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Construction of hydrogen positions

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Generation of Hydrogen Bonds

The generation of hydrogen bonds is one of the major steps in analyzing the energy of a system. This energy term is not usually used in minimization or dynamics because modern parameter sets compute hydrogen bond contributions as a balance between electrostatic attraction and van der Waal repulsion terms. This facility remains useful for the purpose of enery and structural analysis. The process of hydrogen bond generation involves looking at all possible pairs of hydrogen bond donors and acceptors and selecting those which are “good”. The meaning of “good” is determined by parameters to be described below. In addition, the generation routine is responsible for constructing the positions of all uncoordinated hydrogens and adding them into the coordinate list.

The selection of hydrogen bonds involves three checks. First, any good hydrogen bond has a length less than some cutoff. Second, the angle off linearity has a value less than some cutoff. This angle is 180 - D–H...A. Finally, if a hydrogen donor has more than one acceptor which satisfies the above constraints and BEST is specified, the routine will select the one with the lowest energy (normally it will take ALL and let the minimization or dynamics adjust there strengths).

To obtain a more detailed description of the selection process and the process of constructing hydrogen coordinates, the CHARMM paper should be consulted.

Because there are cutoff’s involved with the selection of hydrogen bonds, and because the hydrogen bond list must be updated during dynamics, and because energy must be conserved, switching functions are needed to smooth the transition over a cutoff. Therefore, the specification of hydrogen bond generation also allows the specification of switching function parameters.

One should note that particular choices for the selection process will never conserve energy in a dynamics run. First, one must fix the hydrogen bond list if one uses the extended atom representation. This is necessary as one cannot apply a switching function to the hydrogen bond angle as it is not calculated if the hydrogens are not present. Second, the selection of the best hydrogen bond for a given donor can’t be used, because there is no switching function to smooth the transition between two possible and mutually exclusive hydrogen bonds.

The generation is performed by CHARMM at several different points. One can request the hydrogen bonds be generated explicitly using a hydrogen bond command. This is useful prior to analyzing the system. The hydrogen bonds can be generated during any energy manipulation, see Energy Manipulations: Minimization and Dynamics.

Syntax of the Hydrogen Bond Command

{ HBONds       }        { [IHBFrq integer]  hbond-spec  }
{ UPDAte ...   }        { [IHBFrq  0     ]              }
{ MINImize ... }        {                               }
{ DYNAmics ... }        {                               }
{ ENERgy ...   }        {                               }

hbond-spec ::= [BEST] [DUMMy] [CUTHB real] [CUTHBA real] [ACCE] [INIT]
               [ALL ]                                    [NOAC]

        [HBEXclude] [CTONHB real] [CTOFHB real] [CTONHA real] [CTOFHA real]
        [HBNOexcl ]

Note

The IHBFrq value is remembered. If its value is zero, interpretation of [hbond-spec] will be supressed as well as any modifications to the hbond list.

[SYNTAX HBTRim]

HBTRim  real

[SYNTAX DONOr]
[SYNTAX ACCEptor]


[ DONOr    ]  [ REMOve ] atom-selection [NOANtecedents] [SHOW]
[ ACCeptors]  [ ADD    ]
              [ SET    ]

Purpose of the various hydrogen bond variables

Variable Default Function
ACCE/NOAC ACCE ACCE specifies that acceptor anticedents will be used in an (H-A-AA) angle factor where present in the structure file (from the RTF).
HBEX/HBNOexclude   HBEXclude causes all hydrogen bonds between excluded atoms to be removed in the hbond edit facility. This also includes 1-4 interaction if appropriate as determined by the NBXMode nonbond value. This option is needed for systems where no angle cutoff is applied (as in the AMBER potential).
BEST/ALL ALL BEST turns on selection of best hydrogen bond for a given donor. ALL takes all hydrogen bonds for given donor which satisfy the other conditions.
DUMMy   Sets CUTHB and CUTHBA to zero. This will result in no hydrogen bonds which is desirable when one is not interested in the hydrogen bond energy. The selection will be done very quickly in this case.
CUTHB 4.5 Maximum distance allowed for a hydrogen bond. This distance is measured between the heavy atoms NOTE: a CUTHB value less than 1.0 will disable the HBOND generation code (for efficiency).
CTOFHB CUTHB-0.5 Distance where distance switching function is off Once specified, it will only change if respecified.
CTONHB CTOFHB-0.5 Distance where distance switching function is on. Once specified, it will only change if respecified.
CUTHBA 90.0 Maximum out of line angle allowed for a hydrogen bond. The angle is 180 - D–H...A angle
CTOFHA CUTHBA-20.0 Angle where angle switching function is off Once specified, it will only change if respecified.
CTONHA CTOFHA-20.0 Angle where angle switching function is on. Once specified, it will only change if respecified.
INIT do not INIT specifies that all values and conditions return to the original defaults.

HBTRim

The HBTRim command deletes all hydrogen bonds that have an energy of interaction that is higher than the specified cutoff. This command is used to reduce a list of all hydrogen bonds to that of important hydrogen bonds.

The syntax is;

HBTRim  real

where the real value is the energy cutoff and should usually be negative.

DONER

[ DONOr    ]  [ REMOve ] atom-selection [NOANtecedents] [SHOW]
[ ACCeptors]  [ ADD    ]
              [ SET    ]

These commands modify the list of donors or acceptors contained in the structure file (PSF). By invoking this command before the generation of a hydrogen bond list, the content of this list can be manipulated. The REMOve suboption will delete any current donors/acceptors contained in the selected atoms. The ADD suboption will create additional donors/acceptors based on the selected atoms. The SET suboption will remove all current donors/acceptors and replace the list with the selected set of atoms.

By default, any single atom bonded to a selected atom will be included as the donor/acceptor antecedent. To prevent this, the NOAN keyword can be used to generate simple (radial only) hydrogen bonds. The SHOW keyword causes the new list of donors/acceptors to be displayed.

Note

All modifications listed above are strictly based on the atom-selection, without taking into account any DONOr/ACCeptor specifications in either the PSF or the RTF. If your selection includes aliphatic carbons they may end up as donors/acceptors...

To add water hydrogens as hydrogen bond donors a command like this would be used:

DONOR ADD SELECT RESN TIP3 .AND. HYDROGEN END

To remove all backbone carbonyls as acceptors:

ACCEPTOR REMOVE SELECT ATOM PROTA * O END

Substitution variables NDON/NACC are set to the resulting number of donors/acceptors.

See Input-Output Commands for commands that input/output HBONd information.