* Rotational relaxation time versus
rotational correlation time.
We should note that it is not uncommon
to see the term “rotational correlation time”, often denoted as tc, used in place of the Debye
rotational relaxation time. The
information content of these terms is similar since
r = 3tc but we have observed that some people become
rather fervently attached to the use of one term or the other.
In the original development of the
theories of rotational motion of fluorophores Perrin and others used the
rotational relaxation time, as originally defined by Debye in his studies on
dielectric phenomena. Only later (in
the 1950’s) during the development of nuclear magnetic resonance was the term
rotational correlation time used by Bloch.
It thus seems reasonable for fluorescence practitioners to use r
but certainly adoption of either term should not lead to confusion. In terms of anisotropy and rotational
correlation times, then, the Perrin equation would be: