set outfile [open my_protein.txt w]
set nf [molinfo top get numframes]
set all [atomselect top "resid 54 to 103"]
for {set i 0} {$i<$nf} {incr i} {
$all frame $i
$all update
set sasa [measure sasa 1.4 $all]
puts $outfile "Frame $i, SASA $sasa" 
}
close $outfile