set rdist 3
set hbdist 3.5
set hbang 180

set acceptor "segname AP1 and name N and resid 143 144 145"
set donor "water"

set residue 1 

if { $residue } {
    set selectiontxt "same residue as (${donor} and within ${rdist} of ${acceptor})"
} else {
    set selectiontxt "${donor} and within ${rdist} of ${acceptor}"
}

set accsel [atomselect top "${acceptor}"]
set donsel [atomselect top "${selectiontxt}"]

set output [open "wat_hbonds.txt" w] 

set nframes [molinfo top get numframes] 
for {set i 0} {$i < $nframes} {incr i} {
    animate goto $i
    $donsel update
    
    set hbcurr [measure hbonds $hbdist $hbang $donsel $accsel]
    set idxlst [lindex $hbcurr 0]
    
    if { [llength $idxlst ] } {
        set rsel [atomselect top "index ${idxlst}"]
        set donres [lsort -unique -integer [$rsel get resid]]
        $rsel delete
        puts $output "$i $donres" 
    }
}

$accsel delete
$donsel delete

close $output