FCS and Spatial Correlations on Biological Surfaces

Abstract

Fluorescence correlation spectroscopy (FCS) provides information about the occupation number and the dynamics of molecules in a small volume. Much of the work presented in this volume is focused on extracting information about the dynamics. Indeed, FCS represents one of the most versatile tools for studying dynamics because of the sensitivity and selectivity afforded by fluorescence and the large range of time scales accessible through the fluctuation analysis. The technique is limited at short times by the speed of electronic data collection (nanoseconds) and at long times by the length of time the molecules spend in the observation volume (milliseconds to seconds). This chapter will focus on extracting information about occupation numbers with particular emphasis on interpretations in terms of inter-molecular interactions, such as formation of clusters of like molecules or aggregates of different molecules. With this focus in mind, the dynamic information is secondary and even irrelevant. In fact, it now becomes possible to examinesystems in which temporal fluctuations are either very slow or completely absent. The key is to recognize that information about occupation numbers can be obtained as easily by the analysis of spatial fluctuations as by theanalysis of temporal fluctuations.