B12-Insulin Bioconjugate/Transcobalamin(II)/Insulin Receptor Cover Image For The April Issue Of Clinical Chemistry

A brief post about some free research press (and the new addition to the Cover Gallery). Having already been featured on the cover of the ChemMedChem March 2009 issue (see the New B12-Insulin-TCII-Insulin Receptor Cover Image For This Month's ChemMedChem (March 2009) post) , the side-on view of the B12-Insulin/TCII/Insulin Receptor structure was chosen for this month's cover of Clinical Chemistry. While the originating article itself is not included in the issue (I should have recommended citing the ChemMedChem article in the image caption), several diabetes-related articles are featured in this month's issue.

ON THE COVER: Scientists are investigating ways to develop effective oral insulin therapies. One such model is a vitamin B12-insulin conjugate bound to transcobalamin II and is shown here docked in the insulin receptor. The discovery of easier ways to deliver insulin into the blood stream would improve the lives of the millions of individuals living with diabetes. This month's issue of Clinical Chemistry contains 4 articles related to diabetes. The first 2 articles provide readers with a point/counterpoint discussion of the value of reporting estimated glucose along with Hb A1c. Next is an article on the association of apolipoprotein B with incident type 2 diabetes. Lastly, the development of the first radioimmunoassay for insulin led to a Nobel Prize and is chronicled in this month's Citation Classic feature. (See pages 545, 547, 666, and 671.) Image reproduced with permission from Damian G. Allis and Robert P. Doyle, Department of Chemistry, Syracuse University.

As a brief explanation of the image, this "scene" is meant to show (without proper molecular dynamics simulations to show how well it would work) that the Transcobalamin(II) transport/protection protein for cobalamin/cyanocobalamin (vitamin B12) and the B12-insulin bioconjugate discussed in the ChemMedChem article is small enough to fit within the Insulin Receptor protein such that insulin may still be able to bind to its receptor. This is the final piece of the puzzle in the proposed mechanism (and experimentally demonstrated event) by which the B12-insulin bioconjugate retains all of the benefits of free B12 (transport from the digestive system to the bloodstream) and insulin (proper receptor binding and the subsequent induction of cellular glucose uptake).

The figure caption and April 2010 Table of Contents can be found in PDF format at the Clinical Chemistry website (with a local copy of the PDF also available HERE.

www.somewhereville.com/?page_id=985
www3.interscience.wiley.com/journal/122250806/issue
www.somewhereville.com/?p=511
www.clinchem.org
en.wikipedia.org/wiki/Diabetes
en.wikipedia.org/wiki/Molecular_dynamics
en.wikipedia.org/wiki/Cyanocobalamin
en.wikipedia.org/wiki/Vitamin_B12
en.wikipedia.org/wiki/Bioconjugate
en.wikipedia.org/wiki/Insulin_receptor
www.clinchem.org/content/vol56/issue4/

Exploring the Implications of Vitamin B12 Conjugation to Insulin on Insulin Receptor Binding and Cellular Uptake

In press, in the journal ChemMedChem (and, because I think it's hip, I note that the current "obligatory" image for the wikipedia article for ChemMedChem features the image I made for the review article on the topic addressed in this new study). As with many theory papers (there's some experiment in there, too), this very brief article summarizes several months of cyanocobalamin (B12) parameterization and molecular dynamics (MD) simulations. The purpose of the theory was to address all of the major structural snapshots in the uptake process associated with the insulin-B12 bioconjugate being developed as part of the much heralded oral insulin project in Robert Doyle's group here at Syracuse. These structures include:

1. The structure and dynamic properties of the insulin-B12 bioconjugate
2. The binding of B12 to Transcobalamin II (TCII) (for B12 parameterization)
3. The binding of the insulin-B12 bioconjugate to TCII (and the steric demands therein)
4. The interaction of the insulin-B12 bioconjugate, bound to TCII, with the insulin Receptor (IR)

The quantum chemical (for the B12 geometry and missing force constants) and molecular dynamics (GROMACS with the GROMOS96 (53a6)) simulation work is going to serve as the basis for several posts here (eventually) about parameterization, topology generation, and force field development.

As an example of some of the insights modeling provides, the figure above shows the insulin-B12 bioconjugate (the insulin is divided into A and B chains, the A chain in blue and the important division of the insulin B chain in the front half of the rainbow). Insulin is a rather large-scale example of many of the same molecular issues that arise in the analysis of solid-state molecular crystals by either terahertz or inelastic neutron scattering spectroscopy. The packing of molecules in their crystal lattices can lead to significant changes in molecular geometry, be these changes in the stabilization of higher-energy molecular conformations or even deformations in the covalent framework. In the case of insulin, it is found that the crystal geometry (also the geometry of stored insulin in the body) is quite different from the solution-phase form. It's even worse! The B chain end (B20-B30) in the solid-state geometry covers (protects?) the business-end of the insulin binding region to the Insulin Receptor. One can imagine the difficulty in proposing the original binding model for insulin to its receptor from the original crystal data given that the actual binding region is blocked off in the solid-state form! The "Extended" form in the figure is representative of "multiple other" conformations of the B20-B30 region (which mimics the characterized T-state of insulin), those geometries for which the insulin binding region (blue and green) is completely exposed. This extended geometry is also the one that separates the bulk of the insulin structure from the covalently-linked B12 (at Lys29) and, it is argued from the MD simulations in the paper, enables the B12 to still tightly bind to TCII despite the presence of all this steric bulk.

Amanda K. Petrus1, Damian G. Allis1, Robert P. Smith2, Timothy J. Fairchild3 and Robert P. Doyle1

1. Department of Chemistry, Syracuse University, Syracuse, NY 13244, USA
2. Department of Construction Management and Wood Products Engineering, SUNY, College of Environmental Science and Forestry, Syracuse, NY 13210, USA
3. Department of Exercise Science, Syracuse University, Syracuse, NY 13244, USA

Extract: We recently reported a vitamin B12 (B12) based insulin conjugate that produced significantly decreased blood glucose levels in diabetic STZ-rat models. The results of this study posed a fundamental question, namely what implications does B12 conjugation have on insulin's interaction with its receptor? To explore this question we used a combination of molecular dynamics (MD) simulations and immuno-electron microscopy (IEM).

www3.interscience.wiley.com/journal/110485305/home
en.wikipedia.org
en.wikipedia.org/wiki/Chemmedchem
www3.interscience.wiley.com/journal/116323633/abstract
en.wikipedia.org/wiki/Cyanocobalamin
en.wikipedia.org/wiki/Molecular_dynamics
en.wikipedia.org/wiki/Insulin
chemistry.syr.edu/faculty/doyle.html
chemistry.syr.edu/faculty/doyle_group/index.html
www.syr.edu
en.wikipedia.org/wiki/Quantum_chemistry
www.gromacs.org
en.wikipedia.org/wiki/Terahertz
en.wikipedia.org/wiki/Inelastic_neutron_scattering
chemistry.syr.edu
www.syr.edu
www.esf.edu

DNA-Specific (But Generally Applicable) AMBER With GROMACS 3.3.x: Installation And Notes

The following is the full procedure for installing the AMBER force field port for GROMACS (AMBER-in-GROMACS, AMBER-with-GROMACS, AMBER-on-GROMACS, whatever you want to call it) developed by Eric Sorin at California State University, Long Beach, providing a bit more depth in the installation process (specifically for GROMACS 3.3.x) and a few modified GROMACS files.

As brief background, AMBER (Assisted Model Building and Energy Refinement) is one of THE dominant molecular mechanics/molecular dynamics (MM/MD) force fields used today in biochemical simulations. The motivation for this page (my installing AMBER for use in GROMACS) stems from the current Nanorex focus on Structural DNA Nanotechnology (SDN) modeling, for which we're working on a reduced model force field for large-structure energy minimizations and, importantly, integrating the GROMACS MM/MD package for use via our CAD interface. You can read more about this in the poster presented at FNANO08 this past April. As a force field validated for DNA simulations, AMBER meets our needs of performing atomistic simulations on DNA nanostructures. While NAMD is also a possibility for DNA simulations, GROMACS meets Nanorex's open source needs.

Needless to say, finding that Sorin and co-workers had ported AMBER to GROMACS was a wonderful discovery (and that this same port appears to be driving the Folding@Home project). The installation directions on the AMBER-on-GROMACS website are good, but they don't mention a few important steps that I spent no small amount of time trying to figure out (and I'm definitely not complaining. 99% of the work was the porting, which has been graciously handed to the GROMACS community on a silver platter). Below is the complete set of steps required to get, as the first example, the Dickerson.pdb sample file provided with the porting files to energy minimize and MD correctly. Issues related to DNA-based simulations not using the Dickerson.pdb file will be covered in another post.

Dickerson DNA

QuteMol rendering of Dickerson.pdb (from Drew, H. R., Wing, R. M., Takano, T., Broka, C., Tanaka, S., Itakura, K. & Dickerson, R. E. (1981). Structure of a B-DNA dodecamer: conformation and dynamics. Proc. Natl. Acad. Sci. USA 78, 2179-2183. Protein Data Bank structure 1BNA).

Install/compile a custom installation of GROMACS

The AMBER port to GROMACS does not require modification of the GROMACS code but does require a few changes to force field text files. In the interest of remembering what was changed and what wasn't, I recommend a custom compilation of GROMACS or, at the very least, the installation of another copy of GROMACS you can modify the top directory of without risking changes to an already working GROMACS version.

If you don't know how to compile your own version of GROMACS, I recommend taking a look at Compiling Single-Precision And Double-Precision GROMACS 3.3.3 With OpenMPI 1.2.6 Under OSX 10.5 (Leopard).

My installation directory is /usr/local/gromacs333-amber (which I will refer to here as /ULG-A/ and all directory calls will be to this. Obviously, change all directory calls to match yours.

Download GROMACS-Ported AMBER force field files

I'm running a double-precision MPI version of GROMACS 3.3.3, meaning I'm using the GROMACS-ported AMBER force field files for GROMACS 3.3.1 (May 2006). And get the version "with pdfs." If you're going to be doing these calculations, you should know where the math come from!

Copy vdwradii.dat and aminoacids-NA.dat into /usr/local/gromacs-amber/share/gromacs/top

You will need to be logged in as root to do this (sudo cp FILENAME /ULG-A/top). The AMBER port file vdwradii.dat differs from the vdwradii.dat file in the /ULG-A/top directory of the GROMACS install by the addition of 5 lines:

???  P     0.15
???  LP1   0
???  LP2   0
LYSH MNZ1  0
LYSH MNZ2  0

With no removals or number changes.

The aminoacids.dat file is a little different. In order to use pdb2gmx to generate nucleic acid topology files, GROMACS requires the list of residue codes that reside in aminoacids-NA.dat. So far as I can tell, using aminoacids-NA.dat instead of aminoacids.dat does not change the handling of amino acids (why two files exist for a reason other than amino acid-centric organization is beyond my pay scale), so we'll be using aminoacids-NA.dat exclusively.

Move/delete/rename aminoacids.dat and rename aminoacids-NA.dat to aminoacids.dat

The aminoacids-NA.dat file includes 32 additional residue codes, accounting for the 3' (includes terminal H atom), 5' (included terminal H atom), in-strand (nucleic acid repeat unit) and molecule (individual hydrogen-terminated sugar and nucleic acid) topology information codes for DNA and RNA found in the .rtp files

Copy the ffamber files of interest into the /ULG-A/top directory

If you're doing this in Linux or OSX, you might as well move all of the files you want installed into a single directory and su cp * /ULG-A/top. Note that the .itp and .gro files in your ffamber_v3.3.1 (for the 3.3.x GROMACS) are the same for all of the AMBER force field flavors you can install. Further, all of the ffamber* files have unique names (94, 99, 03, etc.), so you can install all the force field files into /ULG-A/top and use whichever.

Modify /ULG-A/top/FF.dat to reflect the new force field files

The FF.dat file as installed by GROMACS looks like the following:

9
ffG43a1  GROMOS96 43a1 force field
ffG43b1  GROMOS96 43b1 vacuum force field
ffG43a2  GROMOS96 43a2 force field (improved alkane dihedrals)
ffG45a3  GROMOS96 45a3 force field (Schuler JCC 2001 22 1205)
ffG53a5  GROMOS96 53a5 force field (JCC 2004 vol 25 pag 1656)
ffG53a6  GROMOS96 53a6 force field (JCC 2004 vol 25 pag 1656)
ffoplsaa OPLS-AA/L all-atom force field (2001 aminoacid dihedrals)
ffencadv Encad all-atom force field, using scaled-down vacuum charges
ffencads Encad all-atom force field, using full solvent charges    

To include the AMBER force field files (so that these separate files can be called during pdb2gmx), we modify the FF.dat file by (1) incrementing the total number of force fields and (2) adding the force field code and text description to this file. The new FF.dat file will look like this if you added all of the ffamber force field files:

17
ffG43a1  GROMOS96 43a1 force field
ffG43b1  GROMOS96 43b1 vacuum force field
ffG43a2  GROMOS96 43a2 force field (improved alkane dihedrals)
ffG45a3  GROMOS96 45a3 force field (Schuler JCC 2001 22 1205)
ffG53a5  GROMOS96 53a5 force field (JCC 2004 vol 25 pag 1656)
ffG53a6  GROMOS96 53a6 force field (JCC 2004 vol 25 pag 1656)
ffoplsaa OPLS-AA/L all-atom force field (2001 aminoacid dihedrals)
ffencadv Encad all-atom force field, using scaled-down vacuum charges
ffencads Encad all-atom force field, using full solvent charges
ffamber94 AMBER94 Cornell et al. (1995), JACS 117, 5179-5197
ffamber96 AMBER96 Kollman (1996), Acc. Chem. Res. 29, 461-469
ffamberGS AMBER99GS Garcia & Sanbonmatsu (2002), PNAS 99, 2782-2787
ffamberGSs AMBER99GSs Nymeyer & Garcia (2003) PNAS 100, 13934-13939
ffamber99 AMBER99 Wang et al. (2000), J. Comp. Chem. 21, 1049-1074
ffamber99p AMBER99p Sorin & Pande (2005). Biophys. J. 88(4), 2472-2493
ffamber99SB AMBER99sb Hornak et. al (2006). Proteins 65, 712-725
ffamber03 AMBER03 Duan et al. (2003), J. Comp. Chem. 24, 1999-2012

Add and increment numbers as appropriate for the force field you want to use. You can download the complete FF.dat at the following link and comment:

Download amber_FF.txt, delete "amber_", change ".txt" to ".dat", place in /ULG-A/top

source /UGL-A/bin/GMXRC

The GMXRC file contains path-dependent settings for your shell.

In theory, you're all done. At least, this is the end of the installation process according to the official AMBER-on-GROMACS list. The IMPORTANT NOTES section contains relevant information but does not complete the installation. The additional steps are provided below.

Modify the /ULG-A/top/spc.itp file

If you're running through a complete energy minimization calculation with the website installation followed, your first source of error (hopefully) comes in the form of…

Program grompp_amber, VERSION 3.3.3
Source code file: toppush.c, line: 1193

Fatal error:
[ file "/usr/local/gromacs333_amber/share/gromacs/top/spc.itp", line 32 ]:
Atom index (1) in bonds out of bounds (1-0).
This probably means that you have inserted topology section "bonds"
in a part belonging to a different molecule than you intended to.
In that case move the "bonds" section to the right molecule.

The origin of this error is the lack of amberXX water parameters in the spc.itp file (nothing directly to do with the reported GROMACS error). The fix for this is straightforward, simply modifying the spc.itp file in the /ULG-A/top directory as follows below. The original spc.itp file below…

[ moleculetype ]
; molname       nrexcl
SOL             2

[ atoms ]
;   nr   type  resnr residue  atom   cgnr     charge       mass
#ifdef _FF_GROMACS
1     OW      1    SOL     OW      1      -0.82
2     HW      1    SOL    HW1      1       0.41
3     HW      1    SOL    HW2      1       0.41
#endif
#ifdef _FF_GROMOS96
#ifdef HEAVY_H
1     OW      1    SOL     OW      1      -0.82    9.95140
2      H      1    SOL    HW1      1       0.41    4.03200
3      H      1    SOL    HW2      1       0.41    4.03200
#else
1     OW      1    SOL     OW      1      -0.82   15.99940
2      H      1    SOL    HW1      1       0.41    1.00800
3      H      1    SOL    HW2      1       0.41    1.00800
#endif
#endif
#ifdef _FF_OPLS
1  opls_116   1    SOL     OW      1      -0.82
2  opls_117   1    SOL    HW1      1       0.41
3  opls_117   1    SOL    HW2      1       0.41
#endif

#ifdef FLEXIBLE
[ bonds ]
; i     j       funct   length  force.c.
1       2       1       0.1     345000  0.1     345000
1       3       1       0.1     345000  0.1     345000

[angles ]
; i     j       k       funct   angle   force.c.
2       1       3       1       109.47  383     109.47  383
#else
[ settles ]
; OW    funct   doh     dhh
1       1       0.1     0.16330

[ exclusions ]
1       2       3
2       1       3
3       1       2
#endif

is modified to this…

[ moleculetype ]
; molname       nrexcl
SOL             2

[ atoms ]
;   nr   type  resnr residue  atom   cgnr     charge       mass
#ifdef _FF_GROMACS
1     OW      1    SOL     OW      1      -0.82
2     HW      1    SOL    HW1      1       0.41
3     HW      1    SOL    HW2      1       0.41
#endif
#ifdef _FF_GROMOS96
#ifdef HEAVY_H
1     OW      1    SOL     OW      1      -0.82    9.95140
2      H      1    SOL    HW1      1       0.41    4.03200
3      H      1    SOL    HW2      1       0.41    4.03200
#else
1     OW      1    SOL     OW      1      -0.82   15.99940
2      H      1    SOL    HW1      1       0.41    1.00800
3      H      1    SOL    HW2      1       0.41    1.00800
#endif
#endif
#ifdef _FF_OPLS
1  opls_116   1    SOL     OW      1      -0.82
2  opls_117   1    SOL    HW1      1       0.41
3  opls_117   1    SOL    HW2      1       0.41
#endif

#ifdef _FF_AMBER94
; also applies to FF_AMBER96, FF_AMBERGS, and FF_AMBERGSs
1  amber94_42   1  SOL     OW      1      -0.82  15.99940
2  amber94_27   1  SOL    HW1      1       0.41   1.00800
3  amber94_27   1  SOL    HW2      1       0.41   1.00800
#endif

#ifdef _FF_AMBER99
; also applies to FF_AMBER99P, FF_AMBER99SB, FF_AMBER03
1  amber99_54   1  SOL     OW      1      -0.82  15.99940
2  amber99_55   1  SOL    HW1      1       0.41   1.00800
3  amber99_55   1  SOL    HW2      1       0.41   1.00800
#endif



#ifdef FLEXIBLE
[ bonds ]
; i     j       funct   length  force.c.
1       2       1       0.1     345000  0.1     345000
1       3       1       0.1     345000  0.1     345000

[ angles ]
; i     j       k       funct   angle   force.c.
2       1       3       1       109.47  383     109.47  383
#else
[ settles ]
; OW    funct   doh     dhh
1       1       0.1     0.16330

[ exclusions ]
1       2       3
2       1       3
3       1       2
#endif

This FFAMBER-included spc.itp file is downloadable in the following form and comment:

Download amber_spc.txt, delete "amber_" change ".txt" to ".itp", place in /ULG-A/top

At this point, pdb2gmx, editconf, genbox, grompp, and genion should all work without error. The next problem comes with the post-genion grompp, which can't find ion information…

Modify ions.itp

Similar to the spc.itp error, the error that comes up with the inclusion of counterions confounds the mind because everything seems to be in place.

Program grompp_amber, VERSION 3.3.3
Source code file: toppush.c, line: 1396

Fatal error:
No such moleculetype Na

With Na being whatever counterion you're trying. The AMBER-on-GROMACS website reports the following for AMBER ion inclusion:

Our AMBER ports include common ion definitions, which are listed in the ffamber*.rtp files (just below the TIP water models). This allows the AMBER ports to be used without modification or use of the GROMACS ions.itp file. At the moment these include Cl- , IB+, Na+, K+ , Rb+, Cs+, Li+ , Ca2+, Mg2+, Zn2+, Sr2+, and Ba2+. If your pdb file has ions present and pdb2gmx does not properly convert those ions, please check the atom and residue name of your ions and rename them if necessary to agree with the .rtp file. If you are using ion-related GROMACS tools, such as genion, you will need to enter the AMBER ion definition to the ions.itp file in the "top" directory of the GROMACS distribution.

The bases for the ion use are in the .rtp files, but the implementation, at least for GROMACS 3.3.3 and possibly earlier versions, is not as found in these files.

The proper format for both the FFAMBER_94 and FFAMBER_99 ion types in ions.itp is as follows:


#ifdef _FF_AMBER94

[ moleculetype ]
; molname       nrexcl
Na              1
[ atoms ]

; id    at type         res nr  residu name     at name  cgnr   charge    mass
1       amber94_31      1       Na              Na       1      1         22.99000
#endif

#ifdef _FF_AMBER99

[ moleculetype ]
; molname       nrexcl
Na              1
[ atoms ]

; id    at type         res nr  residu name     at name  cgnr   charge    mass
1       amber99_31      1       Na              Na       1      1         22.9900
#endif

This is the format used for the FF_GROMOS96 ions with one change. The atom type for the FF_AMBERXX parameters is NOT the atom label, but instead the AMBER force field label. In the case of Na, this is amber94_31 (for AMBER94) and amber99_31 (for AMBER99). Note that the mass must be specified (you have to scroll down the ions.itp file to see FF_GROMOS96. FF_GROMACS does not require mass specification).

For quick reference, here are the following [ atoms ] specifications for FF_AMBER94 and FF_AMBER_99.

#ifdef _FF_AMBER94
; id    at type         res nr  residu name     at name  cgnr   charge    mass
1       amber94_32      1       IB              IB       1      1         131.00000
1       amber94_56      1       Li              Li       1      1         6.94000
1       amber94_31      1       Na              Na       1      1         22.99000
1       amber94_51      1       K               K        1      1         39.10000
1       amber94_52      1       Rb              Rb       1      1         85.47000
1       amber94_53      1       Cs              Cs       1      1         132.90000
1       amber94_33      1       Mg              Mg       1      2         24.30500
1       amber94_15      1       Ca              Ca       1      2         40.08000
1       amber94_57      1       Zn              Zn       1      2         65.40000
1       amber94_58      1       Sr              Sr       1      2         87.62000
1       amber94_59      1       Ba              Ba       1      2         137.33000
1       amber94_30      1       Cl              Cl       1      -1        35.45000
#endif

#ifdef _FF_AMBER99
; id    at type         res nr  residu name     at name  cgnr   charge    mass
1       amber99_32      1       IB              IB       1      1         131.00000
1       amber99_56      1       Li              Li       1      1         6.94000
1       amber99_31      1       Na              Na       1      1         22.99000
1       amber99_51      1       K               K        1      1         39.10000
1       amber99_52      1       Rb              Rb       1      1         85.47000
1       amber99_53      1       Cs              Cs       1      1         132.90000
1       amber99_33      1       Mg              Mg       1      2         24.30500
1       amber99_15      1       Ca              Ca       1      2         40.08000
1       amber99_57      1       Zn              Zn       1      2         65.40000
1       amber99_58      1       Sr              Sr       1      2         87.62000
1       amber99_59      1       Ba              Ba       1      2         137.33000
1       amber99_30      1       Cl              Cl       1      -1        35.45000
#endif

To save yourself plenty of trouble, download the ions.itp file with all of the ion parameters in the form of the following link:

Download amber_ions.txt, delete "amber_", change ".txt" to ".itp", place in /ULG-A/top

With the ions.itp file complete, your GROMACS energy minimizations and dynamics simulations of amino acid and nucleic acid structures should go without hitch.

My thanks to Alan Wilter S. da Silva, D.Sc. – CCPN Research Associate, Department of Biochemistry, University of Cambridge, for catching a formatting error in the first version of the amber_ions.itp file.

If you find problems, questions, concerns, incompatibilities, etc., please let me know by setting up a user account and posting a comment in this post or email me so I can post whatever you might find.

And don't forget to cite ffAMBER if you use it!

Sorin & Pande (2005). Biophys. J. 88(4), 2472-2493

chemistry.csulb.edu/ffamber
www.gromacs.org
chemistry.csulb.edu/esorin
chemistry.csulb.edu
amber.scripps.edu
www.nanorex.com
en.wikipedia.org/wiki/DNA_nanotechnology
www.somewhereville.com/rescv/fnano08_2008_poster.jpg
www.cs.duke.edu/~reif/FNANO
www.ks.uiuc.edu/Research/namd
en.wikipedia.org/wiki/Open_source
folding.stanford.edu
qutemol.sourceforge.net
www.pdb.org
www.pdb.org/pdb/explore/explore.do?structureId=1BNA
www.linux.org
www.apple.com/macosx
www.gromacs.org/documentation/reference/online/pdb2gmx.html