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A formyl group is the acyl group $\mathrm{H}-\mathrm{C}(=\mathrm{O})-$, attached to the remainder of a molecule through its carbonyl carbon. It contains one carbon, one oxygen and one hydrogen; the terminal dash denotes the attachment bond, not a negative charge. IUPAC retains formyl as a preferred prefix and lists methanoyl as another name for the same substituent.^[ref]

The attachment atom matters. In acetaldehyde, $\mathrm{C}{\mathrm{H}}_3-\mathrm{CHO}$, the formyl group is attached to a carbon and gives an aldehyde. Attaching $\mathrm{HCO}-$ to a nitrogen instead gives an N-formyl derivative, with different chemical surroundings. Thus recognizing a formyl group does not make the entire compound an aldehyde. Nor is a bonded substituent automatically a free radical or an independently existing ion.

A useful molecular-formula calculation starts with methionine, ${\mathrm{C}}_5{\mathrm{H}}_{11}\mathrm{N}{\mathrm{O}}_2\mathrm{S}$. Formally replacing one hydrogen on its amino nitrogen by $\mathrm{CHO}$ adds $\mathrm{CO}$ overall: one hydrogen leaves and the attached group supplies one. The neutral free acid consequently becomes ${\mathrm{C}}_6{\mathrm{H}}_{11}\mathrm{N}{\mathrm{O}}_3\mathrm{S}$, the formula recorded for formylmethionine.^[ref] This is atom bookkeeping, not a preparative reaction or the formula of a residue already bound to transfer RNA.

The formyl group can be derived formally from formic acid, $\mathrm{HCOOH}$, by removing its hydroxyl group at the attachment site. This explains the nomenclature relationship while keeping the connectivity and the chemical species distinct.

Formylmethionine, usually written fMet in bacterial initiation, is methionine whose amino group bears a formyl group, $-\mathrm{C}(=\mathrm{O})\mathrm{H}$. The nitrogen modification makes an amide; it does not replace the sulfur-containing side chain. The free neutral L-form has molecular formula ${\mathrm{C}}_6{\mathrm{H}}_{11}\mathrm{N}{\mathrm{O}}_3\mathrm{S}$.^[ref]

In canonical bacterial initiation, methionine is first attached to an initiator transfer RNA and then formylated on that carrier. Bacterial initiation factor IF2 recognizes the formylated end during assembly at the ribosomal P site. Free formylmethionine, its tRNA-bound form and the initiating residue in a peptide are different chemical states.^[ref]

An original atom balance clarifies the modification. Replacing one amino-nitrogen hydrogen in free methionine, ${\mathrm{C}}_5{\mathrm{H}}_{11}\mathrm{N}{\mathrm{O}}_2\mathrm{S}$, by $\mathrm{CHO}$ gives a net addition of $\mathrm{CO}$ and hence ${\mathrm{C}}_6{\mathrm{H}}_{11}\mathrm{N}{\mathrm{O}}_3\mathrm{S}$. The free-compound formula does not include the transfer RNA or account for its ester linkage.

Formylation alone is not a complete initiation signal. In a primary experiment, anticodon-stem variants of an Escherichia coli initiator were purified, aminoacylated and formylated. Some retained IF2 binding but lost efficient P-site binding and initiation. The carrier's RNA structure therefore contributes recognition information beyond its attached formylmethionine.^[ref]