J. A. D. Jeffreys
Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow GI IXL, Scotland Conventional drawings of the a-helix show the view across the axis of the coil. This aspect requires some skill to draw, as it necessarily involves bonds in the front crossing the projection of those at the rear. In hands that are unskilled the distinction between right., and left-handed helices becomes unrecognizable. Experience has shown that a drawing of the view along the axis, following the method descrihed below, is sufficiently simple for students to reproduce it in an examination. A further advantage is that small errors are not cumulative, but can he damped out as the drawing is developed. The drawing is done in stages. First, guides are drawn linhtlv, .. . or in oencil: then the atoms and bonds are entered murt rtmspicuwjly. Freehand is ;ldrqusre rhruuyhutu. l'wu lints, each 10-IScm (1"-ti", crussat riehr alirle; in tht, nnddle. I t is convenient to relate directions io those of a clock face, with 12 o'clock at the too. The center of the dial. where the I ~ n cross, t is the projcstisn ( i t h e ux~softhehelix. Starting on the outside, at 12 o'clirk, and rnvviue wor hand cl~*.kwise. draw freehand a spiral of decreasing ridius for just over twd turns, ending 1-2 cm (about %") from the center. Atoms a t the near end of the helix lie on the outer part of the curve; perspective makes the remote end apparently smaller. Reference to the threads on a bolt shows that this guide represents a right-handed thread. The guides are now complete. I t is convenient to view the helix from the N-terminal end. An ideal a-helix has 3.6 amino acid residues per turn. This statement can he rephrased; similar atoms repeat at intervals of 100°, or, in the context of this article, in directions 3 hr 20 min apart. Starting a t the outer end, a t 12.00 hr, enter the nitrogen atoms. The proper times are: 12.00; 3.20; 6.40; 10.00; 1.20; 4.40; 8.00; 11.20; 2.40; etc. However, intervals of three and a half hours (10S9,3.4 amino acid residues per turn) are more easily remembered, and are of adequate precision; the times are then: 12.00; 3.30; 7.00; 10.30; 2.00; 5.30; 9.00; 12.30; etc. Every nitrogen atom has its attached hydrogen-which is to
Figure 1. StaR with a freehand drawing af a spiral of decreasing radius far just Over two turns goinq clockwise.
he added later-lying outside the helix; that is, further from the center of the drawing. Each arc between a pair of nitrogen atoms is then divided into three sensibly equal parts by adding two carbon atoms. The one a t the "earlier time" carries a hydrogen atom, and a group R whose nature defines the amino acid. In this projection, for amino acids of the L-series, R always appears at a "later time" than does H. The carbon at the "later time" forms a carbonyl group whose oxygen atom lies nearer the center than its attached carbon. Finally, theH-, and 0-atoms of the amide functions are entered on N, and C, respectively, to form linear N-H . .O=C sequences, and, if possible, the guides are erased. The diagram shows that every unit derived from an amino acid of the type RCHNHzC02H forms two hydrogen bonds, and that all the N-to-H vectors point toward the N-terminal end of the helix; there is no requirement, however, for them to be parallel to the axis of the helix. The introduction of proline, or of any similar amino acid which cannot participate in such hydrogen bonding obviously disrupts the pattern. The mirror image of the diagram is a left-handed helix of 33-amino-acids which is stereochemically, and visibly distinct from a left-handed helix of L-amino acids.
Technical Note
The drawings use an equi-angular spiral, with the guides crossing at the true center. Such a spiral is constructed by setting a shaft normal to the paper, and tying the pen to the shaft with thread. As the pen moves across the paper the thread is wound onto the shaft. True perspective requires a spiral whose radius decreases by a constant fraction at each atom down the chain (for the diagram shown, about 4.9%); however, the rhnins, and the individual atoms are progressively more crowded near the center. In practice, freehand drawings approximate ta an equi-angular spiral.
Fiwre 2. An ideal u-helix has 3.6 amino acld residues Der turn
Volume 60 Number 7 July 1983
549
