[ccp4bb]: REFMAC5 non-default histidine protonation

[Dirk Kostrewa <dirk.kostrewa@psi.ch>]

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Dear CCP4 users,

I've got a histidine that makes a hydrogen bond between its side chain ND1 to 
a main chain N-H. I usually refine proteins in REFMAC5 with calculated 
hydrogens (MAKE HYDR YES) and want to keep this so. In this special case, 
however, the histidine ring is rotated around its Chi2 angle partly out of 
the electron density leaving positive and negative difference densities 
indicating that this ring should be rotated back into its density (2 A 
resolution, R below 0.2). Apparently, a steric clash between the main chain 
N-H and the histidine side chain ND1-H prevents the ring from refining to its 
proper position. Is there any way to tell REFMAC5 about a non-default 
protonation of a residue, thus circumventing to switch off the calculation of 
hydrogens?

Best regards,

Dirk.

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Dirk Kostrewa
Paul Scherrer Institut
Life Sciences, OSRA/007
CH-5232 Villigen PSI, Switzerland
E-mail: dirk.kostrewa@psi.ch
Phone: +41-56-310-4722
Fax: +41-56-310-4556
WWW: http://www.sb.psi.ch
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