Dear Professor Sheldrick
Thankyou very much for the clarity of your posting on SHELX. It is most
helpful to me. I must, and do, accept that the program has no bias towards
any particular structure for oligonucleotides.
Please could you indulge me a little longer while I seek some information
regarding how the program is used ?
Electing to set aside a manual scrutiny of patterns of systematic absence
plus weak presences in an oligonucleotide structure factor file, would it be
possible, in a study using SHELX, for example, for the researchers to supply
the algorithm with the cross-sectional dimensions of a unit cell, chosen to
accord with those of earlier studies, which is larger than might have been
chosen by workers seeking the smallest unit cell consonant with the pattern
of systematic absences plus weak presences ?
If that situation were possible, might that result in an approximate
electron density map, prior to refinement, in which an X or Y axis, or both,
had a scale inflated by the appropriate factor (e.g., root 3 in hexagonal
and R3 lattices, and by 2 on the long side of the unit cell in P212121
lattices) ?
Clarity on this point is very important to me because I notice that the
widely reported diameter of each sugar-phosphate helix in the true
side-by-side model of Lee et al. found in AFM & STM, and in other studies,
at values around 1.2 - 1.3 nm, is just about root 3 smaller than the double
helix diameter at about 2 - 2.2 nm.
Let me thank you in anticipation of your help on this.
Clive Delmonte
