# Re: AMBER: PTRAJ:"hbond" analysis

From: Vineet Pande <pande_vineet.hotmail.com>
Date: Thu, 31 Mar 2005 09:17:57 +0000

Dear Prof. Cheatham:
Thanks a lot for your detailed reply. I think now its much better to
understand for me whats going on.
Regards,
vineet

>From: "Thomas E. Cheatham, III" <cheatham.chpc.utah.edu>
>To: amber.scripps.edu
>Subject: Re: AMBER: PTRAJ:"hbond" analysis
>Date: Wed, 30 Mar 2005 10:40:16 -0700 (Mountain Standard Time)
>
>
> > I Used Ptraj for H-bond analysis. I have the following doubts >>>
>
>Doubts or results?
>
> > acceptor WAT O H1
> > acceptor WAT O H2
> >
> > hbond series HBOND6B time 2 \
> > out wat_hbond_63OE2.dat solventneighbor 5 \
> > solventacceptor WAT O H1 solventacceptor WAT O H2 \
>
>OK, so you are looking at the interaction of *specific* waters (via the
>acceptor command) and general (indistinguishable) waters with the
>solventacceptor command.
>
> > (1) The distances look OK, but ALL the angles are too low. Since I
> > used a cutoff of 120 deg (default), why these 20, 30, 40 etc.
> > degrees are showing up?
>
>By default, the distance cutoff is 3.5 A and the angle is 120.0 degrees,
>but this is an admittedly funny definition of the angle as I currently
>look back at my code.
>
>The angle is calculated as the angle between the
>
> donor -- acceptorH -- acceptor
>
>If you had a linear h-bond, this angle would be 180.0. The angle cutoff
>is, at 120.0, stating that the angle formed must be greater than 120.0.
>
>However, the angle *stored*, for some reason is 180.0 - angle, such that a
>linear hbond would be zero degrees. Therefore, a value of ~30-40 is a
>good hbond...
>
> > (2) What does that last column mean :
> > |.-xxo**o |
>
>This is a schematic of occupancy as a function of time from the beginning
>to the end of the trajectory; the darker the symbol (loosely), the greater
>the occupancy; at the top of the .dat file you will see the definitions,
>but basically for this case...
>
> |.-xxo**o |
>
>...you show partial occupancy at the beginning, getting larger and then
>going away at the end of the trajectory (i.e. it is no longer formed).
>
> > (3) What's the difference between "%occupied", "lifetime" and
> > "maxocc"....?
> >
> > (4) Reading the 1st and 5th rows:
> >
> | 45.83 2.699 (0.11) 17.10 ( 8.78) 17.2 ( 13.0) 68 |.-xxo**o
>
>%occupied is the percent of time the hbond is formed over the trajectory
>(i.e. 45.83%). 2.699 and 17.10 are the distances and angles (along with
>stddev in parenthesis). Lifetime is a measure of the average time the
>hbond is formed, i.e. we sum up all the times from intervals where
>hbond is present and divide by the number of intervals, so this basically
>states that you average lifetime (or amount of time the h-bond was formed)
>at 17.2 ps (assuming you set the time variable correctly). Max occupied
>is the maximum number of (contiguous) frames (minus instantaneous gaps)
>the h-bond was occupied; in your case this is 68 frames * 2 ps or 136 ps.
>
> > IS IT SAFE TO SAY that WAT9563 showed 68+19=87% maximum occupancy
> > and 45.83+18.83=64.66% occupancy.....if its correct, then what's the
> > difference between two options.
>
>Yes, according to your definition of a h-bond (i.e. 3.5 A cutoff and 120
>degree angle criteria) the single water (interacting via H1 or H2) is
>occupied 87% of the time.
>
> > (5) Reading the ultimate last row:
> > solvent donor | 5746 :363.H1 5745 :363.O | 156.83 2.695 (
> > 0.12) 16.56 ( 9.18) 10.6 ( 11.0) 68 |..........|
>
>This is more tricky and will depend your definition of solventdonor (which
>was not in your script/command). Solventdonor provides a way to look at
>the interaction of any of a list of waters or ions or what not with your
>acceptor.
>
>If you assume that solventdonor above was WAT O, this states that a water
>interacted with residue :363 156.8% of the time. How can this be? More
>than a single water can satisfy the criteria; the maximum number that can
>interact with a given donor/acceptor is given by the solventneighbor
>value. If you blow this limit, the program will warn you.
>
>hbond. It is a complicated bit of code but intends to be general.
>
>You could define solventdonor Na+ Na+ solventacceptor Na+ Na+ Na+ to look
>at ion interaction with a residue (ignoring the angle cutoff since it
>doesn't make sense), etc.
>
>Good luck.
>
>
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Received on Thu Mar 31 2005 - 10:53:00 PST
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