Structure and dynamics of acrylodan labeled A-crystallin tetramer formed in the presence of a bile salt (sodium deoxycholate, NaDC) has been studied using fluorescence correlation spectroscopy (FCS) and femtosecond up-conversion techniques. Using FCS it is shown that, the diffusion constant (D t) of the A-crystallin oligomer (mass ∼800 kDa) increases from ∼35 m 2 s -1 to ∼68 m 2 s -1. This corresponds to a decrease in hydrodynamic radius (r h) from ∼6.9 nm to ∼3.3 nm. This corresponds to about 10-fold decrease in molecular mass to ∼80 kDa and suggests formation of a tetramer (since mass of A-crystallin monomer is ∼20 kDa). The steady state emission maximum and average solvation time ( s) of acrylodan labeled at cysteine 131 position of A-crystallin is markedly affected on addition of NaDC, while the tryptophan (trp-9) becomes more exposed. This suggests that NaDC binds near the cys-131 and makes the terminal region of A-crystallin exposed. This may explain the enhanced auto-phosphorylation activity of A-crystallin near the terminus of the 173 amino acid protein (e.g., at the threonine 13, serine 45, or serine 169 and 172) and suggests that phosphorylation at ser-122 (close to cys-131) is relatively less important. © 2012 American Institute of Physics.