AMBER Archive (2008)

Subject: AMBER: improving GAFF sulfonamide parameters

From: Floris Buelens (floris_buelens_at_yahoo.com)
Date: Tue Jun 17 2008 - 04:57:46 CDT

Dear AMBER users and developers, I'm interested in free energy calculations on ligand-protein complexes and would like to thank the developers for their fantastic work on the AMBER and GAFF force fields - I'm in the process of writing up some results for a paper which will show very satisfactory agreement with experiment. However I have come across an issue with GAFF parameters for sulfonamides, for which I'd like to ask for your advice. >From my test panel, I observed a very significant systematic deviation in binding free energy for a subset of sulfonamide-containing compounds. Inspection of MD trajectories shows what look like unnatural angles of the H-N-S and N-S-O bonds, both of which are a little over 90 degrees, as opposed to the ~110 published for HF/6-31+G* calculations (Bharatam et al., Theoretical studies on S ± N interactions in sulfonamides, Tetrahedron 58, 2002, 1759 - 1764). I've attached a minimised structure of the sulfonamide used in the cited study, and here's a quick comparison of the key bond lengths and angles (left is Bharatam et al, right is minimised GAFF): N-S 1.64 1.55 S-H 1.32 1.36 S-O 1.43 1.42 N-H 1 1.01 N-S-H 101.3 108.47 N-S-O 113.1 94.38 H-N-S 108.3 94.39 The most obvious discrepancies seems to be in the N-S-O and H-N-S angles (18.7 and 13.9 degrees off), although difference the N-S bond length is also significant. This is clearly due to the 1-4 interactions between the strongly polarised N-bound hydrogens and S-bound oxygens. I've done some initial testing of angles parameters to reproduce more closely the H-N-S and N-S-O angles (I set equilibrium angles for both to 120 degrees and increased the H-N-S force constant by 50%), and this crude approach appears to resolve something around half of my observed systematic deviation. I'd be very grateful if the GAFF developers could take some time to examine the sulfonamide moeity more closely, or advise me how to generate parameters which do not compromise the force field's consistency? Many thanks for your time, Floris Buelens Department of Crystallography, Birkbeck College, London __________________________________________________________ Sent from Yahoo! Mail. A Smarter Email http://uk.docs.yahoo.com/nowyoucan.html


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