to the biopolymers with beneficial consequences for the elucidation of structure–function relationships in these molecules. The methodology of theoretical conformational analysis and the fruits of its application to the biopolymers, particularly the polypeptides, have been the subject of numerous reviews (Ramachandran & Sasisekharan, 1968; Scheraga, 1968; Venkatachalam & Ramachandran, 1969; Scheraga, 1971; Brant, 1972; Hopflnger, 1973). A critical evaluation of these techniques as applied to the polynucleotides has recently appeared (Olson, 1975 a, b).

Equilibrium data are of importance in describing and understanding of biochemical systems. At present there is marked variation among different investigators in the choice of experimental conditions for the study of similar or identical reactions and in the manner of reporting the data. In many cases the description of the system does not provide all the essential information that would permit reproduction of the experiments. This can lead to confusion and difficulties in correlating the results of different workers.

Equilibrium studies in biochemistry involve special problems that are not encountered in general chemistry. The attainment of biochemical equilibria commonly involves the addition of a specific enzyme to the system to catalyse the reaction studied; sometimes two or more enzymes must be added. In addition the enzyme may require the presence of certain cofactors, such as metal ions. The reactants, or products, or both, may bind or release protons or other ions during the reaction under study.

At even the simplest level we can expect an ATPase mechanism to comprise the following four steps: the binding of ATP, the reaction of ATP with water on the enzyme, and the release of the products ADP and P1. So at the outset techniques are needed to investigate these four processes. The range of techniques needed is soon extended once questions are asked about the role of protons and metal ions, the possibility of a multistep hydrolytic process, multistep substrate and product binding processes, and protein–lipid or protein–protein interactions. Since ATPases and ATP synthases are almost universally involved in some form of energy transduction there is a particular need in an ATPase or ATP synthase reaction to evaluate the equilibrium constants of the steps in the mechanism and to investigate the possibility of alternate reaction pathways. The nature of the coupling process by the protein of the chemical reactions of ATP to the other energetic process, be it muscle contraction, active transport, respiration or photosynthesis, is likewise of profound interest. Finally we would like to know as much as possible about the ATPase or ATP synthase mechanism during the period when the various forms of energy transduction are occurring.

It is only some 50 years ago that biophysicists obtained reliable experimental methods for estimating the molecular weights of biological macromolecules, chiefly as a result of Svedberg's work in developing the ultracentrifuge as an analytical instrument (Svedberg & Pedersen, 1940). Having gained some understanding of the size of proteins, interest soon thereafter turned to the shape and rotational relaxation times of these molecules, and Perrin's work on fluorescence depolarization helped to lay the foundations there (Perrin, 1929). Biophysics had to wait for the development of X-ray spectroscopy of proteins and nucleic acids to provide a picture of the interior structure of biological macromolecules.

The dynamics of macromolecules, the subject of this symposium, are most directly studied by simply looking through a microscope and observing the molecular motion. With a microscope, we can resolve the size and shape of large particles, as well as monitor dynamic motion. For smaller particles, particularly single macromolecules, we cannot resolve the size or shape; but it is still possible to observe the motion, if we can make the particles appear as bright points of light sprinkled dilutely over a dark background. Siedentopf & Zsigmondy (1903) demonstrated this fact with a device which came to be called the ultramicroscope.

The concept of processes in a living organism as of a most complex enzyme-catalysed system of coupled chemical conversions following both physical and chemical laws, opens up new possibilities of cognizance and control of vital processes.

The preceding paper by Douglas Magde has recounted the basic principles of Fluorescence Correlation Spectroscopy (FCS) as originally described (see Magde, Elson & Webb, 1972; Elson & Magde, 1974; Magde, Elson & Webb, 1974 Elson & Webb, 1975; referred to collectively as MEW), and has described the first application to chemical kinetics. In this paper I shall first illustrate the same principles of FCS with a simple graphical demonstration model based on the scheme for application to lateral diffusion in membranes as it was developed in our laboratory by Dr T. J. Herbert; I shall then proceed to discuss some current research in our group organized jointly with Professor E. L. Elson at Cornell.