Hi Kristof, 

One approximate solution would be to measure T1e of MTSL free radical using EPR saturation recovery experiments. Having said that I also say that EPR spectra of these radicals are not straight forward. I myself had stuck up in research during my PhD days on searching for electron spin-lattice and spin-spin relaxation rates for Metal-organic complex Cu-cyclam. I searched for MTSL EPR on google and found a result ( https://openaccess.leidenuniv.nl/bitstream/handle/1887/11002/03.pdf?sequence=9 ) go to page 52 and see the spectra of free MTSL there you will see it is quite complicated. The linewidths (that too of solids) at RT of EPR spectra are too broad. So normally EPR guys go to low temperature to observe signal. 

I will try to have a look at some data from EPR people I know. I will get abck to you soon. 

By the way, I see that you work in proteins, Are you trying to measure distances using Solomon's rate equation with correlation time tc~T1e electron spin lattice relaxation as parameter??

Regards, 

Shashi

Generally speaking, the answer to your question critically depends on the temperature that you want to reach in solid state.

You can get a number of approximates:

A) check for literature values. The T1 of a radical depends mainly on the chemistry around the NO moiety, thus having it free or bound to a protein makes (almost) no difference (unless the NO is pointing in some cavity of the protein)

B) assuming that electron relaxation mechanisms are all dispersed even at X-band, get your hands on a pulsed epr instrument and measure directly on your ssnmr sample at your target temperature. 

C) if you are going to work at room temperature measure by NMRD (http://scitation.aip.org/content/aip/journal/jcp/81/9/10.1063/1.448147)

Cheers, 

E.